Changes v10->v11:
-----------------
- Adhemerval's latest glibc patch currently lives here:
https://github.com/zatrazz/glibc/commits/azanella/getrandom-vdso
It's still in flux (he hasn't posted it to glibc-alpha yet), but that
should still give some idea.
- Substantially increase documentation detail throughout.
- Numerous fixes to style and clarity.
- Add two kselftests - one for the chacha20 assembly implementation, which
compares its output to that of libsodium, and one for the actual vDSO
function and vgetrandom_alloc() syscall, which also has some built-in quick &
dirty benchmarking functionality.
- Add reserved `unsigned long addr` argument to syscall, for future use.
- Enforce that `flags`, `addr`, and `*size_per_each` are all zero on input, so
that they can actually be used in the future.
- Use VM_LOCKONFAULT|VM_NORESERVE so that memory isn't wasted until it's used.
- Set VM_DONTDUMP to prevent state from being written out to core dumps.
- Do mapping and flag setting all in one operation, so that it's not racy, and
so that dropping the mm lock between operations isn't required.
- Due to the above, there's no longer any purpose in selecting
VGETRANDOM_ALLOC_SYSCALL, so remove that.
- Don't update *num or *size_per_each until the mapping has succeeded.
- Introduce vdso_kernel_ulong type for representing the long type that
the kernel uses, even when in 32-bit compat code on a 64-bit kernel.
- Don't use 'bool' type in the vDSO page, in case of compiler quirks resulting
in a different interpretation of the type in different compilation contexts.
--------------
Two statements:
1) Userspace wants faster cryptographically secure random numbers of
arbitrary size, big or small.
2) Userspace is currently unable to safely roll its own RNG with the
same security profile as getrandom().
Statement (1) has been debated for years, with arguments ranging from
"we need faster cryptographically secure card shuffling!" to "the only
things that actually need good randomness are keys, which are few and
far between" to "actually, TLS CBC nonces are frequent" and so on. I
don't intend to wade into that debate substantially, except to note that
recently glibc added arc4random(), whose goal is to return a
cryptographically secure uint32_t, and there are real user reports of it
being too slow. So here we are.
Statement (2) is more interesting. The kernel is the nexus of all
entropic inputs that influence the RNG. It is in the best position, and
probably the only position, to decide anything at all about the current
state of the RNG and of its entropy. One of the things it uniquely knows
about is when reseeding is necessary.
For example, when a virtual machine is forked, restored, or duplicated,
it's imparative that the RNG doesn't generate the same outputs. For this
reason, there's a small protocol between hypervisors and the kernel that
indicates this has happened, alongside some ID, which the RNG uses to
immediately reseed, so as not to return the same numbers. Were userspace
to expand a getrandom() seed from time T1 for the next hour, and at some
point T2 < hour, the virtual machine forked, userspace would continue to
provide the same numbers to two (or more) different virtual machines,
resulting in potential cryptographic catastrophe. Something similar
happens on resuming from hibernation (or even suspend), with various
compromise scenarios there in mind.
There's a more general reason why userspace rolling its own RNG from a
getrandom() seed is fraught. There's a lot of attention paid to this
particular Linuxism we have of the RNG being initialized and thus
non-blocking or uninitialized and thus blocking until it is initialized.
These are our Two Big States that many hold to be the holy
differentiating factor between safe and not safe, between
cryptographically secure and garbage. The fact is, however, that the
distinction between these two states is a hand-wavy wishy-washy inexact
approximation. Outside of a few exceptional cases (e.g. a HW RNG is
available), we actually don't really ever know with any rigor at all
when the RNG is safe and ready (nor when it's compromised). We do the
best we can to "estimate" it, but entropy estimation is fundamentally
impossible in the general case. So really, we're just doing guess work,
and hoping it's good and conservative enough. Let's then assume that
there's always some potential error involved in this differentiator.
In fact, under the surface, the RNG is engineered around a different
principal, and that is trying to *use* new entropic inputs regularly and
at the right specific moments in time. For example, close to boot time,
the RNG reseeds itself more often than later. At certain events, like VM
fork, the RNG reseeds itself immediately. The various heuristics for
when the RNG will use new entropy and how often is really a core aspect
of what the RNG has some potential to do decently enough (and something
that will probably continue to improve in the future from random.c's
present set of algorithms). So in your mind, put away the metal
attachment to the Two Big States, which represent an approximation with
a potential margin of error. Instead keep in mind that the RNG's primary
operating heuristic is how often and exactly when it's going to reseed.
So, if userspace takes a seed from getrandom() at point T1, and uses it
for the next hour (or N megabytes or some other meaningless metric),
during that time, potential errors in the Two Big States approximation
are amplified. During that time potential reseeds are being lost,
forgotten, not reflected in the output stream. That's not good.
The simplest statement you could make is that userspace RNGs that expand
a getrandom() seed at some point T1 are nearly always *worse*, in some
way, than just calling getrandom() every time a random number is
desired.
For those reasons, after some discussion on libc-alpha, glibc's
arc4random() now just calls getrandom() on each invocation. That's
trivially safe, and gives us latitude to then make the safe thing faster
without becoming unsafe at our leasure. Card shuffling isn't
particularly fast, however.
How do we rectify this? By putting a safe implementation of getrandom()
in the vDSO, which has access to whatever information a
particular iteration of random.c is using to make its decisions. I use
that careful language of "particular iteration of random.c", because the
set of things that a vDSO getrandom() implementation might need for making
decisions as good as the kernel's will likely change over time. This
isn't just a matter of exporting certain *data* to userspace. We're not
going to commit to a "data API" where the various heuristics used are
exposed, locking in how the kernel works for decades to come, and then
leave it to various userspaces to roll something on top and shoot
themselves in the foot and have all sorts of complexity disasters.
Rather, vDSO getrandom() is supposed to be the *same exact algorithm*
that runs in the kernel, except it's been hoisted into userspace as
much as possible. And so vDSO getrandom() and kernel getrandom() will
always mirror each other hermetically.
API-wise, the vDSO gains this function:
ssize_t vgetrandom(void *buffer, size_t len, unsigned int flags, void *opaque_state);
The return value and the first 3 arguments are the same as ordinary
getrandom(), while the last argument is a pointer to some state
allocated with vgetrandom_alloc(), explained below. Were all four
arguments passed to the getrandom syscall, nothing different would
happen, and the functions would have the exact same behavior.
Then, we introduce a new syscall:
void *vgetrandom_alloc(unsigned int *num, unsigned int *size_per_each,
unsigned long addr, unsigned int flags);
This takes a hinted number of opaque states in `num`, and returns a
pointer to an array of opaque states, the number actually allocated back
in `num`, and the size in bytes of each one in `size_per_each`, enabling
a libc to slice up the returned array into a state per each thread. (The
`flags` and `addr` arguments, as well as the `*size_per_each` input
value, are reserved for the future and are forced to be zero for now.)
Libc is expected to allocate a chunk of these on first use, and then
dole them out to threads as they're created, allocating more when
needed. The returned address of the first state may be passed to
munmap(2) with a length of `num * size_per_each`, in order to deallocate
the memory.
We very intentionally do *not* leave state allocation up to the caller
of vgetrandom, but provide vgetrandom_alloc for that allocation. There
are too many weird things that can go wrong, and it's important that
vDSO does not provide too generic of a mechanism. It's not going to
store its state in just any old memory address. It'll do it only in ones
it allocates.
Right now this means it's a mlock'd page with WIPEONFORK set. In the
future maybe there will be other interesting page flags or
anti-heartbleed measures, or other platform-specific kernel-specific
things that can be set from the syscall. Again, it's important that the
kernel has a say in how this works rather than agreeing to operate on
any old address; memory isn't neutral.
The interesting meat of the implementation is in lib/vdso/getrandom.c,
as generic C code, and it aims to mainly follow random.c's buffered fast
key erasure logic. Before the RNG is initialized, it falls back to the
syscall. Right now it uses a simple generation counter to make its decisions
on reseeding (though this could be made more extensive over time).
The actual place that has the most work to do is in all of the other
files. Most of the vDSO shared page infrastructure is centered around
gettimeofday, and so the main structs are all in arrays for different
timestamp types, and attached to time namespaces, and so forth. I've
done the best I could to add onto this in an unintrusive way.
In my test results, performance is pretty stellar (around 15x for uint32_t
generation), and it seems to be working. There's an extended example in the
second commit of this series, showing how the syscall and the vDSO function
are meant to be used together.
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: Thomas Gleixner <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Adhemerval Zanella Netto <[email protected]>
Cc: Carlos O'Donell <[email protected]>
Cc: Florian Weimer <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Christian Brauner <[email protected]>
Jason A. Donenfeld (4):
random: add vgetrandom_alloc() syscall
arch: allocate vgetrandom_alloc() syscall number
random: introduce generic vDSO getrandom() implementation
x86: vdso: Wire up getrandom() vDSO implementation
MAINTAINERS | 2 +
arch/alpha/kernel/syscalls/syscall.tbl | 1 +
arch/arm/tools/syscall.tbl | 1 +
arch/arm64/include/asm/unistd.h | 2 +-
arch/arm64/include/asm/unistd32.h | 2 +
arch/ia64/kernel/syscalls/syscall.tbl | 1 +
arch/m68k/kernel/syscalls/syscall.tbl | 1 +
arch/microblaze/kernel/syscalls/syscall.tbl | 1 +
arch/mips/kernel/syscalls/syscall_n32.tbl | 1 +
arch/mips/kernel/syscalls/syscall_n64.tbl | 1 +
arch/mips/kernel/syscalls/syscall_o32.tbl | 1 +
arch/parisc/kernel/syscalls/syscall.tbl | 1 +
arch/powerpc/kernel/syscalls/syscall.tbl | 1 +
arch/s390/kernel/syscalls/syscall.tbl | 1 +
arch/sh/kernel/syscalls/syscall.tbl | 1 +
arch/sparc/kernel/syscalls/syscall.tbl | 1 +
arch/x86/Kconfig | 1 +
arch/x86/entry/syscalls/syscall_32.tbl | 1 +
arch/x86/entry/syscalls/syscall_64.tbl | 1 +
arch/x86/entry/vdso/Makefile | 3 +-
arch/x86/entry/vdso/vdso.lds.S | 2 +
arch/x86/entry/vdso/vgetrandom-chacha.S | 177 +++++++++++
arch/x86/entry/vdso/vgetrandom.c | 17 ++
arch/x86/include/asm/vdso/getrandom.h | 55 ++++
arch/x86/include/asm/vdso/vsyscall.h | 2 +
arch/x86/include/asm/vvar.h | 16 +
arch/xtensa/kernel/syscalls/syscall.tbl | 1 +
drivers/char/random.c | 137 +++++++++
include/linux/syscalls.h | 3 +
include/uapi/asm-generic/unistd.h | 5 +-
include/vdso/datapage.h | 12 +
include/vdso/getrandom.h | 44 +++
include/vdso/types.h | 35 +++
kernel/sys_ni.c | 3 +
lib/vdso/Kconfig | 5 +
lib/vdso/getrandom.c | 205 +++++++++++++
tools/include/uapi/asm-generic/unistd.h | 5 +-
.../arch/mips/entry/syscalls/syscall_n64.tbl | 1 +
.../arch/powerpc/entry/syscalls/syscall.tbl | 1 +
.../perf/arch/s390/entry/syscalls/syscall.tbl | 1 +
.../arch/x86/entry/syscalls/syscall_64.tbl | 1 +
tools/testing/selftests/vDSO/.gitignore | 2 +
tools/testing/selftests/vDSO/Makefile | 11 +
.../testing/selftests/vDSO/vdso_test_chacha.c | 43 +++
.../selftests/vDSO/vdso_test_getrandom.c | 279 ++++++++++++++++++
45 files changed, 1084 insertions(+), 4 deletions(-)
create mode 100644 arch/x86/entry/vdso/vgetrandom-chacha.S
create mode 100644 arch/x86/entry/vdso/vgetrandom.c
create mode 100644 arch/x86/include/asm/vdso/getrandom.h
create mode 100644 include/vdso/getrandom.h
create mode 100644 include/vdso/types.h
create mode 100644 lib/vdso/getrandom.c
create mode 100644 tools/testing/selftests/vDSO/vdso_test_chacha.c
create mode 100644 tools/testing/selftests/vDSO/vdso_test_getrandom.c
--
2.38.1
The vDSO getrandom() works over an opaque per-thread state of an
unexported size, which must be marked as MADV_WIPEONFORK and be
mlock()'d for proper operation. Over time, the nuances of these
allocations may change or grow or even differ based on architectural
features.
The syscall has the signature:
void *vgetrandom_alloc(unsigned int *num, unsigned int *size_per_each,
unsigned long addr, unsigned int flags);
This takes a hinted number of opaque states in `num`, and returns a
pointer to an array of opaque states, the number actually allocated back
in `num`, and the size in bytes of each one in `size_per_each`, enabling
a libc to slice up the returned array into a state per each thread. (The
`flags` and `addr` arguments, as well as the `*size_per_each` input
value, are reserved for the future and are forced to be zero zero for
now.)
Libc is expected to allocate a chunk of these on first use, and then
dole them out to threads as they're created, allocating more when
needed. The returned address of the first state may be passed to
munmap(2) with a length of `num * size_per_each`, in order to deallocate
the memory.
We very intentionally do *not* leave state allocation for vDSO
getrandom() up to userspace itself, but rather provide this new syscall
for such allocations. vDSO getrandom() must not store its state in just
any old memory address, but rather just ones that the kernel specially
allocates for it, leaving the particularities of those allocations up to
the kernel.
The allocation of states is intended to be integrated into libc's thread
management. As an illustrative example, the following code might be used
to do the same outside of libc. Though, vgetrandom_alloc() is not
expected to be exposed outside of libc, and the pthread usage here is
expected to be elided into libc internals. This allocation scheme is
very naive and does not shrink; other implementations may choose to be
more complex.
static void *vgetrandom_alloc(unsigned int *num, unsigned int *size_per_each)
{
*size_per_each = 0; /* Must be zero on input. */
return (void *)syscall(__NR_vgetrandom_alloc, &num, &size_per_each,
0 /* reserved @addr */, 0 /* reserved @flags */);
}
static struct {
pthread_mutex_t lock;
void **states;
size_t len, cap;
} grnd_allocator = {
.lock = PTHREAD_MUTEX_INITIALIZER
};
static void *vgetrandom_get_state(void)
{
void *state = NULL;
pthread_mutex_lock(&grnd_allocator.lock);
if (!grnd_allocator.len) {
size_t new_cap;
unsigned int num = sysconf(_SC_NPROCESSORS_ONLN); /* Could be arbitrary, just a hint. */
unsigned int size_per_each;
void *new_block = vgetrandom_alloc(&num, &size_per_each);
void *new_states;
if (new_block == MAP_FAILED)
goto out;
new_cap = grnd_allocator.cap + num;
new_states = reallocarray(grnd_allocator.states, new_cap, sizeof(*grnd_allocator.states));
if (!new_states) {
munmap(new_block, num * size_per_each);
goto out;
}
grnd_allocator.cap = new_cap;
grnd_allocator.states = new_states;
for (size_t i = 0; i < num; ++i) {
grnd_allocator.states[i] = new_block;
new_block += size_per_each;
}
grnd_allocator.len = num;
}
state = grnd_allocator.states[--grnd_allocator.len];
out:
pthread_mutex_unlock(&grnd_allocator.lock);
return state;
}
static void vgetrandom_put_state(void *state)
{
if (!state)
return;
pthread_mutex_lock(&grnd_allocator.lock);
grnd_allocator.states[grnd_allocator.len++] = state;
pthread_mutex_unlock(&grnd_allocator.lock);
}
Signed-off-by: Jason A. Donenfeld <[email protected]>
---
MAINTAINERS | 1 +
drivers/char/random.c | 130 +++++++++++++++++++++++++++++++++++++++
include/linux/syscalls.h | 3 +
include/vdso/getrandom.h | 16 +++++
kernel/sys_ni.c | 3 +
lib/vdso/Kconfig | 5 ++
6 files changed, 158 insertions(+)
create mode 100644 include/vdso/getrandom.h
diff --git a/MAINTAINERS b/MAINTAINERS
index 256f03904987..3894f947a507 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -17287,6 +17287,7 @@ T: git https://git.kernel.org/pub/scm/linux/kernel/git/crng/random.git
S: Maintained
F: drivers/char/random.c
F: drivers/virt/vmgenid.c
+F: include/vdso/getrandom.h
RAPIDIO SUBSYSTEM
M: Matt Porter <[email protected]>
diff --git a/drivers/char/random.c b/drivers/char/random.c
index e872acc1238f..21692f7f4ea0 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -8,6 +8,7 @@
* into roughly six sections, each with a section header:
*
* - Initialization and readiness waiting.
+ * - vDSO support helpers.
* - Fast key erasure RNG, the "crng".
* - Entropy accumulation and extraction routines.
* - Entropy collection routines.
@@ -39,6 +40,7 @@
#include <linux/blkdev.h>
#include <linux/interrupt.h>
#include <linux/mm.h>
+#include <linux/mman.h>
#include <linux/nodemask.h>
#include <linux/spinlock.h>
#include <linux/kthread.h>
@@ -56,6 +58,9 @@
#include <linux/sched/isolation.h>
#include <crypto/chacha.h>
#include <crypto/blake2s.h>
+#ifdef CONFIG_VDSO_GETRANDOM
+#include <vdso/getrandom.h>
+#endif
#include <asm/processor.h>
#include <asm/irq.h>
#include <asm/irq_regs.h>
@@ -168,6 +173,131 @@ int __cold execute_with_initialized_rng(struct notifier_block *nb)
__func__, (void *)_RET_IP_, crng_init)
+
+/********************************************************************
+ *
+ * vDSO support helpers.
+ *
+ * The actual vDSO function is defined over in lib/vdso/getrandom.c,
+ * but this section contains the kernel-mode helpers to support that.
+ *
+ ********************************************************************/
+
+#ifdef CONFIG_VDSO_GETRANDOM
+/**
+ * sys_vgetrandom_alloc - Allocate opaque states for use with vDSO getrandom().
+ *
+ * @num: On input, a pointer to a suggested hint of how many states to
+ * allocate, and on return the number of states actually allocated.
+ *
+ * @size_per_each: On input, must be zero. On return, the size of each state allocated,
+ * so that the caller can split up the returned allocation into
+ * individual states.
+ *
+ * @addr: Reserved, must be zero.
+ *
+ * @flags: Reserved, must be zero.
+ *
+ * The getrandom() vDSO function in userspace requires an opaque state, which
+ * this function allocates by mapping a certain number of special pages into
+ * the calling process. It takes a hint as to the number of opaque states
+ * desired, and provides the caller with the number of opaque states actually
+ * allocated, the size of each one in bytes, and the address of the first
+ * state, which may be split up into @num states of @size_per_each bytes each,
+ * by adding @size_per_each to the returned first state @num times.
+ *
+ * Returns the address of the first state in the allocation on success, or a
+ * negative error value on failure.
+ *
+ * The returned address of the first state may be passed to munmap(2) with a
+ * length of `(size_t)num * (size_t)size_per_each`, in order to deallocate the
+ * memory, after which it is invalid to pass it to vDSO getrandom().
+ *
+ * States allocated by this function must not be dereferenced, written, read,
+ * or otherwise manipulated. The *only* supported operations are:
+ * - Splitting up the states in intervals of @size_per_each, no more than
+ * @num times from the first state.
+ * - Passing a state to the getrandom() vDSO function's @opaque_state
+ * parameter, but not passing the same state at the same time to two such
+ * calls.
+ * - Passing the first state to munmap(2), as described above.
+ * All other uses are undefined behavior, which is subject to change or removal.
+ */
+SYSCALL_DEFINE4(vgetrandom_alloc, unsigned int __user *, num,
+ unsigned int __user *, size_per_each, unsigned long, addr,
+ unsigned int, flags)
+{
+ struct mm_struct *mm = current->mm;
+ size_t alloc_size, num_states;
+ const size_t state_size = sizeof(struct vgetrandom_state);
+ unsigned long pages_addr, populate, mm_flags;
+ unsigned int num_hint;
+ int ret;
+
+ /*
+ * @flags and @addr are currently unused, so in order to reserve them
+ * for the future, force them to be set to zero by current callers.
+ */
+ if (flags || addr)
+ return -EINVAL;
+
+ /*
+ * Also enforce that *size_per_each is zero on input, in case this becomes
+ * useful later on.
+ */
+ if (get_user(num_hint, size_per_each))
+ return -EFAULT;
+ if (num_hint)
+ return -EINVAL;
+
+ if (get_user(num_hint, num))
+ return -EFAULT;
+
+ num_states = clamp_t(size_t, num_hint, 1, (SIZE_MAX & PAGE_MASK) / state_size);
+ alloc_size = PAGE_ALIGN(num_states * state_size);
+
+ if (mmap_write_lock_killable(mm))
+ return -EINTR;
+ mm_flags = mm->def_flags;
+
+ mm->def_flags |=
+ /*
+ * Don't allow state to be written to swap, to preserve forward secrecy.
+ * This works in conjunction with MAP_LOCKED in do_mmap(), below, which
+ * actually does the locking (and associated permission check and accounting).
+ * Here, VM_LOCKONFAULT together with VM_NORESERVE simply make the mlocking
+ * happen the first time it's actually used, the same as when calling
+ * mlock2(MLOCK_ONFAULT) from userspace.
+ */
+ VM_LOCKONFAULT | VM_NORESERVE |
+
+ /* Don't allow the state to survive forks, to prevent random number re-use. */
+ VM_WIPEONFORK |
+
+ /* Don't write random state into coredumps. */
+ VM_DONTDUMP;
+
+ pages_addr = do_mmap(NULL, 0, alloc_size, PROT_READ | PROT_WRITE,
+ MAP_PRIVATE | MAP_ANONYMOUS | MAP_LOCKED, 0,
+ &populate, NULL);
+
+ mm->def_flags = mm_flags;
+ mmap_write_unlock(mm);
+ if (IS_ERR_VALUE(pages_addr))
+ return pages_addr;
+
+ ret = -EFAULT;
+ if (put_user(alloc_size / state_size, num) || put_user(state_size, size_per_each))
+ goto err_unmap;
+
+ return pages_addr;
+
+err_unmap:
+ vm_munmap(pages_addr, alloc_size);
+ return ret;
+}
+#endif
+
/*********************************************************************
*
* Fast key erasure RNG, the "crng".
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index a34b0f9a9972..dc13b0715836 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -1006,6 +1006,9 @@ asmlinkage long sys_seccomp(unsigned int op, unsigned int flags,
void __user *uargs);
asmlinkage long sys_getrandom(char __user *buf, size_t count,
unsigned int flags);
+asmlinkage long sys_vgetrandom_alloc(unsigned int __user *num,
+ unsigned int __user *size_per_each,
+ unsigned long addr, unsigned int flags);
asmlinkage long sys_memfd_create(const char __user *uname_ptr, unsigned int flags);
asmlinkage long sys_bpf(int cmd, union bpf_attr *attr, unsigned int size);
asmlinkage long sys_execveat(int dfd, const char __user *filename,
diff --git a/include/vdso/getrandom.h b/include/vdso/getrandom.h
new file mode 100644
index 000000000000..e3ceb1976386
--- /dev/null
+++ b/include/vdso/getrandom.h
@@ -0,0 +1,16 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (C) 2022 Jason A. Donenfeld <[email protected]>. All Rights Reserved.
+ */
+
+#ifndef _VDSO_GETRANDOM_H
+#define _VDSO_GETRANDOM_H
+
+/**
+ * struct vgetrandom_state - State used by vDSO getrandom() and allocated by vgetrandom_alloc().
+ *
+ * Currently empty, as the vDSO getrandom() function has not yet been implemented.
+ */
+struct vgetrandom_state { int placeholder; };
+
+#endif /* _VDSO_GETRANDOM_H */
diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
index 860b2dcf3ac4..f28196cb919b 100644
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c
@@ -360,6 +360,9 @@ COND_SYSCALL(pkey_free);
/* memfd_secret */
COND_SYSCALL(memfd_secret);
+/* random */
+COND_SYSCALL(vgetrandom_alloc);
+
/*
* Architecture specific weak syscall entries.
*/
diff --git a/lib/vdso/Kconfig b/lib/vdso/Kconfig
index d883ac299508..3b394fa83f65 100644
--- a/lib/vdso/Kconfig
+++ b/lib/vdso/Kconfig
@@ -31,3 +31,8 @@ config GENERIC_VDSO_TIME_NS
VDSO
endif
+
+config VDSO_GETRANDOM
+ bool
+ help
+ Selected by architectures that support vDSO getrandom().
--
2.38.1
Add vgetrandom_alloc() as syscall 451 (or 561 on alpha) by adding it to
all of the various syscall.tbl and unistd.h files.
Acked-by: Geert Uytterhoeven <[email protected]>
Signed-off-by: Jason A. Donenfeld <[email protected]>
---
arch/alpha/kernel/syscalls/syscall.tbl | 1 +
arch/arm/tools/syscall.tbl | 1 +
arch/arm64/include/asm/unistd.h | 2 +-
arch/arm64/include/asm/unistd32.h | 2 ++
arch/ia64/kernel/syscalls/syscall.tbl | 1 +
arch/m68k/kernel/syscalls/syscall.tbl | 1 +
arch/microblaze/kernel/syscalls/syscall.tbl | 1 +
arch/mips/kernel/syscalls/syscall_n32.tbl | 1 +
arch/mips/kernel/syscalls/syscall_n64.tbl | 1 +
arch/mips/kernel/syscalls/syscall_o32.tbl | 1 +
arch/parisc/kernel/syscalls/syscall.tbl | 1 +
arch/powerpc/kernel/syscalls/syscall.tbl | 1 +
arch/s390/kernel/syscalls/syscall.tbl | 1 +
arch/sh/kernel/syscalls/syscall.tbl | 1 +
arch/sparc/kernel/syscalls/syscall.tbl | 1 +
arch/x86/entry/syscalls/syscall_32.tbl | 1 +
arch/x86/entry/syscalls/syscall_64.tbl | 1 +
arch/xtensa/kernel/syscalls/syscall.tbl | 1 +
include/uapi/asm-generic/unistd.h | 5 ++++-
tools/include/uapi/asm-generic/unistd.h | 5 ++++-
tools/perf/arch/mips/entry/syscalls/syscall_n64.tbl | 1 +
tools/perf/arch/powerpc/entry/syscalls/syscall.tbl | 1 +
tools/perf/arch/s390/entry/syscalls/syscall.tbl | 1 +
tools/perf/arch/x86/entry/syscalls/syscall_64.tbl | 1 +
24 files changed, 31 insertions(+), 3 deletions(-)
diff --git a/arch/alpha/kernel/syscalls/syscall.tbl b/arch/alpha/kernel/syscalls/syscall.tbl
index 8ebacf37a8cf..a4bfd7b53d6f 100644
--- a/arch/alpha/kernel/syscalls/syscall.tbl
+++ b/arch/alpha/kernel/syscalls/syscall.tbl
@@ -490,3 +490,4 @@
558 common process_mrelease sys_process_mrelease
559 common futex_waitv sys_futex_waitv
560 common set_mempolicy_home_node sys_ni_syscall
+561 common vgetrandom_alloc sys_vgetrandom_alloc
diff --git a/arch/arm/tools/syscall.tbl b/arch/arm/tools/syscall.tbl
index ac964612d8b0..e10319cc6c3e 100644
--- a/arch/arm/tools/syscall.tbl
+++ b/arch/arm/tools/syscall.tbl
@@ -464,3 +464,4 @@
448 common process_mrelease sys_process_mrelease
449 common futex_waitv sys_futex_waitv
450 common set_mempolicy_home_node sys_set_mempolicy_home_node
+451 common vgetrandom_alloc sys_vgetrandom_alloc
diff --git a/arch/arm64/include/asm/unistd.h b/arch/arm64/include/asm/unistd.h
index 037feba03a51..64a514f90131 100644
--- a/arch/arm64/include/asm/unistd.h
+++ b/arch/arm64/include/asm/unistd.h
@@ -39,7 +39,7 @@
#define __ARM_NR_compat_set_tls (__ARM_NR_COMPAT_BASE + 5)
#define __ARM_NR_COMPAT_END (__ARM_NR_COMPAT_BASE + 0x800)
-#define __NR_compat_syscalls 451
+#define __NR_compat_syscalls 452
#endif
#define __ARCH_WANT_SYS_CLONE
diff --git a/arch/arm64/include/asm/unistd32.h b/arch/arm64/include/asm/unistd32.h
index 604a2053d006..7285b5a830cc 100644
--- a/arch/arm64/include/asm/unistd32.h
+++ b/arch/arm64/include/asm/unistd32.h
@@ -907,6 +907,8 @@ __SYSCALL(__NR_process_mrelease, sys_process_mrelease)
__SYSCALL(__NR_futex_waitv, sys_futex_waitv)
#define __NR_set_mempolicy_home_node 450
__SYSCALL(__NR_set_mempolicy_home_node, sys_set_mempolicy_home_node)
+#define __NR_vgetrandom_alloc 451
+__SYSCALL(__NR_vgetrandom_alloc, sys_vgetrandom_alloc)
/*
* Please add new compat syscalls above this comment and update
diff --git a/arch/ia64/kernel/syscalls/syscall.tbl b/arch/ia64/kernel/syscalls/syscall.tbl
index 72c929d9902b..5ed9667051fc 100644
--- a/arch/ia64/kernel/syscalls/syscall.tbl
+++ b/arch/ia64/kernel/syscalls/syscall.tbl
@@ -371,3 +371,4 @@
448 common process_mrelease sys_process_mrelease
449 common futex_waitv sys_futex_waitv
450 common set_mempolicy_home_node sys_set_mempolicy_home_node
+451 common vgetrandom_alloc sys_vgetrandom_alloc
diff --git a/arch/m68k/kernel/syscalls/syscall.tbl b/arch/m68k/kernel/syscalls/syscall.tbl
index b1f3940bc298..d9e7ea26dd26 100644
--- a/arch/m68k/kernel/syscalls/syscall.tbl
+++ b/arch/m68k/kernel/syscalls/syscall.tbl
@@ -450,3 +450,4 @@
448 common process_mrelease sys_process_mrelease
449 common futex_waitv sys_futex_waitv
450 common set_mempolicy_home_node sys_set_mempolicy_home_node
+451 common vgetrandom_alloc sys_vgetrandom_alloc
diff --git a/arch/microblaze/kernel/syscalls/syscall.tbl b/arch/microblaze/kernel/syscalls/syscall.tbl
index 820145e47350..c109e307a37b 100644
--- a/arch/microblaze/kernel/syscalls/syscall.tbl
+++ b/arch/microblaze/kernel/syscalls/syscall.tbl
@@ -456,3 +456,4 @@
448 common process_mrelease sys_process_mrelease
449 common futex_waitv sys_futex_waitv
450 common set_mempolicy_home_node sys_set_mempolicy_home_node
+451 common vgetrandom_alloc sys_vgetrandom_alloc
diff --git a/arch/mips/kernel/syscalls/syscall_n32.tbl b/arch/mips/kernel/syscalls/syscall_n32.tbl
index 253ff994ed2e..6d47d8231f7d 100644
--- a/arch/mips/kernel/syscalls/syscall_n32.tbl
+++ b/arch/mips/kernel/syscalls/syscall_n32.tbl
@@ -389,3 +389,4 @@
448 n32 process_mrelease sys_process_mrelease
449 n32 futex_waitv sys_futex_waitv
450 n32 set_mempolicy_home_node sys_set_mempolicy_home_node
+451 n32 vgetrandom_alloc sys_vgetrandom_alloc
diff --git a/arch/mips/kernel/syscalls/syscall_n64.tbl b/arch/mips/kernel/syscalls/syscall_n64.tbl
index 3f1886ad9d80..890e5b51e1fc 100644
--- a/arch/mips/kernel/syscalls/syscall_n64.tbl
+++ b/arch/mips/kernel/syscalls/syscall_n64.tbl
@@ -365,3 +365,4 @@
448 n64 process_mrelease sys_process_mrelease
449 n64 futex_waitv sys_futex_waitv
450 common set_mempolicy_home_node sys_set_mempolicy_home_node
+451 n64 vgetrandom_alloc sys_vgetrandom_alloc
diff --git a/arch/mips/kernel/syscalls/syscall_o32.tbl b/arch/mips/kernel/syscalls/syscall_o32.tbl
index 8f243e35a7b2..de512de148f5 100644
--- a/arch/mips/kernel/syscalls/syscall_o32.tbl
+++ b/arch/mips/kernel/syscalls/syscall_o32.tbl
@@ -438,3 +438,4 @@
448 o32 process_mrelease sys_process_mrelease
449 o32 futex_waitv sys_futex_waitv
450 o32 set_mempolicy_home_node sys_set_mempolicy_home_node
+451 o32 vgetrandom_alloc sys_vgetrandom_alloc
diff --git a/arch/parisc/kernel/syscalls/syscall.tbl b/arch/parisc/kernel/syscalls/syscall.tbl
index 8a99c998da9b..bab1cee627e3 100644
--- a/arch/parisc/kernel/syscalls/syscall.tbl
+++ b/arch/parisc/kernel/syscalls/syscall.tbl
@@ -448,3 +448,4 @@
448 common process_mrelease sys_process_mrelease
449 common futex_waitv sys_futex_waitv
450 common set_mempolicy_home_node sys_set_mempolicy_home_node
+451 common vgetrandom_alloc sys_vgetrandom_alloc
diff --git a/arch/powerpc/kernel/syscalls/syscall.tbl b/arch/powerpc/kernel/syscalls/syscall.tbl
index a0be127475b1..e6c04eda2363 100644
--- a/arch/powerpc/kernel/syscalls/syscall.tbl
+++ b/arch/powerpc/kernel/syscalls/syscall.tbl
@@ -537,3 +537,4 @@
448 common process_mrelease sys_process_mrelease
449 common futex_waitv sys_futex_waitv
450 nospu set_mempolicy_home_node sys_set_mempolicy_home_node
+451 common vgetrandom_alloc sys_vgetrandom_alloc
diff --git a/arch/s390/kernel/syscalls/syscall.tbl b/arch/s390/kernel/syscalls/syscall.tbl
index 799147658dee..5b0b2bea46da 100644
--- a/arch/s390/kernel/syscalls/syscall.tbl
+++ b/arch/s390/kernel/syscalls/syscall.tbl
@@ -453,3 +453,4 @@
448 common process_mrelease sys_process_mrelease sys_process_mrelease
449 common futex_waitv sys_futex_waitv sys_futex_waitv
450 common set_mempolicy_home_node sys_set_mempolicy_home_node sys_set_mempolicy_home_node
+451 common vgetrandom_alloc sys_vgetrandom_alloc sys_vgetrandom_alloc
diff --git a/arch/sh/kernel/syscalls/syscall.tbl b/arch/sh/kernel/syscalls/syscall.tbl
index 2de85c977f54..631f0bac0e9a 100644
--- a/arch/sh/kernel/syscalls/syscall.tbl
+++ b/arch/sh/kernel/syscalls/syscall.tbl
@@ -453,3 +453,4 @@
448 common process_mrelease sys_process_mrelease
449 common futex_waitv sys_futex_waitv
450 common set_mempolicy_home_node sys_set_mempolicy_home_node
+451 common vgetrandom_alloc sys_vgetrandom_alloc
diff --git a/arch/sparc/kernel/syscalls/syscall.tbl b/arch/sparc/kernel/syscalls/syscall.tbl
index 4398cc6fb68d..b4925978adea 100644
--- a/arch/sparc/kernel/syscalls/syscall.tbl
+++ b/arch/sparc/kernel/syscalls/syscall.tbl
@@ -496,3 +496,4 @@
448 common process_mrelease sys_process_mrelease
449 common futex_waitv sys_futex_waitv
450 common set_mempolicy_home_node sys_set_mempolicy_home_node
+451 common vgetrandom_alloc sys_vgetrandom_alloc
diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index 320480a8db4f..f5f863a33824 100644
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl
@@ -455,3 +455,4 @@
448 i386 process_mrelease sys_process_mrelease
449 i386 futex_waitv sys_futex_waitv
450 i386 set_mempolicy_home_node sys_set_mempolicy_home_node
+451 i386 vgetrandom_alloc sys_vgetrandom_alloc
diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index c84d12608cd2..0186f173f0e8 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -372,6 +372,7 @@
448 common process_mrelease sys_process_mrelease
449 common futex_waitv sys_futex_waitv
450 common set_mempolicy_home_node sys_set_mempolicy_home_node
+451 common vgetrandom_alloc sys_vgetrandom_alloc
#
# Due to a historical design error, certain syscalls are numbered differently
diff --git a/arch/xtensa/kernel/syscalls/syscall.tbl b/arch/xtensa/kernel/syscalls/syscall.tbl
index 52c94ab5c205..14d63a119cc2 100644
--- a/arch/xtensa/kernel/syscalls/syscall.tbl
+++ b/arch/xtensa/kernel/syscalls/syscall.tbl
@@ -421,3 +421,4 @@
448 common process_mrelease sys_process_mrelease
449 common futex_waitv sys_futex_waitv
450 common set_mempolicy_home_node sys_set_mempolicy_home_node
+451 common vgetrandom_alloc sys_vgetrandom_alloc
diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
index 45fa180cc56a..9d2e299f3e8a 100644
--- a/include/uapi/asm-generic/unistd.h
+++ b/include/uapi/asm-generic/unistd.h
@@ -886,8 +886,11 @@ __SYSCALL(__NR_futex_waitv, sys_futex_waitv)
#define __NR_set_mempolicy_home_node 450
__SYSCALL(__NR_set_mempolicy_home_node, sys_set_mempolicy_home_node)
+#define __NR_vgetrandom_alloc 451
+__SYSCALL(__NR_vgetrandom_alloc, sys_vgetrandom_alloc)
+
#undef __NR_syscalls
-#define __NR_syscalls 451
+#define __NR_syscalls 452
/*
* 32 bit systems traditionally used different
diff --git a/tools/include/uapi/asm-generic/unistd.h b/tools/include/uapi/asm-generic/unistd.h
index 45fa180cc56a..9d2e299f3e8a 100644
--- a/tools/include/uapi/asm-generic/unistd.h
+++ b/tools/include/uapi/asm-generic/unistd.h
@@ -886,8 +886,11 @@ __SYSCALL(__NR_futex_waitv, sys_futex_waitv)
#define __NR_set_mempolicy_home_node 450
__SYSCALL(__NR_set_mempolicy_home_node, sys_set_mempolicy_home_node)
+#define __NR_vgetrandom_alloc 451
+__SYSCALL(__NR_vgetrandom_alloc, sys_vgetrandom_alloc)
+
#undef __NR_syscalls
-#define __NR_syscalls 451
+#define __NR_syscalls 452
/*
* 32 bit systems traditionally used different
diff --git a/tools/perf/arch/mips/entry/syscalls/syscall_n64.tbl b/tools/perf/arch/mips/entry/syscalls/syscall_n64.tbl
index 3f1886ad9d80..890e5b51e1fc 100644
--- a/tools/perf/arch/mips/entry/syscalls/syscall_n64.tbl
+++ b/tools/perf/arch/mips/entry/syscalls/syscall_n64.tbl
@@ -365,3 +365,4 @@
448 n64 process_mrelease sys_process_mrelease
449 n64 futex_waitv sys_futex_waitv
450 common set_mempolicy_home_node sys_set_mempolicy_home_node
+451 n64 vgetrandom_alloc sys_vgetrandom_alloc
diff --git a/tools/perf/arch/powerpc/entry/syscalls/syscall.tbl b/tools/perf/arch/powerpc/entry/syscalls/syscall.tbl
index e9e0df4f9a61..d58da67a9766 100644
--- a/tools/perf/arch/powerpc/entry/syscalls/syscall.tbl
+++ b/tools/perf/arch/powerpc/entry/syscalls/syscall.tbl
@@ -534,3 +534,4 @@
448 common process_mrelease sys_process_mrelease
449 common futex_waitv sys_futex_waitv
450 nospu set_mempolicy_home_node sys_set_mempolicy_home_node
+451 common vgetrandom_alloc sys_vgetrandom_alloc
diff --git a/tools/perf/arch/s390/entry/syscalls/syscall.tbl b/tools/perf/arch/s390/entry/syscalls/syscall.tbl
index 799147658dee..5b0b2bea46da 100644
--- a/tools/perf/arch/s390/entry/syscalls/syscall.tbl
+++ b/tools/perf/arch/s390/entry/syscalls/syscall.tbl
@@ -453,3 +453,4 @@
448 common process_mrelease sys_process_mrelease sys_process_mrelease
449 common futex_waitv sys_futex_waitv sys_futex_waitv
450 common set_mempolicy_home_node sys_set_mempolicy_home_node sys_set_mempolicy_home_node
+451 common vgetrandom_alloc sys_vgetrandom_alloc sys_vgetrandom_alloc
diff --git a/tools/perf/arch/x86/entry/syscalls/syscall_64.tbl b/tools/perf/arch/x86/entry/syscalls/syscall_64.tbl
index c84d12608cd2..0186f173f0e8 100644
--- a/tools/perf/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/tools/perf/arch/x86/entry/syscalls/syscall_64.tbl
@@ -372,6 +372,7 @@
448 common process_mrelease sys_process_mrelease
449 common futex_waitv sys_futex_waitv
450 common set_mempolicy_home_node sys_set_mempolicy_home_node
+451 common vgetrandom_alloc sys_vgetrandom_alloc
#
# Due to a historical design error, certain syscalls are numbered differently
--
2.38.1
Provide a generic C vDSO getrandom() implementation, which operates on
an opaque state returned by vgetrandom_alloc() and produces random bytes
the same way as getrandom(). This has a the API signature:
ssize_t vgetrandom(void *buffer, size_t len, unsigned int flags, void *opaque_state);
The return value and the first 3 arguments are the same as ordinary
getrandom(), while the last argument is a pointer to the opaque
allocated state. Were all four arguments passed to the getrandom()
syscall, nothing different would happen, and the functions would have
the exact same behavior.
The actual vDSO RNG algorithm implemented is the same one implemented by
drivers/char/random.c, using the same fast-erasure techniques as that.
Should the in-kernel implementation change, so too will the vDSO one.
It requires an implementation of ChaCha20 that does not use any stack,
in order to maintain forward secrecy if a multi-threaded program forks
(though this does not account for a similar issue with SA_SIGINFO
copying registers to the stack), so this is left as an
architecture-specific fill-in. Stack-less ChaCha20 is an easy algorithm
to implement on a variety of architectures, so this shouldn't be too
onerous.
Initially, the state is keyless, and so the first call makes a
getrandom() syscall to generate that key, and then uses it for
subsequent calls. By keeping track of a generation counter, it knows
when its key is invalidated and it should fetch a new one using the
syscall. Later, more than just a generation counter might be used.
Since MADV_WIPEONFORK is set on the opaque state, the key and related
state is wiped during a fork(), so secrets don't roll over into new
processes, and the same state doesn't accidentally generate the same
random stream. The generation counter, as well, is always >0, so that
the 0 counter is a useful indication of a fork() or otherwise
uninitialized state.
If the kernel RNG is not yet initialized, then the vDSO always calls the
syscall, because that behavior cannot be emulated in userspace, but
fortunately that state is short lived and only during early boot. If it
has been initialized, then there is no need to inspect the `flags`
argument, because the behavior does not change post-initialization
regardless of the `flags` value.
Since the opaque state passed to it is mutated, vDSO getrandom() is not
reentrant, when used with the same opaque state, which libc should be
mindful of.
vgetrandom_alloc() and vDSO getrandom() provide the ability for
userspace to generate random bytes quickly and safely, and are intended
to be integrated into libc's thread management. As an illustrative
example, together with the example code from "random: add
vgetrandom_alloc() syscall", the following code might be used to do the
same outside of libc. In a libc, only the non-static vgetrandom()
function at the end would be exported as part of a getrandom()
implementations, and the various pthread-isms are expected to be elided
into libc internals.
static struct {
ssize_t(*fn)(void *buf, size_t len, unsigned long flags, void *state);
pthread_key_t key;
pthread_once_t initialized;
} grnd_ctx = {
.initialized = PTHREAD_ONCE_INIT
};
static void vgetrandom_init(void)
{
if (pthread_key_create(&grnd_ctx.key, vgetrandom_put_state) != 0)
return;
grnd_ctx.fn = vdso_sym("LINUX_2.6", "__vdso_getrandom");
}
ssize_t vgetrandom(void *buf, size_t len, unsigned long flags)
{
void *state;
pthread_once(&grnd_ctx.initialized, vgetrandom_init);
if (!grnd_ctx.fn)
return getrandom(buf, len, flags);
state = pthread_getspecific(grnd_ctx.key);
if (!state) {
state = vgetrandom_get_state();
if (pthread_setspecific(grnd_ctx.key, state) != 0) {
vgetrandom_put_state(state);
state = NULL;
}
if (!state)
return getrandom(buf, len, flags);
}
return grnd_ctx.fn(buf, len, flags, state);
}
Signed-off-by: Jason A. Donenfeld <[email protected]>
---
MAINTAINERS | 1 +
drivers/char/random.c | 7 +
include/vdso/datapage.h | 12 +
include/vdso/getrandom.h | 32 +-
include/vdso/types.h | 35 +++
lib/vdso/getrandom.c | 205 +++++++++++++
tools/testing/selftests/vDSO/.gitignore | 1 +
tools/testing/selftests/vDSO/Makefile | 2 +
.../selftests/vDSO/vdso_test_getrandom.c | 279 ++++++++++++++++++
9 files changed, 572 insertions(+), 2 deletions(-)
create mode 100644 include/vdso/types.h
create mode 100644 lib/vdso/getrandom.c
create mode 100644 tools/testing/selftests/vDSO/vdso_test_getrandom.c
diff --git a/MAINTAINERS b/MAINTAINERS
index 3894f947a507..70dff39fcff9 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -17288,6 +17288,7 @@ S: Maintained
F: drivers/char/random.c
F: drivers/virt/vmgenid.c
F: include/vdso/getrandom.h
+F: lib/vdso/getrandom.c
RAPIDIO SUBSYSTEM
M: Matt Porter <[email protected]>
diff --git a/drivers/char/random.c b/drivers/char/random.c
index 21692f7f4ea0..caa1893c8096 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -60,6 +60,7 @@
#include <crypto/blake2s.h>
#ifdef CONFIG_VDSO_GETRANDOM
#include <vdso/getrandom.h>
+#include <vdso/datapage.h>
#endif
#include <asm/processor.h>
#include <asm/irq.h>
@@ -400,6 +401,9 @@ static void crng_reseed(struct work_struct *work)
if (next_gen == ULONG_MAX)
++next_gen;
WRITE_ONCE(base_crng.generation, next_gen);
+#ifdef CONFIG_VDSO_GETRANDOM
+ smp_store_release(&_vdso_rng_data.generation, next_gen + 1);
+#endif
if (!static_branch_likely(&crng_is_ready))
crng_init = CRNG_READY;
spin_unlock_irqrestore(&base_crng.lock, flags);
@@ -850,6 +854,9 @@ static void __cold _credit_init_bits(size_t bits)
if (static_key_initialized)
execute_in_process_context(crng_set_ready, &set_ready);
atomic_notifier_call_chain(&random_ready_notifier, 0, NULL);
+#ifdef CONFIG_VDSO_GETRANDOM
+ smp_store_release(&_vdso_rng_data.is_ready, true);
+#endif
wake_up_interruptible(&crng_init_wait);
kill_fasync(&fasync, SIGIO, POLL_IN);
pr_notice("crng init done\n");
diff --git a/include/vdso/datapage.h b/include/vdso/datapage.h
index 73eb622e7663..d1f800c1c718 100644
--- a/include/vdso/datapage.h
+++ b/include/vdso/datapage.h
@@ -18,6 +18,7 @@
#include <vdso/time.h>
#include <vdso/time32.h>
#include <vdso/time64.h>
+#include <vdso/types.h>
#ifdef CONFIG_ARCH_HAS_VDSO_DATA
#include <asm/vdso/data.h>
@@ -109,6 +110,16 @@ struct vdso_data {
struct arch_vdso_data arch_data;
};
+/**
+ * struct vdso_rng_data - vdso RNG state information
+ * @generation: counter representing the number of RNG reseeds
+ * @is_ready: boolean signaling whether the RNG is initialized
+ */
+struct vdso_rng_data {
+ vdso_kernel_ulong generation;
+ u8 is_ready;
+};
+
/*
* We use the hidden visibility to prevent the compiler from generating a GOT
* relocation. Not only is going through a GOT useless (the entry couldn't and
@@ -120,6 +131,7 @@ struct vdso_data {
*/
extern struct vdso_data _vdso_data[CS_BASES] __attribute__((visibility("hidden")));
extern struct vdso_data _timens_data[CS_BASES] __attribute__((visibility("hidden")));
+extern struct vdso_rng_data _vdso_rng_data __attribute__((visibility("hidden")));
/*
* The generic vDSO implementation requires that gettimeofday.h
diff --git a/include/vdso/getrandom.h b/include/vdso/getrandom.h
index e3ceb1976386..7dc93d5f72dc 100644
--- a/include/vdso/getrandom.h
+++ b/include/vdso/getrandom.h
@@ -6,11 +6,39 @@
#ifndef _VDSO_GETRANDOM_H
#define _VDSO_GETRANDOM_H
+#include <crypto/chacha.h>
+#include <vdso/types.h>
+
/**
* struct vgetrandom_state - State used by vDSO getrandom() and allocated by vgetrandom_alloc().
*
- * Currently empty, as the vDSO getrandom() function has not yet been implemented.
+ * @batch: One and a half ChaCha20 blocks of buffered RNG output.
+ *
+ * @key: Key to be used for generating next batch.
+ *
+ * @batch_key: Union of the prior two members, which is exactly two full
+ * ChaCha20 blocks in size, so that @batch and @key can be filled
+ * together.
+ *
+ * @generation: Snapshot of @rng_info->generation in the vDSO data page at
+ * the time @key was generated.
+ *
+ * @pos: Offset into @batch of the next available random byte.
+ *
+ * @in_use: Reentrancy guard for reusing a state within the same thread
+ * due to signal handlers.
*/
-struct vgetrandom_state { int placeholder; };
+struct vgetrandom_state {
+ union {
+ struct {
+ u8 batch[CHACHA_BLOCK_SIZE * 3 / 2];
+ u32 key[CHACHA_KEY_SIZE / sizeof(u32)];
+ };
+ u8 batch_key[CHACHA_BLOCK_SIZE * 2];
+ };
+ vdso_kernel_ulong generation;
+ u8 pos;
+ bool in_use;
+};
#endif /* _VDSO_GETRANDOM_H */
diff --git a/include/vdso/types.h b/include/vdso/types.h
new file mode 100644
index 000000000000..ce131463aeff
--- /dev/null
+++ b/include/vdso/types.h
@@ -0,0 +1,35 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (C) 2022 Jason A. Donenfeld <[email protected]>. All Rights Reserved.
+ */
+#ifndef __VDSO_TYPES_H
+#define __VDSO_TYPES_H
+
+#include <linux/types.h>
+
+/**
+ * type vdso_kernel_ulong - unsigned long type that matches kernel's unsigned long
+ *
+ * Data shared between userspace and the kernel must operate the same way in both 64-bit code and in
+ * 32-bit compat code, over the same potentially 64-bit kernel. This type represents the size of an
+ * unsigned long as used by kernel code. This isn't necessarily the same as an unsigned long as used
+ * by userspace, however.
+ *
+ * +-------------------+-------------------+------------------+-------------------+
+ * | 32-bit userspace | 32-bit userspace | 64-bit userspace | 64-bit userspace |
+ * | unsigned long | vdso_kernel_ulong | unsigned long | vdso_kernel_ulong |
+ * +---------------+-------------------+-------------------+------------------+-------------------+
+ * | 32-bit kernel | ✓ same size | ✓ same size |
+ * | unsigned long | | |
+ * +---------------+-------------------+-------------------+------------------+-------------------+
+ * | 64-bit kernel | ✘ different size! | ✓ same size | ✓ same size | ✓ same size |
+ * | unsigned long | | | | |
+ * +---------------+-------------------+-------------------+------------------+-------------------+
+ */
+#ifdef CONFIG_64BIT
+typedef u64 vdso_kernel_ulong;
+#else
+typedef u32 vdso_kernel_ulong;
+#endif
+
+#endif /* __VDSO_TYPES_H */
diff --git a/lib/vdso/getrandom.c b/lib/vdso/getrandom.c
new file mode 100644
index 000000000000..9ca624756432
--- /dev/null
+++ b/lib/vdso/getrandom.c
@@ -0,0 +1,205 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2022 Jason A. Donenfeld <[email protected]>. All Rights Reserved.
+ */
+
+#include <linux/cache.h>
+#include <linux/kernel.h>
+#include <linux/time64.h>
+#include <vdso/datapage.h>
+#include <vdso/getrandom.h>
+#include <asm/vdso/getrandom.h>
+#include <asm/vdso/vsyscall.h>
+
+#define MEMCPY_AND_ZERO_SRC(type, dst, src, len) do { \
+ while (len >= sizeof(type)) { \
+ __put_unaligned_t(type, __get_unaligned_t(type, src), dst); \
+ __put_unaligned_t(type, 0, src); \
+ dst += sizeof(type); \
+ src += sizeof(type); \
+ len -= sizeof(type); \
+ } \
+} while (0)
+
+static void memcpy_and_zero_src(void *dst, void *src, size_t len)
+{
+ if (IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)) {
+ if (IS_ENABLED(CONFIG_64BIT))
+ MEMCPY_AND_ZERO_SRC(u64, dst, src, len);
+ MEMCPY_AND_ZERO_SRC(u32, dst, src, len);
+ MEMCPY_AND_ZERO_SRC(u16, dst, src, len);
+ }
+ MEMCPY_AND_ZERO_SRC(u8, dst, src, len);
+}
+
+/**
+ * __cvdso_getrandom_data - Generic vDSO implementation of getrandom() syscall.
+ * @rng_info: Describes state of kernel RNG, memory shared with kernel.
+ * @buffer: Destination buffer to fill with random bytes.
+ * @len: Size of @buffer in bytes.
+ * @flags: Zero or more GRND_* flags.
+ * @opaque_state: Pointer to an opaque state area.
+ *
+ * This implements a "fast key erasure" RNG using ChaCha20, in the same way that the kernel's
+ * getrandom() syscall does. It periodically reseeds its key from the kernel's RNG, at the same
+ * schedule that the kernel's RNG is reseeded. If the kernel's RNG is not ready, then this always
+ * calls into the syscall.
+ *
+ * @opaque_state *must* be allocated using the vgetrandom_alloc() syscall. Unless external locking
+ * is used, one state must be allocated per thread, as it is not safe to call this function
+ * concurrently with the same @opaque_state. However, it is safe to call this using the same
+ * @opaque_state that is shared between main code and signal handling code, within the same thread.
+ *
+ * Returns the number of random bytes written to @buffer, or a negative value indicating an error.
+ */
+static __always_inline ssize_t
+__cvdso_getrandom_data(const struct vdso_rng_data *rng_info, void *buffer, size_t len,
+ unsigned int flags, void *opaque_state)
+{
+ ssize_t ret = min_t(size_t, INT_MAX & PAGE_MASK /* = MAX_RW_COUNT */, len);
+ struct vgetrandom_state *state = opaque_state;
+ size_t batch_len, nblocks, orig_len = len;
+ unsigned long current_generation;
+ void *orig_buffer = buffer;
+ u32 counter[2] = { 0 };
+ bool in_use;
+
+ /*
+ * If the kernel's RNG is not yet ready, then it's not possible to provide random bytes from
+ * userspace, because A) the various @flags require this to block, or not, depending on
+ * various factors unavailable to userspace, and B) the kernel's behavior before the RNG is
+ * ready is to reseed from the entropy pool at every invocation.
+ */
+ if (unlikely(!READ_ONCE(rng_info->is_ready)))
+ goto fallback_syscall;
+
+ /*
+ * This condition is checked after @rng_info->is_ready, because before the kernel's RNG is
+ * initialized, the @flags parameter may require this to block or return an error, even when
+ * len is zero.
+ */
+ if (unlikely(!len))
+ return 0;
+
+ /*
+ * @state->in_use is basic reentrancy protection against this running in a signal handler
+ * with the same @opaque_state, but obviously not atomic wrt multiple CPUs or more than one
+ * level of reentrancy. If a signal interrupts this after reading @state->in_use, but before
+ * writing @state->in_use, there is still no race, because the signal handler will run to
+ * its completion before returning execution.
+ */
+ in_use = READ_ONCE(state->in_use);
+ if (unlikely(in_use))
+ goto fallback_syscall;
+ WRITE_ONCE(state->in_use, true);
+
+retry_generation:
+ /*
+ * @rng_info->generation must always be read here, as it serializes @state->key with the
+ * kernel's RNG reseeding schedule.
+ */
+ current_generation = READ_ONCE(rng_info->generation);
+
+ /*
+ * If @state->generation doesn't match the kernel RNG's generation, then it means the
+ * kernel's RNG has reseeded, and so @state->key is reseeded as well.
+ */
+ if (unlikely(state->generation != current_generation)) {
+ /*
+ * Write the generation before filling the key, in case of fork. If there is a fork
+ * just after this line, the two forks will get different random bytes from the
+ * syscall, which is good. However, were this line to occur after the getrandom
+ * syscall, then both child and parent could have the same bytes and the same
+ * generation counter, so the fork would not be detected. Therefore, write
+ * @state->generation before the call to the getrandom syscall.
+ */
+ WRITE_ONCE(state->generation, current_generation);
+
+ /* Reseed @state->key using fresh bytes from the kernel. */
+ if (getrandom_syscall(state->key, sizeof(state->key), 0) != sizeof(state->key)) {
+ /*
+ * If the syscall failed to refresh the key, then @state->key is now
+ * invalid, so invalidate the generation so that it is not used again, and
+ * fallback to using the syscall entirely.
+ */
+ WRITE_ONCE(state->generation, 0);
+
+ /*
+ * Set @state->in_use to false only after the last write to @state in the
+ * line above.
+ */
+ WRITE_ONCE(state->in_use, false);
+
+ goto fallback_syscall;
+ }
+
+ /*
+ * Set @state->pos to beyond the end of the batch, so that the batch is refilled
+ * using the new key.
+ */
+ state->pos = sizeof(state->batch);
+ }
+
+ /* Set len to the total amount of bytes that this function is allowed to read, ret. */
+ len = ret;
+more_batch:
+ /*
+ * First use bytes out of @state->batch, which may have been filled by the last call to this
+ * function.
+ */
+ batch_len = min_t(size_t, sizeof(state->batch) - state->pos, len);
+ if (batch_len) {
+ /* Zeroing at the same time as memcpying helps preserve forward secrecy. */
+ memcpy_and_zero_src(buffer, state->batch + state->pos, batch_len);
+ state->pos += batch_len;
+ buffer += batch_len;
+ len -= batch_len;
+ }
+
+ if (!len) {
+ /*
+ * Since @rng_info->generation will never be 0, re-read @state->generation, rather
+ * than using the local current_generation variable, to learn whether a fork
+ * occurred. Primarily, though, this indicates whether the kernel's RNG has
+ * reseeded, in which case generate a new key and start over.
+ */
+ if (unlikely(READ_ONCE(state->generation) != READ_ONCE(rng_info->generation))) {
+ buffer = orig_buffer;
+ goto retry_generation;
+ }
+
+ /*
+ * Set @state->in_use to false only when there will be no more reads or writes of
+ * @state.
+ */
+ WRITE_ONCE(state->in_use, false);
+ return ret;
+ }
+
+ /* Generate blocks of RNG output directly into @buffer while there's enough room left. */
+ nblocks = len / CHACHA_BLOCK_SIZE;
+ if (nblocks) {
+ __arch_chacha20_blocks_nostack(buffer, state->key, counter, nblocks);
+ buffer += nblocks * CHACHA_BLOCK_SIZE;
+ len -= nblocks * CHACHA_BLOCK_SIZE;
+ }
+
+ BUILD_BUG_ON(sizeof(state->batch_key) % CHACHA_BLOCK_SIZE != 0);
+
+ /* Refill the batch and then overwrite the key, in order to preserve forward secrecy. */
+ __arch_chacha20_blocks_nostack(state->batch_key, state->key, counter,
+ sizeof(state->batch_key) / CHACHA_BLOCK_SIZE);
+
+ /* Since the batch was just refilled, set the position back to 0 to indicate a full batch. */
+ state->pos = 0;
+ goto more_batch;
+
+fallback_syscall:
+ return getrandom_syscall(orig_buffer, orig_len, flags);
+}
+
+static __always_inline ssize_t
+__cvdso_getrandom(void *buffer, size_t len, unsigned int flags, void *opaque_state)
+{
+ return __cvdso_getrandom_data(__arch_get_vdso_rng_data(), buffer, len, flags, opaque_state);
+}
diff --git a/tools/testing/selftests/vDSO/.gitignore b/tools/testing/selftests/vDSO/.gitignore
index a8dc51af5a9c..7dbfdec53f3d 100644
--- a/tools/testing/selftests/vDSO/.gitignore
+++ b/tools/testing/selftests/vDSO/.gitignore
@@ -6,3 +6,4 @@ vdso_test_correctness
vdso_test_gettimeofday
vdso_test_getcpu
vdso_standalone_test_x86
+vdso_test_getrandom
diff --git a/tools/testing/selftests/vDSO/Makefile b/tools/testing/selftests/vDSO/Makefile
index d53a4d8008f9..a33b4d200a32 100644
--- a/tools/testing/selftests/vDSO/Makefile
+++ b/tools/testing/selftests/vDSO/Makefile
@@ -11,6 +11,7 @@ ifeq ($(ARCH),$(filter $(ARCH),x86 x86_64))
TEST_GEN_PROGS += $(OUTPUT)/vdso_standalone_test_x86
endif
TEST_GEN_PROGS += $(OUTPUT)/vdso_test_correctness
+TEST_GEN_PROGS += $(OUTPUT)/vdso_test_getrandom
CFLAGS := -std=gnu99
CFLAGS_vdso_standalone_test_x86 := -nostdlib -fno-asynchronous-unwind-tables -fno-stack-protector
@@ -33,3 +34,4 @@ $(OUTPUT)/vdso_test_correctness: vdso_test_correctness.c
vdso_test_correctness.c \
-o $@ \
$(LDFLAGS_vdso_test_correctness)
+$(OUTPUT)/vdso_test_getrandom: parse_vdso.c
diff --git a/tools/testing/selftests/vDSO/vdso_test_getrandom.c b/tools/testing/selftests/vDSO/vdso_test_getrandom.c
new file mode 100644
index 000000000000..528674899b8a
--- /dev/null
+++ b/tools/testing/selftests/vDSO/vdso_test_getrandom.c
@@ -0,0 +1,279 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2022 Jason A. Donenfeld <[email protected]>. All Rights Reserved.
+ */
+
+#include <assert.h>
+#include <pthread.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <time.h>
+#include <unistd.h>
+#include <sys/auxv.h>
+#include <sys/mman.h>
+#include <sys/random.h>
+#include <sys/syscall.h>
+#include <sys/types.h>
+
+#include "../kselftest.h"
+#include "parse_vdso.h"
+
+#ifndef timespecsub
+#define timespecsub(tsp, usp, vsp) \
+ do { \
+ (vsp)->tv_sec = (tsp)->tv_sec - (usp)->tv_sec; \
+ (vsp)->tv_nsec = (tsp)->tv_nsec - (usp)->tv_nsec; \
+ if ((vsp)->tv_nsec < 0) { \
+ (vsp)->tv_sec--; \
+ (vsp)->tv_nsec += 1000000000L; \
+ } \
+ } while (0)
+#endif
+
+static void *vgetrandom_alloc(unsigned int *num, unsigned int *size_per_each)
+{
+ enum { __NR_vgetrandom_alloc = 451 };
+ *size_per_each = 0;
+ return (void *)syscall(__NR_vgetrandom_alloc, num, size_per_each, 0, 0);
+}
+
+static struct {
+ pthread_mutex_t lock;
+ void **states;
+ size_t len, cap;
+} grnd_allocator = {
+ .lock = PTHREAD_MUTEX_INITIALIZER
+};
+
+static void *vgetrandom_get_state(void)
+{
+ void *state = NULL;
+
+ pthread_mutex_lock(&grnd_allocator.lock);
+ if (!grnd_allocator.len) {
+ size_t new_cap;
+ unsigned int num = sysconf(_SC_NPROCESSORS_ONLN); /* Could be arbitrary, just a hint. */
+ unsigned int size_per_each;
+ void *new_block = vgetrandom_alloc(&num, &size_per_each);
+ void *new_states;
+
+ if (new_block == MAP_FAILED)
+ goto out;
+ new_cap = grnd_allocator.cap + num;
+ new_states = reallocarray(grnd_allocator.states, new_cap, sizeof(*grnd_allocator.states));
+ if (!new_states) {
+ munmap(new_block, num * size_per_each);
+ goto out;
+ }
+ grnd_allocator.cap = new_cap;
+ grnd_allocator.states = new_states;
+
+ for (size_t i = 0; i < num; ++i) {
+ grnd_allocator.states[i] = new_block;
+ new_block += size_per_each;
+ }
+ grnd_allocator.len = num;
+ }
+ state = grnd_allocator.states[--grnd_allocator.len];
+
+out:
+ pthread_mutex_unlock(&grnd_allocator.lock);
+ return state;
+}
+
+static void vgetrandom_put_state(void *state)
+{
+ if (!state)
+ return;
+ pthread_mutex_lock(&grnd_allocator.lock);
+ grnd_allocator.states[grnd_allocator.len++] = state;
+ pthread_mutex_unlock(&grnd_allocator.lock);
+}
+
+static struct {
+ ssize_t(*fn)(void *buf, size_t len, unsigned long flags, void *state);
+ pthread_key_t key;
+ pthread_once_t initialized;
+} grnd_ctx = {
+ .initialized = PTHREAD_ONCE_INIT
+};
+
+static void vgetrandom_init(void)
+{
+ if (pthread_key_create(&grnd_ctx.key, vgetrandom_put_state) != 0)
+ return;
+ unsigned long sysinfo_ehdr = getauxval(AT_SYSINFO_EHDR);
+ if (!sysinfo_ehdr) {
+ printf("AT_SYSINFO_EHDR is not present!\n");
+ exit(KSFT_SKIP);
+ }
+ vdso_init_from_sysinfo_ehdr(sysinfo_ehdr);
+ grnd_ctx.fn = (__typeof__(grnd_ctx.fn))vdso_sym("LINUX_2.6", "__vdso_getrandom");
+ if (!grnd_ctx.fn) {
+ printf("__vdso_getrandom is missing!\n");
+ exit(KSFT_FAIL);
+ }
+}
+
+static ssize_t vgetrandom(void *buf, size_t len, unsigned long flags)
+{
+ void *state;
+
+ pthread_once(&grnd_ctx.initialized, vgetrandom_init);
+ state = pthread_getspecific(grnd_ctx.key);
+ if (!state) {
+ state = vgetrandom_get_state();
+ if (pthread_setspecific(grnd_ctx.key, state) != 0) {
+ vgetrandom_put_state(state);
+ state = NULL;
+ }
+ if (!state) {
+ printf("vgetrandom_get_state failed!\n");
+ exit(KSFT_FAIL);
+ }
+ }
+ return grnd_ctx.fn(buf, len, flags, state);
+}
+
+enum { TRIALS = 25000000, THREADS = 256 };
+
+static void *test_vdso_getrandom(void *)
+{
+ for (size_t i = 0; i < TRIALS; ++i) {
+ unsigned int val;
+ ssize_t ret = vgetrandom(&val, sizeof(val), 0);
+ assert(ret == sizeof(val));
+ }
+ return NULL;
+}
+
+static void *test_libc_getrandom(void *)
+{
+ for (size_t i = 0; i < TRIALS; ++i) {
+ unsigned int val;
+ ssize_t ret = getrandom(&val, sizeof(val), 0);
+ assert(ret == sizeof(val));
+ }
+ return NULL;
+}
+
+static void *test_syscall_getrandom(void *)
+{
+ for (size_t i = 0; i < TRIALS; ++i) {
+ unsigned int val;
+ ssize_t ret = syscall(SYS_getrandom, &val, sizeof(val), 0);
+ assert(ret == sizeof(val));
+ }
+ return NULL;
+}
+
+static void bench_single(void)
+{
+ struct timespec start, end, diff;
+
+ clock_gettime(CLOCK_MONOTONIC, &start);
+ test_vdso_getrandom(NULL);
+ clock_gettime(CLOCK_MONOTONIC, &end);
+ timespecsub(&end, &start, &diff);
+ printf(" vdso: %u times in %lu.%09lu seconds\n", TRIALS, diff.tv_sec, diff.tv_nsec);
+
+ clock_gettime(CLOCK_MONOTONIC, &start);
+ test_libc_getrandom(NULL);
+ clock_gettime(CLOCK_MONOTONIC, &end);
+ timespecsub(&end, &start, &diff);
+ printf(" libc: %u times in %lu.%09lu seconds\n", TRIALS, diff.tv_sec, diff.tv_nsec);
+
+ clock_gettime(CLOCK_MONOTONIC, &start);
+ test_syscall_getrandom(NULL);
+ clock_gettime(CLOCK_MONOTONIC, &end);
+ timespecsub(&end, &start, &diff);
+ printf("syscall: %u times in %lu.%09lu seconds\n", TRIALS, diff.tv_sec, diff.tv_nsec);
+}
+
+static void bench_multi(void)
+{
+ struct timespec start, end, diff;
+ pthread_t threads[THREADS];
+
+ clock_gettime(CLOCK_MONOTONIC, &start);
+ for (size_t i = 0; i < THREADS; ++i)
+ assert(pthread_create(&threads[i], NULL, test_vdso_getrandom, NULL) == 0);
+ for (size_t i = 0; i < THREADS; ++i)
+ pthread_join(threads[i], NULL);
+ clock_gettime(CLOCK_MONOTONIC, &end);
+ timespecsub(&end, &start, &diff);
+ printf(" vdso: %u x %u times in %lu.%09lu seconds\n", TRIALS, THREADS, diff.tv_sec, diff.tv_nsec);
+
+ clock_gettime(CLOCK_MONOTONIC, &start);
+ for (size_t i = 0; i < THREADS; ++i)
+ assert(pthread_create(&threads[i], NULL, test_libc_getrandom, NULL) == 0);
+ for (size_t i = 0; i < THREADS; ++i)
+ pthread_join(threads[i], NULL);
+ clock_gettime(CLOCK_MONOTONIC, &end);
+ timespecsub(&end, &start, &diff);
+ printf(" libc: %u x %u times in %lu.%09lu seconds\n", TRIALS, THREADS, diff.tv_sec, diff.tv_nsec);
+
+ clock_gettime(CLOCK_MONOTONIC, &start);
+ for (size_t i = 0; i < THREADS; ++i)
+ assert(pthread_create(&threads[i], NULL, test_syscall_getrandom, NULL) == 0);
+ for (size_t i = 0; i < THREADS; ++i)
+ pthread_join(threads[i], NULL);
+ clock_gettime(CLOCK_MONOTONIC, &end);
+ timespecsub(&end, &start, &diff);
+ printf(" syscall: %u x %u times in %lu.%09lu seconds\n", TRIALS, THREADS, diff.tv_sec, diff.tv_nsec);
+}
+
+static void fill(void)
+{
+ uint8_t weird_size[323929];
+ for (;;)
+ vgetrandom(weird_size, sizeof(weird_size), 0);
+}
+
+static void kselftest(void)
+{
+ uint8_t weird_size[1263];
+
+ ksft_print_header();
+ ksft_set_plan(1);
+
+ for (size_t i = 0; i < 1000; ++i) {
+ ssize_t ret = vgetrandom(weird_size, sizeof(weird_size), 0);
+ if (ret != sizeof(weird_size))
+ exit(KSFT_FAIL);
+ }
+
+ ksft_test_result_pass("getrandom: PASS\n");
+ exit(KSFT_PASS);
+}
+
+static void usage(const char *argv0)
+{
+ fprintf(stderr, "Usage: %s [bench-single|bench-multi|fill]\n", argv0);
+}
+
+int main(int argc, char *argv[])
+{
+ if (argc == 1) {
+ kselftest();
+ return 0;
+ }
+
+ if (argc != 2) {
+ usage(argv[0]);
+ return 1;
+ }
+ if (!strcmp(argv[1], "bench-single"))
+ bench_single();
+ else if (!strcmp(argv[1], "bench-multi"))
+ bench_multi();
+ else if (!strcmp(argv[1], "fill"))
+ fill();
+ else {
+ usage(argv[0]);
+ return 1;
+ }
+ return 0;
+}
--
2.38.1
Hook up the generic vDSO implementation to the x86 vDSO data page. Since
the existing vDSO infrastructure is heavily based on the timekeeping
functionality, which works over arrays of bases, a new macro is
introduced for vvars that are not arrays.
The vDSO function requires a ChaCha20 implementation that does not write
to the stack, yet can still do an entire ChaCha20 permutation, so
provide this using SSE2, since this is userland code that must work on
all x86-64 processors. There's a simple test for this code as well.
Reviewed-by: Samuel Neves <[email protected]> # for vgetrandom-chacha.S
Signed-off-by: Jason A. Donenfeld <[email protected]>
---
arch/x86/Kconfig | 1 +
arch/x86/entry/vdso/Makefile | 3 +-
arch/x86/entry/vdso/vdso.lds.S | 2 +
arch/x86/entry/vdso/vgetrandom-chacha.S | 177 ++++++++++++++++++
arch/x86/entry/vdso/vgetrandom.c | 17 ++
arch/x86/include/asm/vdso/getrandom.h | 55 ++++++
arch/x86/include/asm/vdso/vsyscall.h | 2 +
arch/x86/include/asm/vvar.h | 16 ++
tools/testing/selftests/vDSO/.gitignore | 1 +
tools/testing/selftests/vDSO/Makefile | 9 +
.../testing/selftests/vDSO/vdso_test_chacha.c | 43 +++++
11 files changed, 325 insertions(+), 1 deletion(-)
create mode 100644 arch/x86/entry/vdso/vgetrandom-chacha.S
create mode 100644 arch/x86/entry/vdso/vgetrandom.c
create mode 100644 arch/x86/include/asm/vdso/getrandom.h
create mode 100644 tools/testing/selftests/vDSO/vdso_test_chacha.c
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 67745ceab0db..357148c4a3a4 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -269,6 +269,7 @@ config X86
select HAVE_UNSTABLE_SCHED_CLOCK
select HAVE_USER_RETURN_NOTIFIER
select HAVE_GENERIC_VDSO
+ select VDSO_GETRANDOM if X86_64
select HOTPLUG_SMT if SMP
select IRQ_FORCED_THREADING
select NEED_PER_CPU_EMBED_FIRST_CHUNK
diff --git a/arch/x86/entry/vdso/Makefile b/arch/x86/entry/vdso/Makefile
index 3e88b9df8c8f..2de64e52236a 100644
--- a/arch/x86/entry/vdso/Makefile
+++ b/arch/x86/entry/vdso/Makefile
@@ -27,7 +27,7 @@ VDSO32-$(CONFIG_X86_32) := y
VDSO32-$(CONFIG_IA32_EMULATION) := y
# files to link into the vdso
-vobjs-y := vdso-note.o vclock_gettime.o vgetcpu.o
+vobjs-y := vdso-note.o vclock_gettime.o vgetcpu.o vgetrandom.o vgetrandom-chacha.o
vobjs32-y := vdso32/note.o vdso32/system_call.o vdso32/sigreturn.o
vobjs32-y += vdso32/vclock_gettime.o
vobjs-$(CONFIG_X86_SGX) += vsgx.o
@@ -104,6 +104,7 @@ CFLAGS_REMOVE_vclock_gettime.o = -pg
CFLAGS_REMOVE_vdso32/vclock_gettime.o = -pg
CFLAGS_REMOVE_vgetcpu.o = -pg
CFLAGS_REMOVE_vsgx.o = -pg
+CFLAGS_REMOVE_vgetrandom.o = -pg
#
# X32 processes use x32 vDSO to access 64bit kernel data.
diff --git a/arch/x86/entry/vdso/vdso.lds.S b/arch/x86/entry/vdso/vdso.lds.S
index 4bf48462fca7..1919cc39277e 100644
--- a/arch/x86/entry/vdso/vdso.lds.S
+++ b/arch/x86/entry/vdso/vdso.lds.S
@@ -28,6 +28,8 @@ VERSION {
clock_getres;
__vdso_clock_getres;
__vdso_sgx_enter_enclave;
+ getrandom;
+ __vdso_getrandom;
local: *;
};
}
diff --git a/arch/x86/entry/vdso/vgetrandom-chacha.S b/arch/x86/entry/vdso/vgetrandom-chacha.S
new file mode 100644
index 000000000000..91fbb7ac7af4
--- /dev/null
+++ b/arch/x86/entry/vdso/vgetrandom-chacha.S
@@ -0,0 +1,177 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2022 Jason A. Donenfeld <[email protected]>. All Rights Reserved.
+ */
+
+#include <linux/linkage.h>
+#include <asm/frame.h>
+
+.section .rodata.cst16.CONSTANTS, "aM", @progbits, 16
+.align 16
+CONSTANTS: .octa 0x6b20657479622d323320646e61707865
+.text
+
+/*
+ * Very basic SSE2 implementation of ChaCha20. Produces a given positive number
+ * of blocks of output with a nonce of 0, taking an input key and 8-byte
+ * counter. Importantly does not spill to the stack. Its arguments are:
+ *
+ * rdi: output bytes
+ * rsi: 32-byte key input
+ * rdx: 8-byte counter input/output
+ * rcx: number of 64-byte blocks to write to output
+ */
+SYM_FUNC_START(__arch_chacha20_blocks_nostack)
+
+#define output %rdi
+#define key %rsi
+#define counter %rdx
+#define nblocks %rcx
+#define i %al
+#define state0 %xmm0
+#define state1 %xmm1
+#define state2 %xmm2
+#define state3 %xmm3
+#define copy0 %xmm4
+#define copy1 %xmm5
+#define copy2 %xmm6
+#define copy3 %xmm7
+#define temp %xmm8
+#define one %xmm9
+
+ /* copy0 = "expand 32-byte k" */
+ movaps CONSTANTS(%rip),copy0
+ /* copy1,copy2 = key */
+ movups 0x00(key),copy1
+ movups 0x10(key),copy2
+ /* copy3 = counter || zero nonce */
+ movq 0x00(counter),copy3
+ /* one = 1 || 0 */
+ movq $1,%rax
+ movq %rax,one
+
+.Lblock:
+ /* state0,state1,state2,state3 = copy0,copy1,copy2,copy3 */
+ movdqa copy0,state0
+ movdqa copy1,state1
+ movdqa copy2,state2
+ movdqa copy3,state3
+
+ movb $10,i
+.Lpermute:
+ /* state0 += state1, state3 = rotl32(state3 ^ state0, 16) */
+ paddd state1,state0
+ pxor state0,state3
+ movdqa state3,temp
+ pslld $16,temp
+ psrld $16,state3
+ por temp,state3
+
+ /* state2 += state3, state1 = rotl32(state1 ^ state2, 12) */
+ paddd state3,state2
+ pxor state2,state1
+ movdqa state1,temp
+ pslld $12,temp
+ psrld $20,state1
+ por temp,state1
+
+ /* state0 += state1, state3 = rotl32(state3 ^ state0, 8) */
+ paddd state1,state0
+ pxor state0,state3
+ movdqa state3,temp
+ pslld $8,temp
+ psrld $24,state3
+ por temp,state3
+
+ /* state2 += state3, state1 = rotl32(state1 ^ state2, 7) */
+ paddd state3,state2
+ pxor state2,state1
+ movdqa state1,temp
+ pslld $7,temp
+ psrld $25,state1
+ por temp,state1
+
+ /* state1[0,1,2,3] = state1[0,3,2,1] */
+ pshufd $0x39,state1,state1
+ /* state2[0,1,2,3] = state2[1,0,3,2] */
+ pshufd $0x4e,state2,state2
+ /* state3[0,1,2,3] = state3[2,1,0,3] */
+ pshufd $0x93,state3,state3
+
+ /* state0 += state1, state3 = rotl32(state3 ^ state0, 16) */
+ paddd state1,state0
+ pxor state0,state3
+ movdqa state3,temp
+ pslld $16,temp
+ psrld $16,state3
+ por temp,state3
+
+ /* state2 += state3, state1 = rotl32(state1 ^ state2, 12) */
+ paddd state3,state2
+ pxor state2,state1
+ movdqa state1,temp
+ pslld $12,temp
+ psrld $20,state1
+ por temp,state1
+
+ /* state0 += state1, state3 = rotl32(state3 ^ state0, 8) */
+ paddd state1,state0
+ pxor state0,state3
+ movdqa state3,temp
+ pslld $8,temp
+ psrld $24,state3
+ por temp,state3
+
+ /* state2 += state3, state1 = rotl32(state1 ^ state2, 7) */
+ paddd state3,state2
+ pxor state2,state1
+ movdqa state1,temp
+ pslld $7,temp
+ psrld $25,state1
+ por temp,state1
+
+ /* state1[0,1,2,3] = state1[2,1,0,3] */
+ pshufd $0x93,state1,state1
+ /* state2[0,1,2,3] = state2[1,0,3,2] */
+ pshufd $0x4e,state2,state2
+ /* state3[0,1,2,3] = state3[0,3,2,1] */
+ pshufd $0x39,state3,state3
+
+ decb i
+ jnz .Lpermute
+
+ /* output0 = state0 + copy0 */
+ paddd copy0,state0
+ movups state0,0x00(output)
+ /* output1 = state1 + copy1 */
+ paddd copy1,state1
+ movups state1,0x10(output)
+ /* output2 = state2 + copy2 */
+ paddd copy2,state2
+ movups state2,0x20(output)
+ /* output3 = state3 + copy3 */
+ paddd copy3,state3
+ movups state3,0x30(output)
+
+ /* ++copy3.counter */
+ paddq one,copy3
+
+ /* output += 64, --nblocks */
+ addq $64,output
+ decq nblocks
+ jnz .Lblock
+
+ /* counter = copy3.counter */
+ movq copy3,0x00(counter)
+
+ /* Zero out the potentially sensitive regs, in case nothing uses these again. */
+ pxor state0,state0
+ pxor state1,state1
+ pxor state2,state2
+ pxor state3,state3
+ pxor copy1,copy1
+ pxor copy2,copy2
+ pxor temp,temp
+
+ ret
+SYM_FUNC_END(__arch_chacha20_blocks_nostack)
diff --git a/arch/x86/entry/vdso/vgetrandom.c b/arch/x86/entry/vdso/vgetrandom.c
new file mode 100644
index 000000000000..6045ded5da90
--- /dev/null
+++ b/arch/x86/entry/vdso/vgetrandom.c
@@ -0,0 +1,17 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2022 Jason A. Donenfeld <[email protected]>. All Rights Reserved.
+ */
+#include <linux/types.h>
+
+#include "../../../../lib/vdso/getrandom.c"
+
+ssize_t __vdso_getrandom(void *buffer, size_t len, unsigned int flags, void *state);
+
+ssize_t __vdso_getrandom(void *buffer, size_t len, unsigned int flags, void *state)
+{
+ return __cvdso_getrandom(buffer, len, flags, state);
+}
+
+ssize_t getrandom(void *, size_t, unsigned int, void *)
+ __attribute__((weak, alias("__vdso_getrandom")));
diff --git a/arch/x86/include/asm/vdso/getrandom.h b/arch/x86/include/asm/vdso/getrandom.h
new file mode 100644
index 000000000000..46f99d735ae6
--- /dev/null
+++ b/arch/x86/include/asm/vdso/getrandom.h
@@ -0,0 +1,55 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (C) 2022 Jason A. Donenfeld <[email protected]>. All Rights Reserved.
+ */
+#ifndef __ASM_VDSO_GETRANDOM_H
+#define __ASM_VDSO_GETRANDOM_H
+
+#ifndef __ASSEMBLY__
+
+#include <asm/unistd.h>
+#include <asm/vvar.h>
+
+/**
+ * getrandom_syscall - Invoke the getrandom() syscall.
+ * @buffer: Destination buffer to fill with random bytes.
+ * @len: Size of @buffer in bytes.
+ * @flags: Zero or more GRND_* flags.
+ * Returns the number of random bytes written to @buffer, or a negative value indicating an error.
+ */
+static __always_inline ssize_t getrandom_syscall(void *buffer, size_t len, unsigned int flags)
+{
+ long ret;
+
+ asm ("syscall" : "=a" (ret) :
+ "0" (__NR_getrandom), "D" (buffer), "S" (len), "d" (flags) :
+ "rcx", "r11", "memory");
+
+ return ret;
+}
+
+#define __vdso_rng_data (VVAR(_vdso_rng_data))
+
+static __always_inline const struct vdso_rng_data *__arch_get_vdso_rng_data(void)
+{
+ if (__vdso_data->clock_mode == VDSO_CLOCKMODE_TIMENS)
+ return (void *)&__vdso_rng_data + ((void *)&__timens_vdso_data - (void *)&__vdso_data);
+ return &__vdso_rng_data;
+}
+
+/**
+ * __arch_chacha20_blocks_nostack - Generate ChaCha20 stream without using the stack.
+ * @dst_bytes: Destination buffer to hold @nblocks * 64 bytes of output.
+ * @key: 32-byte input key.
+ * @counter: 8-byte counter, read on input and updated on return.
+ * @nblocks: Number of blocks to generate.
+ *
+ * Generates a given positive number of blocks of ChaCha20 output with nonce=0, and does not write
+ * to any stack or memory outside of the parameters passed to it, in order to mitigate stack data
+ * leaking into forked child processes.
+ */
+extern void __arch_chacha20_blocks_nostack(u8 *dst_bytes, const u32 *key, u32 *counter, size_t nblocks);
+
+#endif /* !__ASSEMBLY__ */
+
+#endif /* __ASM_VDSO_GETRANDOM_H */
diff --git a/arch/x86/include/asm/vdso/vsyscall.h b/arch/x86/include/asm/vdso/vsyscall.h
index be199a9b2676..71c56586a22f 100644
--- a/arch/x86/include/asm/vdso/vsyscall.h
+++ b/arch/x86/include/asm/vdso/vsyscall.h
@@ -11,6 +11,8 @@
#include <asm/vvar.h>
DEFINE_VVAR(struct vdso_data, _vdso_data);
+DEFINE_VVAR_SINGLE(struct vdso_rng_data, _vdso_rng_data);
+
/*
* Update the vDSO data page to keep in sync with kernel timekeeping.
*/
diff --git a/arch/x86/include/asm/vvar.h b/arch/x86/include/asm/vvar.h
index 183e98e49ab9..9d9af37f7cab 100644
--- a/arch/x86/include/asm/vvar.h
+++ b/arch/x86/include/asm/vvar.h
@@ -26,6 +26,8 @@
*/
#define DECLARE_VVAR(offset, type, name) \
EMIT_VVAR(name, offset)
+#define DECLARE_VVAR_SINGLE(offset, type, name) \
+ EMIT_VVAR(name, offset)
#else
@@ -37,6 +39,10 @@ extern char __vvar_page;
extern type timens_ ## name[CS_BASES] \
__attribute__((visibility("hidden"))); \
+#define DECLARE_VVAR_SINGLE(offset, type, name) \
+ extern type vvar_ ## name \
+ __attribute__((visibility("hidden"))); \
+
#define VVAR(name) (vvar_ ## name)
#define TIMENS(name) (timens_ ## name)
@@ -44,12 +50,22 @@ extern char __vvar_page;
type name[CS_BASES] \
__attribute__((section(".vvar_" #name), aligned(16))) __visible
+#define DEFINE_VVAR_SINGLE(type, name) \
+ type name \
+ __attribute__((section(".vvar_" #name), aligned(16))) __visible
+
#endif
/* DECLARE_VVAR(offset, type, name) */
DECLARE_VVAR(128, struct vdso_data, _vdso_data)
+#if !defined(_SINGLE_DATA)
+#define _SINGLE_DATA
+DECLARE_VVAR_SINGLE(640, struct vdso_rng_data, _vdso_rng_data)
+#endif
+
#undef DECLARE_VVAR
+#undef DECLARE_VVAR_SINGLE
#endif
diff --git a/tools/testing/selftests/vDSO/.gitignore b/tools/testing/selftests/vDSO/.gitignore
index 7dbfdec53f3d..30d5c8f0e5c7 100644
--- a/tools/testing/selftests/vDSO/.gitignore
+++ b/tools/testing/selftests/vDSO/.gitignore
@@ -7,3 +7,4 @@ vdso_test_gettimeofday
vdso_test_getcpu
vdso_standalone_test_x86
vdso_test_getrandom
+vdso_test_chacha
diff --git a/tools/testing/selftests/vDSO/Makefile b/tools/testing/selftests/vDSO/Makefile
index a33b4d200a32..1b9057974693 100644
--- a/tools/testing/selftests/vDSO/Makefile
+++ b/tools/testing/selftests/vDSO/Makefile
@@ -3,6 +3,7 @@ include ../lib.mk
uname_M := $(shell uname -m 2>/dev/null || echo not)
ARCH ?= $(shell echo $(uname_M) | sed -e s/i.86/x86/ -e s/x86_64/x86/)
+SODIUM := $(shell pkg-config --libs libsodium 2>/dev/null)
TEST_GEN_PROGS := $(OUTPUT)/vdso_test_gettimeofday $(OUTPUT)/vdso_test_getcpu
TEST_GEN_PROGS += $(OUTPUT)/vdso_test_abi
@@ -12,9 +13,15 @@ TEST_GEN_PROGS += $(OUTPUT)/vdso_standalone_test_x86
endif
TEST_GEN_PROGS += $(OUTPUT)/vdso_test_correctness
TEST_GEN_PROGS += $(OUTPUT)/vdso_test_getrandom
+ifeq ($(uname_M),x86_64)
+ifneq ($(SODIUM),)
+TEST_GEN_PROGS += $(OUTPUT)/vdso_test_chacha
+endif
+endif
CFLAGS := -std=gnu99
CFLAGS_vdso_standalone_test_x86 := -nostdlib -fno-asynchronous-unwind-tables -fno-stack-protector
+CFLAGS_vdso_test_chacha := $(SODIUM) -idirafter $(top_srcdir)/include -idirafter $(top_srcdir)/arch/$(ARCH)/include -D__ASSEMBLY__ -Wa,--noexecstack
LDFLAGS_vdso_test_correctness := -ldl
ifeq ($(CONFIG_X86_32),y)
LDLIBS += -lgcc_s
@@ -35,3 +42,5 @@ $(OUTPUT)/vdso_test_correctness: vdso_test_correctness.c
-o $@ \
$(LDFLAGS_vdso_test_correctness)
$(OUTPUT)/vdso_test_getrandom: parse_vdso.c
+$(OUTPUT)/vdso_test_chacha: CFLAGS += $(CFLAGS_vdso_test_chacha)
+$(OUTPUT)/vdso_test_chacha: $(top_srcdir)/arch/$(ARCH)/entry/vdso/vgetrandom-chacha.S
diff --git a/tools/testing/selftests/vDSO/vdso_test_chacha.c b/tools/testing/selftests/vDSO/vdso_test_chacha.c
new file mode 100644
index 000000000000..1c76aeb3de68
--- /dev/null
+++ b/tools/testing/selftests/vDSO/vdso_test_chacha.c
@@ -0,0 +1,43 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2022 Jason A. Donenfeld <[email protected]>. All Rights Reserved.
+ */
+
+#include <sodium/crypto_stream_chacha20.h>
+#include <sys/random.h>
+#include <string.h>
+#include <stdint.h>
+#include "../kselftest.h"
+
+extern void __arch_chacha20_blocks_nostack(uint8_t *dst_bytes, const uint8_t *key, uint32_t *counter, size_t nblocks);
+
+int main(int argc, char *argv[])
+{
+ enum { TRIALS = 1000, BLOCKS = 128, BLOCK_SIZE = 64 };
+ static const uint8_t nonce[8] = { 0 };
+ uint32_t counter[2];
+ uint8_t key[32];
+ uint8_t output1[BLOCK_SIZE * BLOCKS], output2[BLOCK_SIZE * BLOCKS];
+
+ ksft_print_header();
+ ksft_set_plan(1);
+
+ for (unsigned int trial; trial < TRIALS; ++trial) {
+ if (getrandom(key, sizeof(key), 0) != sizeof(key)) {
+ printf("getrandom() failed!\n");
+ return KSFT_SKIP;
+ }
+ crypto_stream_chacha20(output1, sizeof(output1), nonce, key);
+ for (unsigned int split = 0; split < BLOCKS; ++split) {
+ memset(output2, 'X', sizeof(output2));
+ memset(counter, 0, sizeof(counter));
+ if (split)
+ __arch_chacha20_blocks_nostack(output2, key, counter, split);
+ __arch_chacha20_blocks_nostack(output2 + split * BLOCK_SIZE, key, counter, BLOCKS - split);
+ if (memcmp(output1, output2, sizeof(output1)))
+ return KSFT_FAIL;
+ }
+ }
+ ksft_test_result_pass("chacha: PASS\n");
+ return KSFT_PASS;
+}
--
2.38.1
On Mon, Dec 05, 2022 at 07:34:39PM +0100, Florian Weimer wrote:
> > + * - Passing the first state to munmap(2), as described above.
> > + * All other uses are undefined behavior, which is subject to change or removal
>
> Suggest: “Passing the first state *and total length* to munmap(2)”
Will do.
> Rest of the documentation looks good to me. It addresses my concerns
> about future evolution of this interface.
Great! So hopefully the api is more or less settled now, and we can turn
attention to the glibc part.
Jason
* Jason A. Donenfeld:
> +/********************************************************************
> + *
> + * vDSO support helpers.
> + *
> + * The actual vDSO function is defined over in lib/vdso/getrandom.c,
> + * but this section contains the kernel-mode helpers to support that.
> + *
> + ********************************************************************/
> +
> +#ifdef CONFIG_VDSO_GETRANDOM
> +/**
> + * sys_vgetrandom_alloc - Allocate opaque states for use with vDSO getrandom().
> + *
> + * @num: On input, a pointer to a suggested hint of how many states to
> + * allocate, and on return the number of states actually allocated.
> + *
> + * @size_per_each: On input, must be zero. On return, the size of each state allocated,
> + * so that the caller can split up the returned allocation into
> + * individual states.
> + *
> + * @addr: Reserved, must be zero.
> + *
> + * @flags: Reserved, must be zero.
> + *
> + * The getrandom() vDSO function in userspace requires an opaque state, which
> + * this function allocates by mapping a certain number of special pages into
> + * the calling process. It takes a hint as to the number of opaque states
> + * desired, and provides the caller with the number of opaque states actually
> + * allocated, the size of each one in bytes, and the address of the first
> + * state, which may be split up into @num states of @size_per_each bytes each,
> + * by adding @size_per_each to the returned first state @num times.
> + *
> + * Returns the address of the first state in the allocation on success, or a
> + * negative error value on failure.
> + *
> + * The returned address of the first state may be passed to munmap(2) with a
> + * length of `(size_t)num * (size_t)size_per_each`, in order to deallocate the
> + * memory, after which it is invalid to pass it to vDSO getrandom().
> + *
> + * States allocated by this function must not be dereferenced, written, read,
> + * or otherwise manipulated. The *only* supported operations are:
> + * - Splitting up the states in intervals of @size_per_each, no more than
> + * @num times from the first state.
> + * - Passing a state to the getrandom() vDSO function's @opaque_state
> + * parameter, but not passing the same state at the same time to two such
> + * calls.
> + * - Passing the first state to munmap(2), as described above.
> + * All other uses are undefined behavior, which is subject to change or removal
Suggest: “Passing the first state *and total length* to munmap(2)”
Rest of the documentation looks good to me. It addresses my concerns
about future evolution of this interface.
Thanks,
Florian
* Jason A. Donenfeld:
> +retry_generation:
> + /*
> + * @rng_info->generation must always be read here, as it serializes @state->key with the
> + * kernel's RNG reseeding schedule.
> + */
> + current_generation = READ_ONCE(rng_info->generation);
> + if (unlikely(READ_ONCE(state->generation) != READ_ONCE(rng_info-
I'm pretty sure you need some sort of barrier here. We have a similar
TM-lite construct in glibc ld.so for locating link maps by address, and
there the compiler performed reordering.
_dl_find_object miscompilation on powerpc64le
<https://sourceware.org/bugzilla/show_bug.cgi?id=28745>
I'm not familiar with READ_ONCE, but Documentation/atomic_t.txt suggests
it's a “regular LOAD”, and include/asm-generic/rwonce.h concurs.
Likewise, the signal safety mechanism needs compiler barriers (signal
fences).
I'm also not sure how READ_ONCE realizes atomic 64-bit reads on 32-bit
platforms. i386 can do them in user space via the FPU worst-case (if
the control word hasn't been corrupted). CMPXCHG8B is not applicable
here because it's a read-only mapping. Maybe add a comment at least
about that “strong prevailing wind”?
Thanks,
Florian
On Mon, Dec 5, 2022 at 3:01 AM Jason A. Donenfeld <[email protected]> wrote:
> + mm->def_flags |=
> + /*
> + * Don't allow state to be written to swap, to preserve forward secrecy.
> + * This works in conjunction with MAP_LOCKED in do_mmap(), below, which
> + * actually does the locking (and associated permission check and accounting).
> + * Here, VM_LOCKONFAULT together with VM_NORESERVE simply make the mlocking
> + * happen the first time it's actually used, the same as when calling
> + * mlock2(MLOCK_ONFAULT) from userspace.
> + */
> + VM_LOCKONFAULT | VM_NORESERVE |
Have you checked the interaction with this line in dup_mmap()?
"tmp->vm_flags &= ~(VM_LOCKED | VM_LOCKONFAULT);"
As the mlock.2 manpage says, "Memory locks are not inherited by a
child created via fork(2)". I think the intention here is that the VMA
should stay unswappable after fork(), right?
Of course, trying to reserve more mlocked memory in fork() would also
be problematic...
* Jason A. Donenfeld:
> Hi Florian,
>
> On Mon, Dec 05, 2022 at 07:56:47PM +0100, Florian Weimer wrote:
>> * Jason A. Donenfeld:
>>
>> > +retry_generation:
>> > + /*
>> > + * @rng_info->generation must always be read here, as it serializes @state->key with the
>> > + * kernel's RNG reseeding schedule.
>> > + */
>> > + current_generation = READ_ONCE(rng_info->generation);
>>
>> > + if (unlikely(READ_ONCE(state->generation) != READ_ONCE(rng_info-
>>
>> I'm pretty sure you need some sort of barrier here. We have a similar
>> TM-lite construct in glibc ld.so for locating link maps by address, and
>> there the compiler performed reordering.
>>
>> _dl_find_object miscompilation on powerpc64le
>> <https://sourceware.org/bugzilla/show_bug.cgi?id=28745>
>>
>> I'm not familiar with READ_ONCE, but Documentation/atomic_t.txt suggests
>> it's a “regular LOAD”, and include/asm-generic/rwonce.h concurs.
>
> Do you mean compiler barriers or SMP barriers?
Compiler barrier.
>> Likewise, the signal safety mechanism needs compiler barriers (signal
>> fences).
>
> READ_ONCE() should prevent the compiler from reordering the read.
READ_ONCE looks just like a volatile read. Other reads can be ordered
around it.
For example, this:
void f1 (int, int, int);
extern int a;
extern int b;
void
f2 (volatile int *p)
{
int a1 = a;
int p1 = *p;
int b1 = b;
return f1 (a1, p1, b1);
}
Turns into:
.globl f2
.type f2, @function
f2:
movl (%rdi), %esi
movl b(%rip), %edx
movl a(%rip), %edi
jmp f1
Looks like compiler reodering to me.
>> I'm also not sure how READ_ONCE realizes atomic 64-bit reads on 32-bit
>> platforms. i386 can do them in user space via the FPU worst-case (if
>> the control word hasn't been corrupted). CMPXCHG8B is not applicable
>> here because it's a read-only mapping. Maybe add a comment at least
>> about that “strong prevailing wind”?
>
> There's read tearing in that case, which isn't super, but perhaps not
> all together harmful.
Maybe add a comment that it was considered?
Thanks,
Florian
Hi Jann,
On Mon, Dec 05, 2022 at 08:13:36PM +0100, Jann Horn wrote:
> On Mon, Dec 5, 2022 at 3:01 AM Jason A. Donenfeld <[email protected]> wrote:
> > + mm->def_flags |=
> > + /*
> > + * Don't allow state to be written to swap, to preserve forward secrecy.
> > + * This works in conjunction with MAP_LOCKED in do_mmap(), below, which
> > + * actually does the locking (and associated permission check and accounting).
> > + * Here, VM_LOCKONFAULT together with VM_NORESERVE simply make the mlocking
> > + * happen the first time it's actually used, the same as when calling
> > + * mlock2(MLOCK_ONFAULT) from userspace.
> > + */
> > + VM_LOCKONFAULT | VM_NORESERVE |
>
> Have you checked the interaction with this line in dup_mmap()?
> "tmp->vm_flags &= ~(VM_LOCKED | VM_LOCKONFAULT);"
>
> As the mlock.2 manpage says, "Memory locks are not inherited by a
> child created via fork(2)". I think the intention here is that the VMA
> should stay unswappable after fork(), right?
>
> Of course, trying to reserve more mlocked memory in fork() would also
> be problematic...
Thanks for pointing that out! Indeed that seems problematic.
Fortunately, the use of WIPEONFORK at the same time as LOCKONFAULT means
that memory doesn't actually need to be reserved in fork() itself. So
something like the below seems correct and doable.
Jason
diff --git a/kernel/fork.c b/kernel/fork.c
index ec57cae58ff1..cd53ffff615d 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -656,7 +656,9 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
tmp->anon_vma = NULL;
} else if (anon_vma_fork(tmp, mpnt))
goto fail_nomem_anon_vma_fork;
- tmp->vm_flags &= ~(VM_LOCKED | VM_LOCKONFAULT);
+ if ((tmp->vm_flags & (VM_LOCKONFAULT | VM_WIPEONFORK)) !=
+ (VM_LOCKONFAULT | VM_WIPEONFORK))
+ tmp->vm_flags &= ~(VM_LOCKED | VM_LOCKONFAULT);
file = tmp->vm_file;
if (file) {
struct address_space *mapping = file->f_mapping;
* Jason A. Donenfeld:
> Hi Jann,
>
> On Mon, Dec 05, 2022 at 08:13:36PM +0100, Jann Horn wrote:
>> On Mon, Dec 5, 2022 at 3:01 AM Jason A. Donenfeld <[email protected]> wrote:
>> > + mm->def_flags |=
>> > + /*
>> > + * Don't allow state to be written to swap, to preserve forward secrecy.
>> > + * This works in conjunction with MAP_LOCKED in do_mmap(), below, which
>> > + * actually does the locking (and associated permission check and accounting).
>> > + * Here, VM_LOCKONFAULT together with VM_NORESERVE simply make the mlocking
>> > + * happen the first time it's actually used, the same as when calling
>> > + * mlock2(MLOCK_ONFAULT) from userspace.
>> > + */
>> > + VM_LOCKONFAULT | VM_NORESERVE |
>>
>> Have you checked the interaction with this line in dup_mmap()?
>> "tmp->vm_flags &= ~(VM_LOCKED | VM_LOCKONFAULT);"
>>
>> As the mlock.2 manpage says, "Memory locks are not inherited by a
>> child created via fork(2)". I think the intention here is that the VMA
>> should stay unswappable after fork(), right?
>>
>> Of course, trying to reserve more mlocked memory in fork() would also
>> be problematic...
>
> Thanks for pointing that out! Indeed that seems problematic.
> Fortunately, the use of WIPEONFORK at the same time as LOCKONFAULT means
> that memory doesn't actually need to be reserved in fork() itself. So
> something like the below seems correct and doable.
>
> Jason
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index ec57cae58ff1..cd53ffff615d 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -656,7 +656,9 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
> tmp->anon_vma = NULL;
> } else if (anon_vma_fork(tmp, mpnt))
> goto fail_nomem_anon_vma_fork;
> - tmp->vm_flags &= ~(VM_LOCKED | VM_LOCKONFAULT);
> + if ((tmp->vm_flags & (VM_LOCKONFAULT | VM_WIPEONFORK)) !=
> + (VM_LOCKONFAULT | VM_WIPEONFORK))
> + tmp->vm_flags &= ~(VM_LOCKED | VM_LOCKONFAULT);
> file = tmp->vm_file;
> if (file) {
> struct address_space *mapping = file->f_mapping;
Still it's a bit concerning that calling getrandom (the libc function)
now apparently can kill the process if the system is under severe memory
pressure. In many cases, that's okay, but we wouldn't want that for
PID 1, for example. vm.overcommit_memory=2 mode is supposed to prevent
such crashes, and I think NORESERVE (not shown here) sidesteps that.
Thanks,
Florian
Hi Florian,
On Mon, Dec 05, 2022 at 09:06:06PM +0100, Florian Weimer wrote:
> * Jason A. Donenfeld:
>
> > Hi Jann,
> >
> > On Mon, Dec 05, 2022 at 08:13:36PM +0100, Jann Horn wrote:
> >> On Mon, Dec 5, 2022 at 3:01 AM Jason A. Donenfeld <[email protected]> wrote:
> >> > + mm->def_flags |=
> >> > + /*
> >> > + * Don't allow state to be written to swap, to preserve forward secrecy.
> >> > + * This works in conjunction with MAP_LOCKED in do_mmap(), below, which
> >> > + * actually does the locking (and associated permission check and accounting).
> >> > + * Here, VM_LOCKONFAULT together with VM_NORESERVE simply make the mlocking
> >> > + * happen the first time it's actually used, the same as when calling
> >> > + * mlock2(MLOCK_ONFAULT) from userspace.
> >> > + */
> >> > + VM_LOCKONFAULT | VM_NORESERVE |
> >>
> >> Have you checked the interaction with this line in dup_mmap()?
> >> "tmp->vm_flags &= ~(VM_LOCKED | VM_LOCKONFAULT);"
> >>
> >> As the mlock.2 manpage says, "Memory locks are not inherited by a
> >> child created via fork(2)". I think the intention here is that the VMA
> >> should stay unswappable after fork(), right?
> >>
> >> Of course, trying to reserve more mlocked memory in fork() would also
> >> be problematic...
> >
> > Thanks for pointing that out! Indeed that seems problematic.
> > Fortunately, the use of WIPEONFORK at the same time as LOCKONFAULT means
> > that memory doesn't actually need to be reserved in fork() itself. So
> > something like the below seems correct and doable.
> >
> > Jason
> >
> > diff --git a/kernel/fork.c b/kernel/fork.c
> > index ec57cae58ff1..cd53ffff615d 100644
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -656,7 +656,9 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
> > tmp->anon_vma = NULL;
> > } else if (anon_vma_fork(tmp, mpnt))
> > goto fail_nomem_anon_vma_fork;
> > - tmp->vm_flags &= ~(VM_LOCKED | VM_LOCKONFAULT);
> > + if ((tmp->vm_flags & (VM_LOCKONFAULT | VM_WIPEONFORK)) !=
> > + (VM_LOCKONFAULT | VM_WIPEONFORK))
> > + tmp->vm_flags &= ~(VM_LOCKED | VM_LOCKONFAULT);
> > file = tmp->vm_file;
> > if (file) {
> > struct address_space *mapping = file->f_mapping;
>
> Still it's a bit concerning that calling getrandom (the libc function)
> now apparently can kill the process if the system is under severe memory
> pressure. In many cases, that's okay, but we wouldn't want that for
> PID 1, for example. vm.overcommit_memory=2 mode is supposed to prevent
> such crashes, and I think NORESERVE (not shown here) sidesteps that.
Right. Setting VM_NORESERVE this way sidesteps it. Passing MAP_NORESERVE
to do_mmap() would make it respect vm.overcommit_memory=2, but then we'd
face problems at fork() time, as Jann pointed out, when we might go down
the path of trying to mlock memory from the fork() handler, and that
seems not so desirable. But moreover, the overcommitment potentially
makes the allocation scheme a lot simpler for libcs. Do any init daemons
actually use vm.overcommit_memory=2? Is this actually something to care
about?
If this isn't something we really care about so much, then my little
diff above should suffice, and this all remains very simple. I suspect
that's the case, because there are several VMAs that get set with
VM_NORESERVE already in the kernel.
If this is something we need to care about, then perhaps it's worth
rethinking the problem space from its basic goals:
- This memory must not be written to swap. Even if we added some flag to
zero that part of swap when paging back in, that wouldn't cut it,
because it's often very hard to "truly" zero out disk writes (and
nobody wants to TRIM so often). (Rationale: forward secrecy.)
- It would be "nice" if untouched allocations didn't actually take
up any memory.
- This needs to be wiped on fork.
Complications thus far encountered:
- VM_LOCKED|VM_LOCKONFAULT isn't inherited by forks.
- You're worried about forcing VM_LOCKONFAULT (rightfully or wrongly,
as yet established).
However, there are two useful characteristics of this series that we
might be able to exploit in order to arrive at a solution:
1) Due to being wiped during fork(), the code is already robust to
having the contents of those pages zeroed out midway through.
2) In the absolute worst case of whatever contingency we're coding for,
we have the option to fallback to the getrandom() syscall, and
everything is fine.
So, putting together the basic goals with the complications thus far
encountered, and trying to make use of (1) and (2), what if we introduce
a VM_DROPPABLE flag. The semantics would be:
a) It never is written out to swap.
b) No memory is pre-reserved / committed.
c) Under memory pressure, mm can just drop the pages and make them zero.
d) If there's not enough memory to service a page fault, it's not fatal,
and no signal is sent. Instead, writes are simply lost, and reads
return zero, as if the page was dropped.
e) It is inherited by fork.
f) The pages are zeroed on fork (like VM_WIPEONFORK).
g) It doesn't count against the mlock budget, since nothing is locked.
Then, as an emergent restriction, we require that each opaque_state
never straddle two pages, by returning a rounded up size_per_each.
What do you think of this plan? It's harder to implement, so I still
would prefer the simpler diff I sent to Jann above. But if you're really
convinced that disrespecting vm.overcommit_memory=2 is abominable, then
maybe this more complicated plan could work. Plus, semantic (g) has its
own advantages alone.
I'm CC'ing linux-mm about this matter, as I'm sure they'll have
something to contribute here. (And linux-mm@, if your reaction is "why
do we need this syscall at all, can't userspace just bla bla bla bla",
please read the cover letter of the series, this patch's commit message,
and prior discussion on that topic, so we don't have to rehash that.)
Jason