From: dcashman <[email protected]>
Address Space Layout Randomization (ASLR) provides a barrier to exploitation of user-space processes in the presence of security vulnerabilities by making it more difficult to find desired code/data which could help an attack. This is done by adding a random offset to the location of regions in the process address space, with a greater range of potential offset values corresponding to better protection/a larger search-space for brute force, but also to greater potential for fragmentation.
The offset added to the mmap_base address, which provides the basis for the majority of the mappings for a process, is set once on process exec in arch_pick_mmap_layout() and is done via hard-coded per-arch values, which reflect, hopefully, the best compromise for all systems. The trade-off between increased entropy in the offset value generation and the corresponding increased variability in address space fragmentation is not absolute, however, and some platforms may tolerate higher amounts of entropy. This patch introduces both new Kconfig values and a sysctl interface which may be used to change the amount of entropy used for offset generation on a system.
The direct motivation for this change was in response to the libstagefright vulnerabilities that affected Android, specifically to information provided by Google's project zero at:
http://googleprojectzero.blogspot.com/2015/09/stagefrightened.html
The attack presented therein, by Google's project zero, specifically targeted the limited randomness used to generate the offset added to the mmap_base address in order to craft a brute-force-based attack. Concretely, the attack was against the mediaserver process, which was limited to respawning every 5 seconds, on an arm device. The hard-coded 8 bits used resulted in an average expected success rate of defeating the mmap ASLR after just over 10 minutes (128 tries at 5 seconds a piece). With this patch, and an accompanying increase in the entropy value to 16 bits, the same attack would take an average expected time of over 45 hours (32768 tries), which makes it both less feasible and more likely to be noticed.
The introduced Kconfig and sysctl options are limited by per-arch minimum and maximum values, the minimum of which was chosen to match the current hard-coded value and the maximum of which was chosen so as to give the greatest flexibility without generating an invalid mmap_base address, generally a 3-4 bits less than the number of bits in the user-space accessible virtual address space.
When decided whether or not to change the default value, a system developer should consider that mmap_base address could be placed anywhere up to 2^(value) bits away from the non-randomized location, which would introduce variable-sized areas above and below the mmap_base address such that the maximum vm_area_struct size may be reduced, preventing very large allocations.
Changes in v3:
* moved sysctl from /proc/sys/kernel to /proc/sys/vm
* added to arch/x86 (both 32 and 64 bit)
* added to arch/arm64
* added ability for arch to specify default value in between max - min
dcashman (4):
mm: mmap: Add new /proc tunable for mmap_base ASLR.
arm: mm: support ARCH_MMAP_RND_BITS.
arm64: mm: support ARCH_MMAP_RND_BITS.
x86: mm: support ARCH_MMAP_RND_BITS.
Documentation/sysctl/vm.txt | 29 ++++++++++++++++++++
arch/Kconfig | 64 +++++++++++++++++++++++++++++++++++++++++++++
arch/arm/Kconfig | 10 +++++++
arch/arm/mm/mmap.c | 3 +--
arch/arm64/Kconfig | 23 ++++++++++++++++
arch/arm64/mm/mmap.c | 6 +++--
arch/x86/Kconfig | 16 ++++++++++++
arch/x86/mm/mmap.c | 12 ++++-----
include/linux/mm.h | 11 ++++++++
kernel/sysctl.c | 22 ++++++++++++++++
mm/mmap.c | 12 +++++++++
11 files changed, 198 insertions(+), 10 deletions(-)
--
2.6.0.rc2.230.g3dd15c0
From: dcashman <[email protected]>
ASLR currently only uses 8 bits to generate the random offset for the
mmap base address on 32 bit architectures. This value was chosen to
prevent a poorly chosen value from dividing the address space in such
a way as to prevent large allocations. This may not be an issue on all
platforms. Allow the specification of a minimum number of bits so that
platforms desiring greater ASLR protection may determine where to place
the trade-off.
Signed-off-by: Daniel Cashman <[email protected]>
---
Documentation/sysctl/vm.txt | 29 ++++++++++++++++++++
arch/Kconfig | 64 +++++++++++++++++++++++++++++++++++++++++++++
include/linux/mm.h | 11 ++++++++
kernel/sysctl.c | 22 ++++++++++++++++
mm/mmap.c | 12 +++++++++
5 files changed, 138 insertions(+)
diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
index f72370b..d77a81a 100644
--- a/Documentation/sysctl/vm.txt
+++ b/Documentation/sysctl/vm.txt
@@ -42,6 +42,8 @@ Currently, these files are in /proc/sys/vm:
- min_slab_ratio
- min_unmapped_ratio
- mmap_min_addr
+- mmap_rnd_bits
+- mmap_rnd_compat_bits
- nr_hugepages
- nr_overcommit_hugepages
- nr_trim_pages (only if CONFIG_MMU=n)
@@ -485,6 +487,33 @@ against future potential kernel bugs.
==============================================================
+mmap_rnd_bits:
+
+This value can be used to select the number of bits to use to
+determine the random offset to the base address of vma regions
+resulting from mmap allocations on architectures which support
+tuning address space randomization. This value will be bounded
+by the architecture's minimum and maximum supported values.
+
+This value can be changed after boot using the
+/proc/sys/kernel/mmap_rnd_bits tunable
+
+==============================================================
+
+mmap_rnd_compat_bits:
+
+This value can be used to select the number of bits to use to
+determine the random offset to the base address of vma regions
+resulting from mmap allocations for applications run in
+compatibility mode on architectures which support tuning address
+space randomization. This value will be bounded by the
+architecture's minimum and maximum supported values.
+
+This value can be changed after boot using the
+/proc/sys/kernel/mmap_rnd_compat_bits tunable
+
+==============================================================
+
nr_hugepages
Change the minimum size of the hugepage pool.
diff --git a/arch/Kconfig b/arch/Kconfig
index 4e949e5..141823f 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -511,6 +511,70 @@ config ARCH_HAS_ELF_RANDOMIZE
- arch_mmap_rnd()
- arch_randomize_brk()
+config HAVE_ARCH_MMAP_RND_BITS
+ bool
+ help
+ An arch should select this symbol if it supports setting a variable
+ number of bits for use in establishing the base address for mmap
+ allocations and provides values for both:
+ - ARCH_MMAP_RND_BITS_MIN
+ - ARCH_MMAP_RND_BITS_MAX
+
+config ARCH_MMAP_RND_BITS_MIN
+ int
+
+config ARCH_MMAP_RND_BITS_MAX
+ int
+
+config ARCH_MMAP_RND_BITS_DEFAULT
+ int
+
+config ARCH_MMAP_RND_BITS
+ int "Number of bits to use for ASLR of mmap base address" if EXPERT
+ range ARCH_MMAP_RND_BITS_MIN ARCH_MMAP_RND_BITS_MAX
+ default ARCH_MMAP_RND_BITS_DEFAULT if ARCH_MMAP_RND_BITS_DEFAULT
+ default ARCH_MMAP_RND_BITS_MIN
+ depends on HAVE_ARCH_MMAP_RND_BITS
+ help
+ This value can be used to select the number of bits to use to
+ determine the random offset to the base address of vma regions
+ resulting from mmap allocations. This value will be bounded
+ by the architecture's minimum and maximum supported values.
+
+ This value can be changed after boot using the
+ /proc/sys/kernel/mmap_rnd_bits tunable
+
+config HAVE_ARCH_MMAP_RND_COMPAT_BITS
+ bool
+ help
+ An arch should select this symbol if it supports running applications
+ in compatibility mode, supports setting a variable number of bits for
+ use in establishing the base address for mmap allocations, and
+ provides values for both:
+ - ARCH_MMAP_RND_COMPAT_BITS_MIN
+ - ARCH_MMAP_RND_COMPAT_BITS_MAX
+
+config ARCH_MMAP_RND_COMPAT_BITS_MIN
+ int
+
+config ARCH_MMAP_RND_COMPAT_BITS_MAX
+ int
+
+config ARCH_MMAP_RND_COMPAT_BITS
+ int "Number of bits to use for ASLR of mmap base address for compatible applications" if EXPERT
+ range ARCH_MMAP_RND_COMPAT_BITS_MIN ARCH_MMAP_RND_COMPAT_BITS_MAX
+ default ARCH_MMAP_RND_COMPAT_BITS_MIN
+ depends on HAVE_ARCH_MMAP_RND_COMPAT_BITS
+ help
+ This value can be used to select the number of bits to use to
+ determine the random offset to the base address of vma regions
+ resulting from mmap allocations for compatible applications This
+ value will be bounded by the architecture's minimum and maximum
+ supported values.
+
+ This value can be changed after boot using the
+ /proc/sys/kernel/mmap_rnd_compat_bits tunable
+
config HAVE_COPY_THREAD_TLS
bool
help
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 00bad77..7d39828 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -51,6 +51,17 @@ extern int sysctl_legacy_va_layout;
#define sysctl_legacy_va_layout 0
#endif
+#ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS
+extern int mmap_rnd_bits_min;
+extern int mmap_rnd_bits_max;
+extern int mmap_rnd_bits;
+#endif
+#ifdef CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS
+extern int mmap_rnd_compat_bits_min;
+extern int mmap_rnd_compat_bits_max;
+extern int mmap_rnd_compat_bits;
+#endif
+
#include <asm/page.h>
#include <asm/pgtable.h>
#include <asm/processor.h>
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index dc6858d..40e5de6 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1568,6 +1568,28 @@ static struct ctl_table vm_table[] = {
.mode = 0644,
.proc_handler = proc_doulongvec_minmax,
},
+#ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS
+ {
+ .procname = "mmap_rnd_bits",
+ .data = &mmap_rnd_bits,
+ .maxlen = sizeof(mmap_rnd_bits),
+ .mode = 0644,
+ .proc_handler = proc_dointvec_minmax,
+ .extra1 = &mmap_rnd_bits_min,
+ .extra2 = &mmap_rnd_bits_max,
+ },
+#endif
+#ifdef CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS
+ {
+ .procname = "mmap_rnd_compat_bits",
+ .data = &mmap_rnd_compat_bits,
+ .maxlen = sizeof(mmap_rnd_compat_bits),
+ .mode = 0644,
+ .proc_handler = proc_dointvec_minmax,
+ .extra1 = &mmap_rnd_compat_bits_min,
+ .extra2 = &mmap_rnd_compat_bits_max,
+ },
+#endif
{ }
};
diff --git a/mm/mmap.c b/mm/mmap.c
index 2ce04a6..aa49841 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -58,6 +58,18 @@
#define arch_rebalance_pgtables(addr, len) (addr)
#endif
+#ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS
+int mmap_rnd_bits_min = CONFIG_ARCH_MMAP_RND_BITS_MIN;
+int mmap_rnd_bits_max = CONFIG_ARCH_MMAP_RND_BITS_MAX;
+int mmap_rnd_bits = CONFIG_ARCH_MMAP_RND_BITS;
+#endif
+#ifdef CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS
+int mmap_rnd_compat_bits_min = CONFIG_ARCH_MMAP_RND_COMPAT_BITS_MIN;
+int mmap_rnd_compat_bits_max = CONFIG_ARCH_MMAP_RND_COMPAT_BITS_MAX;
+int mmap_rnd_compat_bits = CONFIG_ARCH_MMAP_RND_COMPAT_BITS;
+#endif
+
+
static void unmap_region(struct mm_struct *mm,
struct vm_area_struct *vma, struct vm_area_struct *prev,
unsigned long start, unsigned long end);
--
2.6.0.rc2.230.g3dd15c0
From: dcashman <[email protected]>
arm: arch_mmap_rnd() uses a hard-code value of 8 to generate the
random offset for the mmap base address. This value represents a
compromise between increased ASLR effectiveness and avoiding
address-space fragmentation. Replace it with a Kconfig option, which
is sensibly bounded, so that platform developers may choose where to
place this compromise. Keep 8 as the minimum acceptable value.
Signed-off-by: Daniel Cashman <[email protected]>
---
arch/arm/Kconfig | 10 ++++++++++
arch/arm/mm/mmap.c | 3 +--
2 files changed, 11 insertions(+), 2 deletions(-)
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 0365cbb..ca2e43a 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -35,6 +35,7 @@ config ARM
select HAVE_ARCH_BITREVERSE if (CPU_32v7M || CPU_32v7) && !CPU_32v6
select HAVE_ARCH_JUMP_LABEL if !XIP_KERNEL && !CPU_ENDIAN_BE32
select HAVE_ARCH_KGDB if !CPU_ENDIAN_BE32
+ select HAVE_ARCH_MMAP_RND_BITS
select HAVE_ARCH_SECCOMP_FILTER if (AEABI && !OABI_COMPAT)
select HAVE_ARCH_TRACEHOOK
select HAVE_BPF_JIT
@@ -306,6 +307,15 @@ config MMU
Select if you want MMU-based virtualised addressing space
support by paged memory management. If unsure, say 'Y'.
+config ARCH_MMAP_RND_BITS_MIN
+ default 8
+
+config ARCH_MMAP_RND_BITS_MAX
+ default 14 if MMU && PAGE_OFFSET=0x40000000
+ default 15 if MMU && PAGE_OFFSET=0x80000000
+ default 16 if MMU
+ default 8
+
#
# The "ARM system type" choice list is ordered alphabetically by option
# text. Please add new entries in the option alphabetic order.
diff --git a/arch/arm/mm/mmap.c b/arch/arm/mm/mmap.c
index 407dc78..c938693 100644
--- a/arch/arm/mm/mmap.c
+++ b/arch/arm/mm/mmap.c
@@ -173,8 +173,7 @@ unsigned long arch_mmap_rnd(void)
{
unsigned long rnd;
- /* 8 bits of randomness in 20 address space bits */
- rnd = (unsigned long)get_random_int() % (1 << 8);
+ rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits);
return rnd << PAGE_SHIFT;
}
--
2.6.0.rc2.230.g3dd15c0
From: dcashman <[email protected]>
arm64: arch_mmap_rnd() uses STACK_RND_MASK to generate the
random offset for the mmap base address. This value represents a
compromise between increased ASLR effectiveness and avoiding
address-space fragmentation. Replace it with a Kconfig option, which
is sensibly bounded, so that platform developers may choose where to
place this compromise. Keep default values as new minimums.
Signed-off-by: Daniel Cashman <[email protected]>
---
arch/arm64/Kconfig | 23 +++++++++++++++++++++++
arch/arm64/mm/mmap.c | 6 ++++--
2 files changed, 27 insertions(+), 2 deletions(-)
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 9ac16a4..be38e4c 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -51,6 +51,8 @@ config ARM64
select HAVE_ARCH_JUMP_LABEL
select HAVE_ARCH_KASAN if SPARSEMEM_VMEMMAP
select HAVE_ARCH_KGDB
+ select HAVE_ARCH_MMAP_RND_BITS
+ select HAVE_ARCH_MMAP_RND_COMPAT_BITS if COMPAT
select HAVE_ARCH_SECCOMP_FILTER
select HAVE_ARCH_TRACEHOOK
select HAVE_BPF_JIT
@@ -104,6 +106,27 @@ config ARCH_PHYS_ADDR_T_64BIT
config MMU
def_bool y
+config ARCH_MMAP_RND_BITS_MIN
+ default 15 if ARM64_64K_PAGES
+ default 19
+
+config ARCH_MMAP_RND_BITS_MAX
+ default 20 if ARM64_64K_PAGES && ARCH_VA_BITS=39
+ default 24 if ARCH_VA_BITS=39
+ default 23 if ARM64_64K_PAGES && ARCH_VA_BITS=42
+ default 27 if ARCH_VA_BITS=42
+ default 29 if ARM64_64K_PAGES && ARCH_VA_BITS=48
+ default 33 if ARCH_VA_BITS=48
+ default 15 if ARM64_64K_PAGES
+ default 19
+
+config ARCH_MMAP_RND_COMPAT_BITS_MIN
+ default 7 if ARM64_64K_PAGES
+ default 11
+
+config ARCH_MMAP_RND_COMPAT_BITS_MAX
+ default 16
+
config NO_IOPORT_MAP
def_bool y if !PCI
diff --git a/arch/arm64/mm/mmap.c b/arch/arm64/mm/mmap.c
index ed17747..b84d5b1 100644
--- a/arch/arm64/mm/mmap.c
+++ b/arch/arm64/mm/mmap.c
@@ -51,8 +51,10 @@ unsigned long arch_mmap_rnd(void)
{
unsigned long rnd;
- rnd = (unsigned long)get_random_int() & STACK_RND_MASK;
-
+ if (test_thread_flag(TIF_32BIT))
+ rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_compat_bits);
+ else
+ rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits);
return rnd << PAGE_SHIFT;
}
--
2.6.0.rc2.230.g3dd15c0
From: dcashman <[email protected]>
x86: arch_mmap_rnd() uses hard-coded values, 8 for 32-bit and 28 for
64-bit, to generate the random offset for the mmap base address.
This value represents a compromise between increased ASLR
effectiveness and avoiding address-space fragmentation. Replace it
with a Kconfig option, which is sensibly bounded, so that platform
developers may choose where to place this compromise. Keep default
values as new minimums.
Signed-off-by: Daniel Cashman <[email protected]>
---
arch/x86/Kconfig | 16 ++++++++++++++++
arch/x86/mm/mmap.c | 12 ++++++------
2 files changed, 22 insertions(+), 6 deletions(-)
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index db3622f..12768c4 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -82,6 +82,8 @@ config X86
select HAVE_ARCH_KASAN if X86_64 && SPARSEMEM_VMEMMAP
select HAVE_ARCH_KGDB
select HAVE_ARCH_KMEMCHECK
+ select HAVE_ARCH_MMAP_RND_BITS
+ select HAVE_ARCH_MMAP_RND_COMPAT_BITS if COMPAT
select HAVE_ARCH_SECCOMP_FILTER
select HAVE_ARCH_SOFT_DIRTY if X86_64
select HAVE_ARCH_TRACEHOOK
@@ -183,6 +185,20 @@ config HAVE_LATENCYTOP_SUPPORT
config MMU
def_bool y
+config ARCH_MMAP_RND_BITS_MIN
+ default 28 if 64BIT
+ default 8
+
+config ARCH_MMAP_RND_BITS_MAX
+ default 32 if 64BIT
+ default 16
+
+config ARCH_MMAP_RND_COMPAT_BITS_MIN
+ default 8
+
+config ARCH_MMAP_RND_COMPAT_BITS_MAX
+ default 16
+
config SBUS
bool
diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
index 844b06d..647fecf 100644
--- a/arch/x86/mm/mmap.c
+++ b/arch/x86/mm/mmap.c
@@ -69,14 +69,14 @@ unsigned long arch_mmap_rnd(void)
{
unsigned long rnd;
- /*
- * 8 bits of randomness in 32bit mmaps, 20 address space bits
- * 28 bits of randomness in 64bit mmaps, 40 address space bits
- */
if (mmap_is_ia32())
- rnd = (unsigned long)get_random_int() % (1<<8);
+#ifdef CONFIG_COMPAT
+ rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_compat_bits);
+#else
+ rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits);
+#endif
else
- rnd = (unsigned long)get_random_int() % (1<<28);
+ rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits);
return rnd << PAGE_SHIFT;
}
--
2.6.0.rc2.230.g3dd15c0
On Wed, Nov 18, 2015 at 2:48 PM, Daniel Cashman <[email protected]> wrote:
> From: dcashman <[email protected]>
>
> ASLR currently only uses 8 bits to generate the random offset for the
> mmap base address on 32 bit architectures. This value was chosen to
> prevent a poorly chosen value from dividing the address space in such
> a way as to prevent large allocations. This may not be an issue on all
> platforms. Allow the specification of a minimum number of bits so that
> platforms desiring greater ASLR protection may determine where to place
> the trade-off.
>
> Signed-off-by: Daniel Cashman <[email protected]>
> ---
> Documentation/sysctl/vm.txt | 29 ++++++++++++++++++++
> arch/Kconfig | 64 +++++++++++++++++++++++++++++++++++++++++++++
> include/linux/mm.h | 11 ++++++++
> kernel/sysctl.c | 22 ++++++++++++++++
> mm/mmap.c | 12 +++++++++
> 5 files changed, 138 insertions(+)
>
> diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
> index f72370b..d77a81a 100644
> --- a/Documentation/sysctl/vm.txt
> +++ b/Documentation/sysctl/vm.txt
> @@ -42,6 +42,8 @@ Currently, these files are in /proc/sys/vm:
> - min_slab_ratio
> - min_unmapped_ratio
> - mmap_min_addr
> +- mmap_rnd_bits
> +- mmap_rnd_compat_bits
> - nr_hugepages
> - nr_overcommit_hugepages
> - nr_trim_pages (only if CONFIG_MMU=n)
> @@ -485,6 +487,33 @@ against future potential kernel bugs.
>
> ==============================================================
>
> +mmap_rnd_bits:
> +
> +This value can be used to select the number of bits to use to
> +determine the random offset to the base address of vma regions
> +resulting from mmap allocations on architectures which support
> +tuning address space randomization. This value will be bounded
> +by the architecture's minimum and maximum supported values.
> +
> +This value can be changed after boot using the
> +/proc/sys/kernel/mmap_rnd_bits tunable
Here and several places below also need the swap from
"/proc/sys/kernel/..." to ".../vm/...".
> +
> +==============================================================
> +
> +mmap_rnd_compat_bits:
> +
> +This value can be used to select the number of bits to use to
> +determine the random offset to the base address of vma regions
> +resulting from mmap allocations for applications run in
> +compatibility mode on architectures which support tuning address
> +space randomization. This value will be bounded by the
> +architecture's minimum and maximum supported values.
> +
> +This value can be changed after boot using the
> +/proc/sys/kernel/mmap_rnd_compat_bits tunable
> +
> +==============================================================
> +
> nr_hugepages
>
> Change the minimum size of the hugepage pool.
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 4e949e5..141823f 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -511,6 +511,70 @@ config ARCH_HAS_ELF_RANDOMIZE
> - arch_mmap_rnd()
> - arch_randomize_brk()
>
> +config HAVE_ARCH_MMAP_RND_BITS
> + bool
> + help
> + An arch should select this symbol if it supports setting a variable
> + number of bits for use in establishing the base address for mmap
> + allocations and provides values for both:
> + - ARCH_MMAP_RND_BITS_MIN
> + - ARCH_MMAP_RND_BITS_MAX
> +
> +config ARCH_MMAP_RND_BITS_MIN
> + int
> +
> +config ARCH_MMAP_RND_BITS_MAX
> + int
> +
> +config ARCH_MMAP_RND_BITS_DEFAULT
> + int
> +
> +config ARCH_MMAP_RND_BITS
> + int "Number of bits to use for ASLR of mmap base address" if EXPERT
> + range ARCH_MMAP_RND_BITS_MIN ARCH_MMAP_RND_BITS_MAX
> + default ARCH_MMAP_RND_BITS_DEFAULT if ARCH_MMAP_RND_BITS_DEFAULT
> + default ARCH_MMAP_RND_BITS_MIN
> + depends on HAVE_ARCH_MMAP_RND_BITS
> + help
> + This value can be used to select the number of bits to use to
> + determine the random offset to the base address of vma regions
> + resulting from mmap allocations. This value will be bounded
> + by the architecture's minimum and maximum supported values.
> +
> + This value can be changed after boot using the
> + /proc/sys/kernel/mmap_rnd_bits tunable
> +
> +config HAVE_ARCH_MMAP_RND_COMPAT_BITS
> + bool
> + help
> + An arch should select this symbol if it supports running applications
> + in compatibility mode, supports setting a variable number of bits for
> + use in establishing the base address for mmap allocations, and
> + provides values for both:
> + - ARCH_MMAP_RND_COMPAT_BITS_MIN
> + - ARCH_MMAP_RND_COMPAT_BITS_MAX
> +
> +config ARCH_MMAP_RND_COMPAT_BITS_MIN
> + int
> +
> +config ARCH_MMAP_RND_COMPAT_BITS_MAX
> + int
> +
> +config ARCH_MMAP_RND_COMPAT_BITS
> + int "Number of bits to use for ASLR of mmap base address for compatible applications" if EXPERT
> + range ARCH_MMAP_RND_COMPAT_BITS_MIN ARCH_MMAP_RND_COMPAT_BITS_MAX
> + default ARCH_MMAP_RND_COMPAT_BITS_MIN
> + depends on HAVE_ARCH_MMAP_RND_COMPAT_BITS
> + help
> + This value can be used to select the number of bits to use to
> + determine the random offset to the base address of vma regions
> + resulting from mmap allocations for compatible applications This
> + value will be bounded by the architecture's minimum and maximum
> + supported values.
> +
> + This value can be changed after boot using the
> + /proc/sys/kernel/mmap_rnd_compat_bits tunable
> +
> config HAVE_COPY_THREAD_TLS
> bool
> help
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 00bad77..7d39828 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -51,6 +51,17 @@ extern int sysctl_legacy_va_layout;
> #define sysctl_legacy_va_layout 0
> #endif
>
> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS
> +extern int mmap_rnd_bits_min;
> +extern int mmap_rnd_bits_max;
> +extern int mmap_rnd_bits;
> +#endif
> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS
> +extern int mmap_rnd_compat_bits_min;
> +extern int mmap_rnd_compat_bits_max;
> +extern int mmap_rnd_compat_bits;
> +#endif
> +
> #include <asm/page.h>
> #include <asm/pgtable.h>
> #include <asm/processor.h>
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index dc6858d..40e5de6 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -1568,6 +1568,28 @@ static struct ctl_table vm_table[] = {
> .mode = 0644,
> .proc_handler = proc_doulongvec_minmax,
> },
> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS
> + {
> + .procname = "mmap_rnd_bits",
> + .data = &mmap_rnd_bits,
> + .maxlen = sizeof(mmap_rnd_bits),
> + .mode = 0644,
> + .proc_handler = proc_dointvec_minmax,
> + .extra1 = &mmap_rnd_bits_min,
> + .extra2 = &mmap_rnd_bits_max,
> + },
> +#endif
> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS
> + {
> + .procname = "mmap_rnd_compat_bits",
> + .data = &mmap_rnd_compat_bits,
> + .maxlen = sizeof(mmap_rnd_compat_bits),
> + .mode = 0644,
> + .proc_handler = proc_dointvec_minmax,
> + .extra1 = &mmap_rnd_compat_bits_min,
> + .extra2 = &mmap_rnd_compat_bits_max,
> + },
> +#endif
> { }
> };
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 2ce04a6..aa49841 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -58,6 +58,18 @@
> #define arch_rebalance_pgtables(addr, len) (addr)
> #endif
>
> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS
> +int mmap_rnd_bits_min = CONFIG_ARCH_MMAP_RND_BITS_MIN;
> +int mmap_rnd_bits_max = CONFIG_ARCH_MMAP_RND_BITS_MAX;
> +int mmap_rnd_bits = CONFIG_ARCH_MMAP_RND_BITS;
> +#endif
> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS
> +int mmap_rnd_compat_bits_min = CONFIG_ARCH_MMAP_RND_COMPAT_BITS_MIN;
> +int mmap_rnd_compat_bits_max = CONFIG_ARCH_MMAP_RND_COMPAT_BITS_MAX;
I think the min/max values should be const, since they're determined
at build time and should never change.
> +int mmap_rnd_compat_bits = CONFIG_ARCH_MMAP_RND_COMPAT_BITS;
> +#endif
> +
> +
> static void unmap_region(struct mm_struct *mm,
> struct vm_area_struct *vma, struct vm_area_struct *prev,
> unsigned long start, unsigned long end);
> --
> 2.6.0.rc2.230.g3dd15c0
>
-Kees
--
Kees Cook
Chrome OS Security
> I think the min/max values should be const, since they're determined
> at build time and should never change.
Ok. Also, I just submitted the patch-set again with [PATCH v3] instead
of [PATCH] so I'd prefer discussion there; sorry for the mistake.
-Dan
On Wed, Nov 18, 2015 at 2:48 PM, Daniel Cashman <[email protected]> wrote:
> From: dcashman <[email protected]>
>
> x86: arch_mmap_rnd() uses hard-coded values, 8 for 32-bit and 28 for
> 64-bit, to generate the random offset for the mmap base address.
> This value represents a compromise between increased ASLR
> effectiveness and avoiding address-space fragmentation. Replace it
> with a Kconfig option, which is sensibly bounded, so that platform
> developers may choose where to place this compromise. Keep default
> values as new minimums.
>
> Signed-off-by: Daniel Cashman <[email protected]>
> ---
> arch/x86/Kconfig | 16 ++++++++++++++++
> arch/x86/mm/mmap.c | 12 ++++++------
> 2 files changed, 22 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index db3622f..12768c4 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -82,6 +82,8 @@ config X86
> select HAVE_ARCH_KASAN if X86_64 && SPARSEMEM_VMEMMAP
> select HAVE_ARCH_KGDB
> select HAVE_ARCH_KMEMCHECK
> + select HAVE_ARCH_MMAP_RND_BITS
> + select HAVE_ARCH_MMAP_RND_COMPAT_BITS if COMPAT
> select HAVE_ARCH_SECCOMP_FILTER
> select HAVE_ARCH_SOFT_DIRTY if X86_64
> select HAVE_ARCH_TRACEHOOK
> @@ -183,6 +185,20 @@ config HAVE_LATENCYTOP_SUPPORT
> config MMU
> def_bool y
>
> +config ARCH_MMAP_RND_BITS_MIN
> + default 28 if 64BIT
> + default 8
> +
> +config ARCH_MMAP_RND_BITS_MAX
> + default 32 if 64BIT
> + default 16
> +
> +config ARCH_MMAP_RND_COMPAT_BITS_MIN
> + default 8
> +
> +config ARCH_MMAP_RND_COMPAT_BITS_MAX
> + default 16
> +
> config SBUS
> bool
>
> diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
> index 844b06d..647fecf 100644
> --- a/arch/x86/mm/mmap.c
> +++ b/arch/x86/mm/mmap.c
> @@ -69,14 +69,14 @@ unsigned long arch_mmap_rnd(void)
> {
> unsigned long rnd;
>
> - /*
> - * 8 bits of randomness in 32bit mmaps, 20 address space bits
> - * 28 bits of randomness in 64bit mmaps, 40 address space bits
> - */
> if (mmap_is_ia32())
> - rnd = (unsigned long)get_random_int() % (1<<8);
> +#ifdef CONFIG_COMPAT
> + rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_compat_bits);
> +#else
> + rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits);
> +#endif
Oh, that is a weird side-effect of x86's compat: a 32-bit build isn't
compat, so it needs the native bits. I'm trying to resist bikeshedding
the names again, so probably this should stay as-is.
-Kees
> else
> - rnd = (unsigned long)get_random_int() % (1<<28);
> + rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits);
>
> return rnd << PAGE_SHIFT;
> }
> --
> 2.6.0.rc2.230.g3dd15c0
>
--
Kees Cook
Chrome OS Security
On Wed, Nov 18, 2015 at 3:23 PM, Daniel Cashman <[email protected]> wrote:
>> I think the min/max values should be const, since they're determined
>> at build time and should never change.
>
> Ok. Also, I just submitted the patch-set again with [PATCH v3] instead
> of [PATCH] so I'd prefer discussion there; sorry for the mistake.
Oops, yeah, just saw that come in after I already sent my comments. :P
-Kees
--
Kees Cook
Chrome OS Security