Hi,
Repeating the commit logs for patch 4 here:
Dedicated caches are available For fixed size allocations via
kmem_cache_alloc(), but for dynamically sized allocations there is only
the global kmalloc API's set of buckets available. This means it isn't
possible to separate specific sets of dynamically sized allocations into
a separate collection of caches.
This leads to a use-after-free exploitation weakness in the Linux
kernel since many heap memory spraying/grooming attacks depend on using
userspace-controllable dynamically sized allocations to collide with
fixed size allocations that end up in same cache.
While CONFIG_RANDOM_KMALLOC_CACHES provides a probabilistic defense
against these kinds of "type confusion" attacks, including for fixed
same-size heap objects, we can create a complementary deterministic
defense for dynamically sized allocations.
In order to isolate user-controllable sized allocations from system
allocations, introduce kmem_buckets_create(), which behaves like
kmem_cache_create(). (The next patch will introduce kmem_buckets_alloc(),
which behaves like kmem_cache_alloc().)
Allows for confining allocations to a dedicated set of sized caches
(which have the same layout as the kmalloc caches).
This can also be used in the future once codetag allocation annotations
exist to implement per-caller allocation cache isolation[0] even for
dynamic allocations.
Link: https://lore.kernel.org/lkml/202402211449.401382D2AF@keescook [0]
After the implemetation are 2 example patches of how this could be used
for some repeat "offenders" that get used in exploits. There are more to
be isolated beyond just these. Repeating the commit log for patch 8 here:
The msg subsystem is a common target for exploiting[1][2][3][4][5][6]
use-after-free type confusion flaws in the kernel for both read and
write primitives. Avoid having a user-controlled size cache share the
global kmalloc allocator by using a separate set of kmalloc buckets.
Link: https://blog.hacktivesecurity.com/index.php/2022/06/13/linux-kernel-exploit-development-1day-case-study/ [1]
Link: https://hardenedvault.net/blog/2022-11-13-msg_msg-recon-mitigation-ved/ [2]
Link: https://www.willsroot.io/2021/08/corctf-2021-fire-of-salvation-writeup.html [3]
Link: https://a13xp0p0v.github.io/2021/02/09/CVE-2021-26708.html [4]
Link: https://google.github.io/security-research/pocs/linux/cve-2021-22555/writeup.html [5]
Link: https://zplin.me/papers/ELOISE.pdf [6]
-Kees
v2: significant rewrite, generalized the buckets type, added kvmalloc style
v1: https://lore.kernel.org/lkml/[email protected]/
Kees Cook (9):
slab: Introduce kmem_buckets typedef
slub: Plumb kmem_buckets into __do_kmalloc_node()
util: Introduce __kvmalloc_node() that can take kmem_buckets argument
slab: Introduce kmem_buckets_create()
slab: Introduce kmem_buckets_alloc()
slub: Introduce kmem_buckets_alloc_track_caller()
slab: Introduce kmem_buckets_valloc()
ipc, msg: Use dedicated slab buckets for alloc_msg()
mm/util: Use dedicated slab buckets for memdup_user()
include/linux/slab.h | 50 +++++++++++++++++++++-------
ipc/msgutil.c | 13 +++++++-
lib/fortify_kunit.c | 2 +-
mm/slab.h | 6 ++--
mm/slab_common.c | 77 ++++++++++++++++++++++++++++++++++++++++++--
mm/slub.c | 14 ++++----
mm/util.c | 23 +++++++++----
7 files changed, 154 insertions(+), 31 deletions(-)
--
2.34.1
To perform allocations with the buckets allocated with
kmem_buckets_create(), introduce kmem_buckets_alloc() which behaves
like kmem_cache_alloc().
Signed-off-by: Kees Cook <[email protected]>
---
Cc: Vlastimil Babka <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Pekka Enberg <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Hyeonggon Yoo <[email protected]>
Cc: [email protected]
---
include/linux/slab.h | 6 ++++++
1 file changed, 6 insertions(+)
diff --git a/include/linux/slab.h b/include/linux/slab.h
index 058d0e3cd181..08d248f9a1ba 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -600,6 +600,12 @@ static __always_inline __alloc_size(1) void *kmalloc(size_t size, gfp_t flags)
return __kmalloc(size, flags);
}
+static __always_inline __alloc_size(2)
+void *kmem_buckets_alloc(kmem_buckets *b, size_t size, gfp_t flags)
+{
+ return __kmalloc_node(b, size, flags, NUMA_NO_NODE);
+}
+
static __always_inline __alloc_size(1) void *kmalloc_node(size_t size, gfp_t flags, int node)
{
if (__builtin_constant_p(size) && size) {
--
2.34.1
To be able to choose which buckets to allocate from, make the buckets
available to the lower level kmalloc interfaces.
Signed-off-by: Kees Cook <[email protected]>
---
Cc: Vlastimil Babka <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Pekka Enberg <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Hyeonggon Yoo <[email protected]>
Cc: [email protected]
Cc: [email protected]
---
include/linux/slab.h | 8 ++++----
lib/fortify_kunit.c | 2 +-
mm/slab.h | 6 ++++--
mm/slab_common.c | 2 +-
mm/slub.c | 12 ++++++------
5 files changed, 16 insertions(+), 14 deletions(-)
diff --git a/include/linux/slab.h b/include/linux/slab.h
index 55059faf166c..1cc1a7637b56 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -508,8 +508,8 @@ static __always_inline void kfree_bulk(size_t size, void **p)
kmem_cache_free_bulk(NULL, size, p);
}
-void *__kmalloc_node(size_t size, gfp_t flags, int node) __assume_kmalloc_alignment
- __alloc_size(1);
+void *__kmalloc_node(kmem_buckets *b, size_t size, gfp_t flags, int node)
+ __assume_kmalloc_alignment __alloc_size(2);
void *kmem_cache_alloc_node(struct kmem_cache *s, gfp_t flags, int node) __assume_slab_alignment
__malloc;
@@ -608,7 +608,7 @@ static __always_inline __alloc_size(1) void *kmalloc_node(size_t size, gfp_t fla
kmalloc_caches[kmalloc_type(flags, _RET_IP_)][index],
flags, node, size);
}
- return __kmalloc_node(size, flags, node);
+ return __kmalloc_node(NULL, size, flags, node);
}
/**
@@ -686,7 +686,7 @@ static inline __alloc_size(1, 2) void *kmalloc_array_node(size_t n, size_t size,
return NULL;
if (__builtin_constant_p(n) && __builtin_constant_p(size))
return kmalloc_node(bytes, flags, node);
- return __kmalloc_node(bytes, flags, node);
+ return __kmalloc_node(NULL, bytes, flags, node);
}
static inline __alloc_size(1, 2) void *kcalloc_node(size_t n, size_t size, gfp_t flags, int node)
diff --git a/lib/fortify_kunit.c b/lib/fortify_kunit.c
index 2e4fedc81621..c44400b577f3 100644
--- a/lib/fortify_kunit.c
+++ b/lib/fortify_kunit.c
@@ -182,7 +182,7 @@ static void alloc_size_##allocator##_dynamic_test(struct kunit *test) \
checker(expected_size, __kmalloc(alloc_size, gfp), \
kfree(p)); \
checker(expected_size, \
- __kmalloc_node(alloc_size, gfp, NUMA_NO_NODE), \
+ __kmalloc_node(NULL, alloc_size, gfp, NUMA_NO_NODE), \
kfree(p)); \
\
orig = kmalloc(alloc_size, gfp); \
diff --git a/mm/slab.h b/mm/slab.h
index 54deeb0428c6..931f261bde48 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -404,16 +404,18 @@ static inline unsigned int size_index_elem(unsigned int bytes)
* KMALLOC_MAX_CACHE_SIZE and the caller must check that.
*/
static inline struct kmem_cache *
-kmalloc_slab(size_t size, gfp_t flags, unsigned long caller)
+kmalloc_slab(kmem_buckets *b, size_t size, gfp_t flags, unsigned long caller)
{
unsigned int index;
+ if (!b)
+ b = &kmalloc_caches[kmalloc_type(flags, caller)];
if (size <= 192)
index = kmalloc_size_index[size_index_elem(size)];
else
index = fls(size - 1);
- return kmalloc_caches[kmalloc_type(flags, caller)][index];
+ return (*b)[index];
}
gfp_t kmalloc_fix_flags(gfp_t flags);
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 8787cf17d6e4..1d0f25b6ae91 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -698,7 +698,7 @@ size_t kmalloc_size_roundup(size_t size)
* The flags don't matter since size_index is common to all.
* Neither does the caller for just getting ->object_size.
*/
- return kmalloc_slab(size, GFP_KERNEL, 0)->object_size;
+ return kmalloc_slab(NULL, size, GFP_KERNEL, 0)->object_size;
}
/* Above the smaller buckets, size is a multiple of page size. */
diff --git a/mm/slub.c b/mm/slub.c
index 2ef88bbf56a3..71220b4b1f79 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3959,7 +3959,7 @@ void *kmalloc_large_node(size_t size, gfp_t flags, int node)
EXPORT_SYMBOL(kmalloc_large_node);
static __always_inline
-void *__do_kmalloc_node(size_t size, gfp_t flags, int node,
+void *__do_kmalloc_node(kmem_buckets *b, size_t size, gfp_t flags, int node,
unsigned long caller)
{
struct kmem_cache *s;
@@ -3975,7 +3975,7 @@ void *__do_kmalloc_node(size_t size, gfp_t flags, int node,
if (unlikely(!size))
return ZERO_SIZE_PTR;
- s = kmalloc_slab(size, flags, caller);
+ s = kmalloc_slab(b, size, flags, caller);
ret = slab_alloc_node(s, NULL, flags, node, caller, size);
ret = kasan_kmalloc(s, ret, size, flags);
@@ -3983,22 +3983,22 @@ void *__do_kmalloc_node(size_t size, gfp_t flags, int node,
return ret;
}
-void *__kmalloc_node(size_t size, gfp_t flags, int node)
+void *__kmalloc_node(kmem_buckets *b, size_t size, gfp_t flags, int node)
{
- return __do_kmalloc_node(size, flags, node, _RET_IP_);
+ return __do_kmalloc_node(b, size, flags, node, _RET_IP_);
}
EXPORT_SYMBOL(__kmalloc_node);
void *__kmalloc(size_t size, gfp_t flags)
{
- return __do_kmalloc_node(size, flags, NUMA_NO_NODE, _RET_IP_);
+ return __do_kmalloc_node(NULL, size, flags, NUMA_NO_NODE, _RET_IP_);
}
EXPORT_SYMBOL(__kmalloc);
void *__kmalloc_node_track_caller(size_t size, gfp_t flags,
int node, unsigned long caller)
{
- return __do_kmalloc_node(size, flags, node, caller);
+ return __do_kmalloc_node(NULL, size, flags, node, caller);
}
EXPORT_SYMBOL(__kmalloc_node_track_caller);
--
2.34.1
For better capturing the caller details for allocation wrappers,
introduce kmem_buckets_alloc_track_caller() by plumbing the
buckets into the existing *_track_caller() interfaces.
Signed-off-by: Kees Cook <[email protected]>
---
Cc: Vlastimil Babka <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Pekka Enberg <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Hyeonggon Yoo <[email protected]>
Cc: [email protected]
---
include/linux/slab.h | 11 +++++++----
mm/slub.c | 4 ++--
2 files changed, 9 insertions(+), 6 deletions(-)
diff --git a/include/linux/slab.h b/include/linux/slab.h
index 08d248f9a1ba..7d84f875dcf4 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -606,6 +606,9 @@ void *kmem_buckets_alloc(kmem_buckets *b, size_t size, gfp_t flags)
return __kmalloc_node(b, size, flags, NUMA_NO_NODE);
}
+#define kmem_buckets_alloc_track_caller(b, size, flags) \
+ __kmalloc_node_track_caller(b, size, flags, NUMA_NO_NODE, _RET_IP_)
+
static __always_inline __alloc_size(1) void *kmalloc_node(size_t size, gfp_t flags, int node)
{
if (__builtin_constant_p(size) && size) {
@@ -670,10 +673,10 @@ static inline __alloc_size(1, 2) void *kcalloc(size_t n, size_t size, gfp_t flag
return kmalloc_array(n, size, flags | __GFP_ZERO);
}
-void *__kmalloc_node_track_caller(size_t size, gfp_t flags, int node,
- unsigned long caller) __alloc_size(1);
+void *__kmalloc_node_track_caller(kmem_buckets *b, size_t size, gfp_t flags, int node,
+ unsigned long caller) __alloc_size(2);
#define kmalloc_node_track_caller(size, flags, node) \
- __kmalloc_node_track_caller(size, flags, node, \
+ __kmalloc_node_track_caller(NULL, size, flags, node, \
_RET_IP_)
/*
@@ -685,7 +688,7 @@ void *__kmalloc_node_track_caller(size_t size, gfp_t flags, int node,
* request comes from.
*/
#define kmalloc_track_caller(size, flags) \
- __kmalloc_node_track_caller(size, flags, \
+ __kmalloc_node_track_caller(NULL, size, flags, \
NUMA_NO_NODE, _RET_IP_)
static inline __alloc_size(1, 2) void *kmalloc_array_node(size_t n, size_t size, gfp_t flags,
diff --git a/mm/slub.c b/mm/slub.c
index 71220b4b1f79..ae54ec452a11 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3995,10 +3995,10 @@ void *__kmalloc(size_t size, gfp_t flags)
}
EXPORT_SYMBOL(__kmalloc);
-void *__kmalloc_node_track_caller(size_t size, gfp_t flags,
+void *__kmalloc_node_track_caller(kmem_buckets *b, size_t size, gfp_t flags,
int node, unsigned long caller)
{
- return __do_kmalloc_node(NULL, size, flags, node, caller);
+ return __do_kmalloc_node(b, size, flags, node, caller);
}
EXPORT_SYMBOL(__kmalloc_node_track_caller);
--
2.34.1
For allocations that may need to fallback to vmalloc, add
kmem_buckets_valloc().
Signed-off-by: Kees Cook <[email protected]>
---
Cc: Vlastimil Babka <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Pekka Enberg <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Hyeonggon Yoo <[email protected]>
Cc: [email protected]
---
include/linux/slab.h | 6 ++++++
1 file changed, 6 insertions(+)
diff --git a/include/linux/slab.h b/include/linux/slab.h
index 7d84f875dcf4..0cf72861d5fa 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -740,6 +740,12 @@ static inline __alloc_size(1) void *kzalloc_node(size_t size, gfp_t flags, int n
void * __alloc_size(2)
__kvmalloc_node(kmem_buckets *b, size_t size, gfp_t flags, int node);
+static __always_inline __alloc_size(2)
+void *kmem_buckets_valloc(kmem_buckets *b, size_t size, gfp_t flags)
+{
+ return __kvmalloc_node(b, size, flags, NUMA_NO_NODE);
+}
+
static inline __alloc_size(1) void *kvmalloc_node(size_t size, gfp_t flags, int node)
{
return __kvmalloc_node(NULL, size, flags, node);
--
2.34.1
Both memdup_user() and vmemdup_user() handle allocations that are
regularly used for exploiting use-after-free type confusion flaws in
the kernel (e.g. prctl() PR_SET_VMA_ANON_NAME[1] and setxattr[2][3][4]
respectively).
Since both are designed for contents coming from userspace, it allows
for userspace-controlled allocation sizes. Use a dedicated set of kmalloc
buckets so these allocations do not share caches with the global kmalloc
buckets.
After a fresh boot under Ubuntu 23.10, we can see the caches are already
in active use:
# grep ^memdup /proc/slabinfo
memdup_user-8k 4 4 8192 4 8 : ...
memdup_user-4k 8 8 4096 8 8 : ...
memdup_user-2k 16 16 2048 16 8 : ...
memdup_user-1k 0 0 1024 16 4 : ...
memdup_user-512 0 0 512 16 2 : ...
memdup_user-256 0 0 256 16 1 : ...
memdup_user-128 0 0 128 32 1 : ...
memdup_user-64 256 256 64 64 1 : ...
memdup_user-32 512 512 32 128 1 : ...
memdup_user-16 1024 1024 16 256 1 : ...
memdup_user-8 2048 2048 8 512 1 : ...
memdup_user-192 0 0 192 21 1 : ...
memdup_user-96 168 168 96 42 1 : ...
Link: https://starlabs.sg/blog/2023/07-prctl-anon_vma_name-an-amusing-heap-spray/ [1]
Link: https://duasynt.com/blog/linux-kernel-heap-spray [2]
Link: https://etenal.me/archives/1336 [3]
Link: https://github.com/a13xp0p0v/kernel-hack-drill/blob/master/drill_exploit_uaf.c [4]
Signed-off-by: Kees Cook <[email protected]>
---
Cc: Andrew Morton <[email protected]>
Cc: "GONG, Ruiqi" <[email protected]>
Cc: Xiu Jianfeng <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: Kent Overstreet <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Matteo Rizzo <[email protected]>
Cc: [email protected]
---
mm/util.c | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)
diff --git a/mm/util.c b/mm/util.c
index 02c895b87a28..25b9122022a7 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -181,6 +181,16 @@ char *kmemdup_nul(const char *s, size_t len, gfp_t gfp)
}
EXPORT_SYMBOL(kmemdup_nul);
+static kmem_buckets *user_buckets __ro_after_init;
+
+static int __init init_user_buckets(void)
+{
+ user_buckets = kmem_buckets_create("memdup_user", 0, 0, 0, INT_MAX, NULL);
+
+ return 0;
+}
+subsys_initcall(init_user_buckets);
+
/**
* memdup_user - duplicate memory region from user space
*
@@ -194,7 +204,7 @@ void *memdup_user(const void __user *src, size_t len)
{
void *p;
- p = kmalloc_track_caller(len, GFP_USER | __GFP_NOWARN);
+ p = kmem_buckets_alloc_track_caller(user_buckets, len, GFP_USER | __GFP_NOWARN);
if (!p)
return ERR_PTR(-ENOMEM);
@@ -220,7 +230,7 @@ void *vmemdup_user(const void __user *src, size_t len)
{
void *p;
- p = kvmalloc(len, GFP_USER);
+ p = kmem_buckets_valloc(user_buckets, len, GFP_USER);
if (!p)
return ERR_PTR(-ENOMEM);
--
2.34.1
The msg subsystem is a common target for exploiting[1][2][3][4][5][6]
use-after-free type confusion flaws in the kernel for both read and
write primitives. Avoid having a user-controlled size cache share the
global kmalloc allocator by using a separate set of kmalloc buckets.
Link: https://blog.hacktivesecurity.com/index.php/2022/06/13/linux-kernel-exploit-development-1day-case-study/ [1]
Link: https://hardenedvault.net/blog/2022-11-13-msg_msg-recon-mitigation-ved/ [2]
Link: https://www.willsroot.io/2021/08/corctf-2021-fire-of-salvation-writeup.html [3]
Link: https://a13xp0p0v.github.io/2021/02/09/CVE-2021-26708.html [4]
Link: https://google.github.io/security-research/pocs/linux/cve-2021-22555/writeup.html [5]
Link: https://zplin.me/papers/ELOISE.pdf [6]
Link: https://syst3mfailure.io/wall-of-perdition/
Signed-off-by: Kees Cook <[email protected]>
---
Cc: "GONG, Ruiqi" <[email protected]>
Cc: Xiu Jianfeng <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: Kent Overstreet <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Matteo Rizzo <[email protected]>
---
ipc/msgutil.c | 13 ++++++++++++-
1 file changed, 12 insertions(+), 1 deletion(-)
diff --git a/ipc/msgutil.c b/ipc/msgutil.c
index d0a0e877cadd..f392f30a057a 100644
--- a/ipc/msgutil.c
+++ b/ipc/msgutil.c
@@ -42,6 +42,17 @@ struct msg_msgseg {
#define DATALEN_MSG ((size_t)PAGE_SIZE-sizeof(struct msg_msg))
#define DATALEN_SEG ((size_t)PAGE_SIZE-sizeof(struct msg_msgseg))
+static kmem_buckets *msg_buckets __ro_after_init;
+
+static int __init init_msg_buckets(void)
+{
+ msg_buckets = kmem_buckets_create("msg_msg", 0, SLAB_ACCOUNT,
+ sizeof(struct msg_msg),
+ DATALEN_MSG, NULL);
+
+ return 0;
+}
+subsys_initcall(init_msg_buckets);
static struct msg_msg *alloc_msg(size_t len)
{
@@ -50,7 +61,7 @@ static struct msg_msg *alloc_msg(size_t len)
size_t alen;
alen = min(len, DATALEN_MSG);
- msg = kmalloc(sizeof(*msg) + alen, GFP_KERNEL_ACCOUNT);
+ msg = kmem_buckets_alloc(msg_buckets, sizeof(*msg) + alen, GFP_KERNEL);
if (msg == NULL)
return NULL;
--
2.34.1
On 2024/03/05 18:10, Kees Cook wrote:
> Hi,
>
> Repeating the commit logs for patch 4 here:
>
> Dedicated caches are available For fixed size allocations via
> kmem_cache_alloc(), but for dynamically sized allocations there is only
> the global kmalloc API's set of buckets available. This means it isn't
> possible to separate specific sets of dynamically sized allocations into
> a separate collection of caches.
>
> This leads to a use-after-free exploitation weakness in the Linux
> kernel since many heap memory spraying/grooming attacks depend on using
> userspace-controllable dynamically sized allocations to collide with
> fixed size allocations that end up in same cache.
>
> While CONFIG_RANDOM_KMALLOC_CACHES provides a probabilistic defense
> against these kinds of "type confusion" attacks, including for fixed
> same-size heap objects, we can create a complementary deterministic
> defense for dynamically sized allocations.
>
> In order to isolate user-controllable sized allocations from system
> allocations, introduce kmem_buckets_create(), which behaves like
> kmem_cache_create(). (The next patch will introduce kmem_buckets_alloc(),
> which behaves like kmem_cache_alloc().)
So can I say the vision here would be to make all the kernel interfaces
that handles user space input to use separated caches? Which looks like
creating a "grey zone" in the middle of kernel space (trusted) and user
space (untrusted) memory. I've also thought that maybe hardening on the
"border" could be more efficient and targeted than a mitigation that
affects globally, e.g. CONFIG_RANDOM_KMALLOC_CACHES.
On Wed, Mar 06, 2024 at 09:47:36AM +0800, GONG, Ruiqi wrote:
>
>
> On 2024/03/05 18:10, Kees Cook wrote:
> > Hi,
> >
> > Repeating the commit logs for patch 4 here:
> >
> > Dedicated caches are available For fixed size allocations via
> > kmem_cache_alloc(), but for dynamically sized allocations there is only
> > the global kmalloc API's set of buckets available. This means it isn't
> > possible to separate specific sets of dynamically sized allocations into
> > a separate collection of caches.
> >
> > This leads to a use-after-free exploitation weakness in the Linux
> > kernel since many heap memory spraying/grooming attacks depend on using
> > userspace-controllable dynamically sized allocations to collide with
> > fixed size allocations that end up in same cache.
> >
> > While CONFIG_RANDOM_KMALLOC_CACHES provides a probabilistic defense
> > against these kinds of "type confusion" attacks, including for fixed
> > same-size heap objects, we can create a complementary deterministic
> > defense for dynamically sized allocations.
> >
> > In order to isolate user-controllable sized allocations from system
> > allocations, introduce kmem_buckets_create(), which behaves like
> > kmem_cache_create(). (The next patch will introduce kmem_buckets_alloc(),
> > which behaves like kmem_cache_alloc().)
>
> So can I say the vision here would be to make all the kernel interfaces
> that handles user space input to use separated caches? Which looks like
> creating a "grey zone" in the middle of kernel space (trusted) and user
> space (untrusted) memory. I've also thought that maybe hardening on the
> "border" could be more efficient and targeted than a mitigation that
> affects globally, e.g. CONFIG_RANDOM_KMALLOC_CACHES.
I think it ends up having a similar effect, yes. The more copies that
move to memdup_user(), the more coverage is created. The main point is to
just not share caches between different kinds of allocations. The most
abused version of this is the userspace size-controllable allocations,
which this targets. The existing caches (which could still be used for
type confusion attacks when the sizes are sufficiently similar) have a
good chance of being mitigated by CONFIG_RANDOM_KMALLOC_CACHES already,
so this proposed change is just complementary, IMO.
-Kees
--
Kees Cook
On 2024/03/08 4:31, Kees Cook wrote:
> On Wed, Mar 06, 2024 at 09:47:36AM +0800, GONG, Ruiqi wrote:
>>
>>
>> On 2024/03/05 18:10, Kees Cook wrote:
>>> Hi,
>>>
>>> Repeating the commit logs for patch 4 here:
>>>
>>> Dedicated caches are available For fixed size allocations via
>>> kmem_cache_alloc(), but for dynamically sized allocations there is only
>>> the global kmalloc API's set of buckets available. This means it isn't
>>> possible to separate specific sets of dynamically sized allocations into
>>> a separate collection of caches.
>>>
>>> This leads to a use-after-free exploitation weakness in the Linux
>>> kernel since many heap memory spraying/grooming attacks depend on using
>>> userspace-controllable dynamically sized allocations to collide with
>>> fixed size allocations that end up in same cache.
>>>
>>> While CONFIG_RANDOM_KMALLOC_CACHES provides a probabilistic defense
>>> against these kinds of "type confusion" attacks, including for fixed
>>> same-size heap objects, we can create a complementary deterministic
>>> defense for dynamically sized allocations.
>>>
>>> In order to isolate user-controllable sized allocations from system
>>> allocations, introduce kmem_buckets_create(), which behaves like
>>> kmem_cache_create(). (The next patch will introduce kmem_buckets_alloc(),
>>> which behaves like kmem_cache_alloc().)
>>
>> So can I say the vision here would be to make all the kernel interfaces
>> that handles user space input to use separated caches? Which looks like
>> creating a "grey zone" in the middle of kernel space (trusted) and user
>> space (untrusted) memory. I've also thought that maybe hardening on the
>> "border" could be more efficient and targeted than a mitigation that
>> affects globally, e.g. CONFIG_RANDOM_KMALLOC_CACHES.
>
> I think it ends up having a similar effect, yes. The more copies that
> move to memdup_user(), the more coverage is created. The main point is to
> just not share caches between different kinds of allocations. The most
> abused version of this is the userspace size-controllable allocations,
> which this targets.
I agree. Currently if we want to fulfill a more strict separation
between user-space manageable memory and other memory in kernel space,
technically speaking for fixed size allocations we could transform them
into using dedicated caches (i.e. kmem_cache_create()), but for dynamic
size allocations I don't think of any solution. With the APIs provided
by this patch set, we've got something that works.
> ... The existing caches (which could still be used for
> type confusion attacks when the sizes are sufficiently similar) have a
> good chance of being mitigated by CONFIG_RANDOM_KMALLOC_CACHES already,
> so this proposed change is just complementary, IMO.
Maybe in the future we could require that all user-kernel interfaces
that make use of SLAB caches should use either kmem_cache_create() or
kmem_buckets_create()? ;)
>
> -Kees
>
On 3/5/24 11:10 AM, Kees Cook wrote:
> Hi,
>
> Repeating the commit logs for patch 4 here:
>
> Dedicated caches are available For fixed size allocations via
> kmem_cache_alloc(), but for dynamically sized allocations there is only
> the global kmalloc API's set of buckets available. This means it isn't
> possible to separate specific sets of dynamically sized allocations into
> a separate collection of caches.
>
> This leads to a use-after-free exploitation weakness in the Linux
> kernel since many heap memory spraying/grooming attacks depend on using
> userspace-controllable dynamically sized allocations to collide with
> fixed size allocations that end up in same cache.
>
> While CONFIG_RANDOM_KMALLOC_CACHES provides a probabilistic defense
> against these kinds of "type confusion" attacks, including for fixed
> same-size heap objects, we can create a complementary deterministic
> defense for dynamically sized allocations.
>
> In order to isolate user-controllable sized allocations from system
> allocations, introduce kmem_buckets_create(), which behaves like
> kmem_cache_create(). (The next patch will introduce kmem_buckets_alloc(),
> which behaves like kmem_cache_alloc().)
>
> Allows for confining allocations to a dedicated set of sized caches
> (which have the same layout as the kmalloc caches).
>
> This can also be used in the future once codetag allocation annotations
> exist to implement per-caller allocation cache isolation[0] even for
> dynamic allocations.
>
> Link: https://lore.kernel.org/lkml/202402211449.401382D2AF@keescook [0]
>
> After the implemetation are 2 example patches of how this could be used
> for some repeat "offenders" that get used in exploits. There are more to
> be isolated beyond just these. Repeating the commit log for patch 8 here:
>
> The msg subsystem is a common target for exploiting[1][2][3][4][5][6]
> use-after-free type confusion flaws in the kernel for both read and
> write primitives. Avoid having a user-controlled size cache share the
> global kmalloc allocator by using a separate set of kmalloc buckets.
>
> Link: https://blog.hacktivesecurity.com/index.php/2022/06/13/linux-kernel-exploit-development-1day-case-study/ [1]
> Link: https://hardenedvault.net/blog/2022-11-13-msg_msg-recon-mitigation-ved/ [2]
> Link: https://www.willsroot.io/2021/08/corctf-2021-fire-of-salvation-writeup.html [3]
> Link: https://a13xp0p0v.github.io/2021/02/09/CVE-2021-26708.html [4]
> Link: https://google.github.io/security-research/pocs/linux/cve-2021-22555/writeup.html [5]
> Link: https://zplin.me/papers/ELOISE.pdf [6]
Hi Kees,
after reading [1] I think the points should be addressed, mainly about the
feasibility of converting users manually. On a related technical note I
worry what will become of /proc/slabinfo when we convert non-trivial amounts
of users.
Also would interested to hear Jann Horn et al.'s opinion, and whether the
SLAB_VIRTUAL effort will continue?
Thanks,
Vlastimil
[1]
https://dustri.org/b/notes-on-the-slab-introduce-dedicated-bucket-allocator-series.html
> -Kees
>
> v2: significant rewrite, generalized the buckets type, added kvmalloc style
> v1: https://lore.kernel.org/lkml/[email protected]/
>
> Kees Cook (9):
> slab: Introduce kmem_buckets typedef
> slub: Plumb kmem_buckets into __do_kmalloc_node()
> util: Introduce __kvmalloc_node() that can take kmem_buckets argument
> slab: Introduce kmem_buckets_create()
> slab: Introduce kmem_buckets_alloc()
> slub: Introduce kmem_buckets_alloc_track_caller()
> slab: Introduce kmem_buckets_valloc()
> ipc, msg: Use dedicated slab buckets for alloc_msg()
> mm/util: Use dedicated slab buckets for memdup_user()
>
> include/linux/slab.h | 50 +++++++++++++++++++++-------
> ipc/msgutil.c | 13 +++++++-
> lib/fortify_kunit.c | 2 +-
> mm/slab.h | 6 ++--
> mm/slab_common.c | 77 ++++++++++++++++++++++++++++++++++++++++++--
> mm/slub.c | 14 ++++----
> mm/util.c | 23 +++++++++----
> 7 files changed, 154 insertions(+), 31 deletions(-)
>
On Mon, Mar 25, 2024 at 10:03:23AM +0100, Vlastimil Babka wrote:
> On 3/5/24 11:10 AM, Kees Cook wrote:
> > Hi,
> >
> > Repeating the commit logs for patch 4 here:
> >
> > Dedicated caches are available For fixed size allocations via
> > kmem_cache_alloc(), but for dynamically sized allocations there is only
> > the global kmalloc API's set of buckets available. This means it isn't
> > possible to separate specific sets of dynamically sized allocations into
> > a separate collection of caches.
> >
> > This leads to a use-after-free exploitation weakness in the Linux
> > kernel since many heap memory spraying/grooming attacks depend on using
> > userspace-controllable dynamically sized allocations to collide with
> > fixed size allocations that end up in same cache.
> >
> > While CONFIG_RANDOM_KMALLOC_CACHES provides a probabilistic defense
> > against these kinds of "type confusion" attacks, including for fixed
> > same-size heap objects, we can create a complementary deterministic
> > defense for dynamically sized allocations.
> >
> > In order to isolate user-controllable sized allocations from system
> > allocations, introduce kmem_buckets_create(), which behaves like
> > kmem_cache_create(). (The next patch will introduce kmem_buckets_alloc(),
> > which behaves like kmem_cache_alloc().)
> >
> > Allows for confining allocations to a dedicated set of sized caches
> > (which have the same layout as the kmalloc caches).
> >
> > This can also be used in the future once codetag allocation annotations
> > exist to implement per-caller allocation cache isolation[0] even for
> > dynamic allocations.
> >
> > Link: https://lore.kernel.org/lkml/202402211449.401382D2AF@keescook [0]
> >
> > After the implemetation are 2 example patches of how this could be used
> > for some repeat "offenders" that get used in exploits. There are more to
> > be isolated beyond just these. Repeating the commit log for patch 8 here:
> >
> > The msg subsystem is a common target for exploiting[1][2][3][4][5][6]
> > use-after-free type confusion flaws in the kernel for both read and
> > write primitives. Avoid having a user-controlled size cache share the
> > global kmalloc allocator by using a separate set of kmalloc buckets.
> >
> > Link: https://blog.hacktivesecurity.com/index.php/2022/06/13/linux-kernel-exploit-development-1day-case-study/ [1]
> > Link: https://hardenedvault.net/blog/2022-11-13-msg_msg-recon-mitigation-ved/ [2]
> > Link: https://www.willsroot.io/2021/08/corctf-2021-fire-of-salvation-writeup.html [3]
> > Link: https://a13xp0p0v.github.io/2021/02/09/CVE-2021-26708.html [4]
> > Link: https://google.github.io/security-research/pocs/linux/cve-2021-22555/writeup.html [5]
> > Link: https://zplin.me/papers/ELOISE.pdf [6]
>
> Hi Kees,
>
> after reading [1] I think the points should be addressed, mainly about the
> feasibility of converting users manually.
Sure, I can do that.
Adding Julien to this thread... Julien can you please respond to LKML
patches in email? It's much easier to keep things in a single thread. :)
] This is playing wack-a-mole
Kind of, but not really. These patches provide a mechanism for having
dedicated dynamically-sized slab caches (to match kmem_cache_create(),
which only works for fixed-size allocations). This is needed to expand
the codetag work into doing per-call-site allocations, as I detailed
here[1].
Also, adding uses manually isn't very difficult, as can be seen in the
examples I included. In fact, my examples between v1 and v2 collapsed
from 3 to 2, because covering memdup_user() actually covered 2 known
allocation paths (attrs and vma names), and given its usage pattern,
will cover more in the future without changes.
] something like AUTOSLAB would be better
Yes, that's the goal of [1]. This is a prerequisite for that, as
mentioned in the cover letter.
] The slabs needs to be pinned
Yes, and this is a general problem[2] with all kmalloc allocations, though.
This isn't unique to to this patch series. SLAB_VIRTUAL solves it, and
is under development.
] Lacks guard pages
Yes, and again, this is a general problem with all kmalloc allocations.
Solving it, like SLAB_VIRTUAL, would be a complementary hardening
improvement to the allocator generally.
] PAX_USERCOPY has been marking these sites since 2012
Either it's whack-a-mole or it's not. :) PAX_USERCOPY shows that it _is_
possible to mark all sites. Regardless, like AUTOSLAB, PAX_USERCOPY isn't
upstream, and its current implementation is an unpublished modification
to a GPL project. I look forward to someone proposing it for inclusion
in Linux, but for now we can work with the patches where an effort _has_
been made to upstream them for the benefit of the entire ecosystem.
] What about CONFIG_KMALLOC_SPLIT_VARSIZE
This proposed improvement is hampered by not having dedicated
_dynamically_ sized kmem caches, which this series provides. And with
codetag-split allocations[1], the goals of CONFIG_KMALLOC_SPLIT_VARSIZE
are more fully realized, providing much more complete coverage.
] I have no idea how the community around the Linux kernel works with
] their email-based workflows
Step 1: reply to the proposal in email instead of (or perhaps in
addition to) making blog posts. :)
> On a related technical note I
> worry what will become of /proc/slabinfo when we convert non-trivial amounts
> of users.
It gets longer. :) And potentially makes the codetag /proc file
redundant. All that said, there are very few APIs in the kernel where
userspace can control both the size and contents of an allocation.
> Also would interested to hear Jann Horn et al.'s opinion, and whether the
> SLAB_VIRTUAL effort will continue?
SLAB_VIRTUAL is needed to address the reclamation UAF gap, and is
still being developed. I don't intend to let it fall off the radar.
(Which is why I included Jann and Matteo in CC originally.)
In the meantime, adding this series as-is kills two long-standing
exploitation methodologies, and paves the way to providing very
fine-grained caches using codetags (which I imagine would be entirely
optional and trivial to control with a boot param).
-Kees
[1] https://lore.kernel.org/lkml/202402211449.401382D2AF@keescook/
[2] https://googleprojectzero.blogspot.com/2021/10/how-simple-linux-kernel-memory.html
--
Kees Cook
On Mon, Mar 25, 2024 at 10:03:23AM +0100, Vlastimil Babka wrote:
> On 3/5/24 11:10 AM, Kees Cook wrote:
> > Hi,
> >
> > Repeating the commit logs for patch 4 here:
> >
> > Dedicated caches are available For fixed size allocations via
> > kmem_cache_alloc(), but for dynamically sized allocations there is only
> > the global kmalloc API's set of buckets available. This means it isn't
> > possible to separate specific sets of dynamically sized allocations into
> > a separate collection of caches.
> >
> > This leads to a use-after-free exploitation weakness in the Linux
> > kernel since many heap memory spraying/grooming attacks depend on using
> > userspace-controllable dynamically sized allocations to collide with
> > fixed size allocations that end up in same cache.
> >
> > While CONFIG_RANDOM_KMALLOC_CACHES provides a probabilistic defense
> > against these kinds of "type confusion" attacks, including for fixed
> > same-size heap objects, we can create a complementary deterministic
> > defense for dynamically sized allocations.
> >
> > In order to isolate user-controllable sized allocations from system
> > allocations, introduce kmem_buckets_create(), which behaves like
> > kmem_cache_create(). (The next patch will introduce kmem_buckets_alloc(),
> > which behaves like kmem_cache_alloc().)
> >
> > Allows for confining allocations to a dedicated set of sized caches
> > (which have the same layout as the kmalloc caches).
> >
> > This can also be used in the future once codetag allocation annotations
> > exist to implement per-caller allocation cache isolation[0] even for
> > dynamic allocations.
> >
> > Link: https://lore.kernel.org/lkml/202402211449.401382D2AF@keescook [0]
> >
> > After the implemetation are 2 example patches of how this could be used
> > for some repeat "offenders" that get used in exploits. There are more to
> > be isolated beyond just these. Repeating the commit log for patch 8 here:
> >
> > The msg subsystem is a common target for exploiting[1][2][3][4][5][6]
> > use-after-free type confusion flaws in the kernel for both read and
> > write primitives. Avoid having a user-controlled size cache share the
> > global kmalloc allocator by using a separate set of kmalloc buckets.
> >
> > Link: https://blog.hacktivesecurity.com/index.php/2022/06/13/linux-kernel-exploit-development-1day-case-study/ [1]
> > Link: https://hardenedvault.net/blog/2022-11-13-msg_msg-recon-mitigation-ved/ [2]
> > Link: https://www.willsroot.io/2021/08/corctf-2021-fire-of-salvation-writeup.html [3]
> > Link: https://a13xp0p0v.github.io/2021/02/09/CVE-2021-26708.html [4]
> > Link: https://google.github.io/security-research/pocs/linux/cve-2021-22555/writeup.html [5]
> > Link: https://zplin.me/papers/ELOISE.pdf [6]
>
> Hi Kees,
>
> after reading [1] I think the points should be addressed, mainly about the
> feasibility of converting users manually. On a related technical note I
> worry what will become of /proc/slabinfo when we convert non-trivial amounts
> of users.
There shouldn't be any need to convert users to this interface - just
leverage the alloc_hooks() macro.
On Mon, Mar 25, 2024 at 03:32:12PM -0400, Kent Overstreet wrote:
> On Mon, Mar 25, 2024 at 10:03:23AM +0100, Vlastimil Babka wrote:
> > On 3/5/24 11:10 AM, Kees Cook wrote:
> > > Hi,
> > >
> > > Repeating the commit logs for patch 4 here:
> > >
> > > Dedicated caches are available For fixed size allocations via
> > > kmem_cache_alloc(), but for dynamically sized allocations there is only
> > > the global kmalloc API's set of buckets available. This means it isn't
> > > possible to separate specific sets of dynamically sized allocations into
> > > a separate collection of caches.
> > >
> > > This leads to a use-after-free exploitation weakness in the Linux
> > > kernel since many heap memory spraying/grooming attacks depend on using
> > > userspace-controllable dynamically sized allocations to collide with
> > > fixed size allocations that end up in same cache.
> > >
> > > While CONFIG_RANDOM_KMALLOC_CACHES provides a probabilistic defense
> > > against these kinds of "type confusion" attacks, including for fixed
> > > same-size heap objects, we can create a complementary deterministic
> > > defense for dynamically sized allocations.
> > >
> > > In order to isolate user-controllable sized allocations from system
> > > allocations, introduce kmem_buckets_create(), which behaves like
> > > kmem_cache_create(). (The next patch will introduce kmem_buckets_alloc(),
> > > which behaves like kmem_cache_alloc().)
> > >
> > > Allows for confining allocations to a dedicated set of sized caches
> > > (which have the same layout as the kmalloc caches).
> > >
> > > This can also be used in the future once codetag allocation annotations
> > > exist to implement per-caller allocation cache isolation[0] even for
> > > dynamic allocations.
> > >
> > > Link: https://lore.kernel.org/lkml/202402211449.401382D2AF@keescook [0]
> > >
> > > After the implemetation are 2 example patches of how this could be used
> > > for some repeat "offenders" that get used in exploits. There are more to
> > > be isolated beyond just these. Repeating the commit log for patch 8 here:
> > >
> > > The msg subsystem is a common target for exploiting[1][2][3][4][5][6]
> > > use-after-free type confusion flaws in the kernel for both read and
> > > write primitives. Avoid having a user-controlled size cache share the
> > > global kmalloc allocator by using a separate set of kmalloc buckets.
> > >
> > > Link: https://blog.hacktivesecurity.com/index.php/2022/06/13/linux-kernel-exploit-development-1day-case-study/ [1]
> > > Link: https://hardenedvault.net/blog/2022-11-13-msg_msg-recon-mitigation-ved/ [2]
> > > Link: https://www.willsroot.io/2021/08/corctf-2021-fire-of-salvation-writeup.html [3]
> > > Link: https://a13xp0p0v.github.io/2021/02/09/CVE-2021-26708.html [4]
> > > Link: https://google.github.io/security-research/pocs/linux/cve-2021-22555/writeup.html [5]
> > > Link: https://zplin.me/papers/ELOISE.pdf [6]
> >
> > Hi Kees,
> >
> > after reading [1] I think the points should be addressed, mainly about the
> > feasibility of converting users manually. On a related technical note I
> > worry what will become of /proc/slabinfo when we convert non-trivial amounts
> > of users.
>
> There shouldn't be any need to convert users to this interface - just
> leverage the alloc_hooks() macro.
I expect to do both -- using the alloc_hooks() macro to do
per-call-site-allocation caches will certainly have a non-trivial amount
of memory usage overhead, and not all systems will want it. We can have
a boot param to choose between per-site and normal, though normal can
include a handful of these manually identified places.
--
Kees Cook
25 March 2024 at 19:24, "Kees Cook" <[email protected]> wrote:
>
> On Mon, Mar 25, 2024 at 10:03:23AM +0100, Vlastimil Babka wrote:
>
> >
> > On 3/5/24 11:10 AM, Kees Cook wrote:
> >
> > Hi,
> >
> >
> >
> > Repeating the commit logs for patch 4 here:
> >
> >
> >
> > Dedicated caches are available For fixed size allocations via
> >
> > kmem_cache_alloc(), but for dynamically sized allocations there is only
> >
> > the global kmalloc API's set of buckets available. This means it isn't
> >
> > possible to separate specific sets of dynamically sized allocations into
> >
> > a separate collection of caches.
> >
> >
> >
> > This leads to a use-after-free exploitation weakness in the Linux
> >
> > kernel since many heap memory spraying/grooming attacks depend on using
> >
> > userspace-controllable dynamically sized allocations to collide with
> >
> > fixed size allocations that end up in same cache.
> >
> >
> >
> > While CONFIG_RANDOM_KMALLOC_CACHES provides a probabilistic defense
> >
> > against these kinds of "type confusion" attacks, including for fixed
> >
> > same-size heap objects, we can create a complementary deterministic
> >
> > defense for dynamically sized allocations.
> >
> >
> >
> > In order to isolate user-controllable sized allocations from system
> >
> > allocations, introduce kmem_buckets_create(), which behaves like
> >
> > kmem_cache_create(). (The next patch will introduce kmem_buckets_alloc(),
> >
> > which behaves like kmem_cache_alloc().)
> >
> >
> >
> > Allows for confining allocations to a dedicated set of sized caches
> >
> > (which have the same layout as the kmalloc caches).
> >
> >
> >
> > This can also be used in the future once codetag allocation annotations
> >
> > exist to implement per-caller allocation cache isolation[0] even for
> >
> > dynamic allocations.
> >
> >
> >
> > Link: https://lore.kernel.org/lkml/202402211449.401382D2AF@keescook [0]
> >
> >
> >
> > After the implemetation are 2 example patches of how this could be used
> >
> > for some repeat "offenders" that get used in exploits. There are more to
> >
> > be isolated beyond just these. Repeating the commit log for patch 8 here:
> >
> >
> >
> > The msg subsystem is a common target for exploiting[1][2][3][4][5][6]
> >
> > use-after-free type confusion flaws in the kernel for both read and
> >
> > write primitives. Avoid having a user-controlled size cache share the
> >
> > global kmalloc allocator by using a separate set of kmalloc buckets.
> >
> >
> >
> > Link: https://blog.hacktivesecurity.com/index.php/2022/06/13/linux-kernel-exploit-development-1day-case-study/ [1]
> >
> > Link: https://hardenedvault.net/blog/2022-11-13-msg_msg-recon-mitigation-ved/ [2]
> >
> > Link: https://www.willsroot.io/2021/08/corctf-2021-fire-of-salvation-writeup.html [3]
> >
> > Link: https://a13xp0p0v.github.io/2021/02/09/CVE-2021-26708.html [4]
> >
> > Link: https://google.github.io/security-research/pocs/linux/cve-2021-22555/writeup.html [5]
> >
> > Link: https://zplin.me/papers/ELOISE.pdf [6]
> >
> >
> >
> > Hi Kees,
> >
> >
> >
> > after reading [1] I think the points should be addressed, mainly about the
> >
> > feasibility of converting users manually.
> >
>
> Sure, I can do that.
>
> Adding Julien to this thread... Julien can you please respond to LKML
>
> patches in email? It's much easier to keep things in a single thread. :)
>
> ] This is playing wack-a-mole
>
> Kind of, but not really. These patches provide a mechanism for having
>
> dedicated dynamically-sized slab caches (to match kmem_cache_create(),
>
> which only works for fixed-size allocations). This is needed to expand
>
> the codetag work into doing per-call-site allocations, as I detailed
>
> here[1].
>
> Also, adding uses manually isn't very difficult, as can be seen in the
>
> examples I included. In fact, my examples between v1 and v2 collapsed
>
> from 3 to 2, because covering memdup_user() actually covered 2 known
>
> allocation paths (attrs and vma names), and given its usage pattern,
>
> will cover more in the future without changes.
It's not about difficulty, it's about scale. There are hundreds of interesting structures: I'm worried that no one will take the time to add a separate bucket for each of them, chase their call-sites down, and monitor every single newly added structures to check if they are "interesting" and should benefit from their own bucket as well.
>
> ] something like AUTOSLAB would be better
>
> Yes, that's the goal of [1]. This is a prerequisite for that, as
>
> mentioned in the cover letter.
This series looks unrelated to [1] to me: the former adds a mechanism to add buckets and expects developers to manually make use of them, while the latter is about adding infrastructure to automate call-site-based segregation.
> ] The slabs needs to be pinned
>
> Yes, and this is a general problem[2] with all kmalloc allocations, though.
>
> This isn't unique to to this patch series. SLAB_VIRTUAL solves it, and
>
> is under development.
Then it would be nice to mention it in the serie, as an acknowledged limitation.
> ] Lacks guard pages
>
> Yes, and again, this is a general problem with all kmalloc allocations.
>
> Solving it, like SLAB_VIRTUAL, would be a complementary hardening
>
> improvement to the allocator generally.
Then it would also be nice to mention it, because currently it's unclear that those limitations are both known and will be properly addressed.
>
> ] PAX_USERCOPY has been marking these sites since 2012
>
> Either it's whack-a-mole or it's not. :)
This annotation was added 12 years ago in PaX, and while it was state of the art back then, I think that in 2024 we can do better than this.
> PAX_USERCOPY shows that it _is_ possible to mark all sites.
It shows that it's possible to annotate some sites (17 in grsecurity-3.1-4.9.9-201702122044.patch), and while it has a similar approach to your series, its annotations aren't conveying the same meaning.
> Regardless, like AUTOSLAB, PAX_USERCOPY isn't
>
> upstream, and its current implementation is an unpublished modification
>
> to a GPL project. I look forward to someone proposing it for inclusion
>
> in Linux, but for now we can work with the patches where an effort _has_
>
> been made to upstream them for the benefit of the entire ecosystem.
>
> ] What about CONFIG_KMALLOC_SPLIT_VARSIZE
>
> This proposed improvement is hampered by not having dedicated
>
> _dynamically_ sized kmem caches, which this series provides. And with
>
> codetag-split allocations[1], the goals of CONFIG_KMALLOC_SPLIT_VARSIZE
>
> are more fully realized, providing much more complete coverage.
CONFIG_KMALLOC_SPLIT_VARSIZE has been bypassed dozen of times in various ways as part of Google's kernelCTF.
Your series is, to my understanding, a weaker form of it. So I'm not super-convinced that it's the right approach to mitigate UAF.
Do you think it would be possible for Google to add this series to its kernelCTF, so gather empirical data on how feasible/easy it is to bypass it?
>
> ] I have no idea how the community around the Linux kernel works with
>
> ] their email-based workflows
>
> Step 1: reply to the proposal in email instead of (or perhaps in
>
> addition to) making blog posts. :)
>
> >
> > On a related technical note I
> >
> > worry what will become of /proc/slabinfo when we convert non-trivial amounts
> >
> > of users.
> >
>
> It gets longer. :) And potentially makes the codetag /proc file
>
> redundant. All that said, there are very few APIs in the kernel where
>
> userspace can control both the size and contents of an allocation.
>
> >
> > Also would interested to hear Jann Horn et al.'s opinion, and whether the
> >
> > SLAB_VIRTUAL effort will continue?
> >
>
> SLAB_VIRTUAL is needed to address the reclamation UAF gap, and is
>
> still being developed. I don't intend to let it fall off the radar.
>
> (Which is why I included Jann and Matteo in CC originally.)
>
> In the meantime, adding this series as-is kills two long-standing
>
> exploitation methodologies, and paves the way to providing very
>
> fine-grained caches using codetags (which I imagine would be entirely
>
> optional and trivial to control with a boot param).
>
> -Kees
>
> [1] https://lore.kernel.org/lkml/202402211449.401382D2AF@keescook/
>
> [2] https://googleprojectzero.blogspot.com/2021/10/how-simple-linux-kernel-memory.html
>
> --
>
> Kees Cook
>
On Tue, Mar 26, 2024 at 06:07:07PM +0000, [email protected] wrote:
> 25 March 2024 at 19:24, "Kees Cook" <[email protected]> wrote:
> > On Mon, Mar 25, 2024 at 10:03:23AM +0100, Vlastimil Babka wrote:
> > > On 3/5/24 11:10 AM, Kees Cook wrote:
> > > Hi,
> > >
> > > Repeating the commit logs for patch 4 here:
> > >
> > > Dedicated caches are available For fixed size allocations via
> > > kmem_cache_alloc(), but for dynamically sized allocations there is only
> > > the global kmalloc API's set of buckets available. This means it isn't
> > > possible to separate specific sets of dynamically sized allocations into
> > > a separate collection of caches.
> > >
> > > This leads to a use-after-free exploitation weakness in the Linux
> > > kernel since many heap memory spraying/grooming attacks depend on using
> > > userspace-controllable dynamically sized allocations to collide with
> > > fixed size allocations that end up in same cache.
> > >
> > > While CONFIG_RANDOM_KMALLOC_CACHES provides a probabilistic defense
> > > against these kinds of "type confusion" attacks, including for fixed
> > > same-size heap objects, we can create a complementary deterministic
> > > defense for dynamically sized allocations.
> > >
> > > In order to isolate user-controllable sized allocations from system
> > > allocations, introduce kmem_buckets_create(), which behaves like
> > > kmem_cache_create(). (The next patch will introduce kmem_buckets_alloc(),
> > > which behaves like kmem_cache_alloc().)
> > >
> > > Allows for confining allocations to a dedicated set of sized caches
> > > (which have the same layout as the kmalloc caches).
> > >
> > > This can also be used in the future once codetag allocation annotations
> > > exist to implement per-caller allocation cache isolation[0] even for
> > > dynamic allocations.
> > >
> > > Link: https://lore.kernel.org/lkml/202402211449.401382D2AF@keescook [0]
> > >
> > > After the implemetation are 2 example patches of how this could be used
> > > for some repeat "offenders" that get used in exploits. There are more to
> > > be isolated beyond just these. Repeating the commit log for patch 8 here:
> > >
> > > The msg subsystem is a common target for exploiting[1][2][3][4][5][6]
> > > use-after-free type confusion flaws in the kernel for both read and
> > > write primitives. Avoid having a user-controlled size cache share the
> > > global kmalloc allocator by using a separate set of kmalloc buckets.
> > >
> > > Link: https://blog.hacktivesecurity.com/index.php/2022/06/13/linux-kernel-exploit-development-1day-case-study/ [1]
> > > Link: https://hardenedvault.net/blog/2022-11-13-msg_msg-recon-mitigation-ved/ [2]
> > > Link: https://www.willsroot.io/2021/08/corctf-2021-fire-of-salvation-writeup.html [3]
> > > Link: https://a13xp0p0v.github.io/2021/02/09/CVE-2021-26708.html [4]
> > > Link: https://google.github.io/security-research/pocs/linux/cve-2021-22555/writeup.html [5]
> > > Link: https://zplin.me/papers/ELOISE.pdf [6]
> > >
> > > Hi Kees,
> > >
> > > after reading [1] I think the points should be addressed, mainly about the
> > > feasibility of converting users manually.
> >
> > Sure, I can do that.
> > Adding Julien to this thread... Julien can you please respond to LKML
> > patches in email? It's much easier to keep things in a single thread. :)
> >
> > ] This is playing wack-a-mole
> > Kind of, but not really. These patches provide a mechanism for having
> > dedicated dynamically-sized slab caches (to match kmem_cache_create(),
> > which only works for fixed-size allocations). This is needed to expand
> > the codetag work into doing per-call-site allocations, as I detailed
> > here[1].
> >
> > Also, adding uses manually isn't very difficult, as can be seen in the
> > examples I included. In fact, my examples between v1 and v2 collapsed
> > from 3 to 2, because covering memdup_user() actually covered 2 known
> > allocation paths (attrs and vma names), and given its usage pattern,
> > will cover more in the future without changes.
>
> It's not about difficulty, it's about scale. There are hundreds of interesting structures: I'm worried that no one will take the time to add a separate bucket for each of them, chase their call-sites down, and monitor every single newly added structures to check if they are "interesting" and should benefit from their own bucket as well.
Very few are both: 1) dynamically sized, and 2) coming from userspace,
so I think the scale is fine.
> > ] something like AUTOSLAB would be better
> > Yes, that's the goal of [1]. This is a prerequisite for that, as
> > mentioned in the cover letter.
>
> This series looks unrelated to [1] to me: the former adds a mechanism to add buckets and expects developers to manually make use of them, while the latter is about adding infrastructure to automate call-site-based segregation.
Right -- but for call-site-based separation, there is currently no way
to separate _dynamically_ sized allocations; only fixed size (via
kmem_cache_create()). This series adds the ability for call-site-based
separation to also use kmem_bucket_create(). Call-site-based
separation isn't possible without this series.
>
> > ] The slabs needs to be pinned
> > Yes, and this is a general problem[2] with all kmalloc allocations, though.
> > This isn't unique to to this patch series. SLAB_VIRTUAL solves it, and
> > is under development.
>
> Then it would be nice to mention it in the serie, as an acknowledged limitation.
Sure, I can update the cover letter.
>
> > ] Lacks guard pages
> > Yes, and again, this is a general problem with all kmalloc allocations.
> > Solving it, like SLAB_VIRTUAL, would be a complementary hardening
> > improvement to the allocator generally.
>
> Then it would also be nice to mention it, because currently it's unclear that those limitations are both known and will be properly addressed.
Sure. For both this and pinning, the issues are orthogonal, so it didn't
seem useful to distract from what the series was doing, but I can
explicitly mention them going forward.
>
> > ] PAX_USERCOPY has been marking these sites since 2012
> > Either it's whack-a-mole or it's not. :)
>
> This annotation was added 12 years ago in PaX, and while it was state of the art back then, I think that in 2024 we can do better than this.
Agreed. Here's my series to start that. :)
> > PAX_USERCOPY shows that it _is_ possible to mark all sites.
>
> It shows that it's possible to annotate some sites (17 in grsecurity-3.1-4.9.9-201702122044.patch), and while it has a similar approach to your series, its annotations aren't conveying the same meaning.
Sure, GFP_USERCOPY is separate.
> > Regardless, like AUTOSLAB, PAX_USERCOPY isn't
> > upstream, and its current implementation is an unpublished modification
> > to a GPL project. I look forward to someone proposing it for inclusion
> > in Linux, but for now we can work with the patches where an effort _has_
> > been made to upstream them for the benefit of the entire ecosystem.
> > ] What about CONFIG_KMALLOC_SPLIT_VARSIZE
> > This proposed improvement is hampered by not having dedicated
> > _dynamically_ sized kmem caches, which this series provides. And with
> > codetag-split allocations[1], the goals of CONFIG_KMALLOC_SPLIT_VARSIZE
> > are more fully realized, providing much more complete coverage.
>
> CONFIG_KMALLOC_SPLIT_VARSIZE has been bypassed dozen of times in various ways as part of Google's kernelCTF.
> Your series is, to my understanding, a weaker form of it. So I'm not super-convinced that it's the right approach to mitigate UAF.
This series doesn't do anything that CONFIG_KMALLOC_SPLIT_VARSIZE does.
The call-site-separation series (which would depend on this series)
would do that work.
> Do you think it would be possible for Google to add this series to its kernelCTF, so gather empirical data on how feasible/easy it is to bypass it?
Sure, feel free to make that happen. :) But again, I'm less interested
in this series as a _standalone_ solution. It's a prerequisite for
call-site-based allocation separation. As part of it, though, we can
plug the blatant exploitation methods that currently exist.
-Kees
--
Kees Cook