[PATCH v3 0/6] slab: Introduce dedicated bucket allocator

Kees Cook
2024-04-24T21:41:07Z

Hi,

Series change history:

v3:
- clarify rationale and purpose in commit log
- rebase to -next (CONFIG_CODE_TAGGING)
- simplify calling styles and split out bucket plumbing more cleanly
- consolidate kmem_buckets_*() family introduction patches
v2: https://lore.kernel.org/lkml/20240305100933.it.923-kees@kernel.org/
v1: https://lore.kernel.org/lkml/20240304184252.work.496-kees@kernel.org/

For the cover letter, I'm repeating commit log for patch 4 here, which has
additional clarifications and rationale since v2:

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 that are directly user
controlled. Addressing these cases is limited in scope, so isolation these
kinds of interfaces will not become an unbounded game of whack-a-mole. For
example, pass through memdup_user(), making isolation there very
effective.

In order to isolate user-controllable sized allocations from system
allocations, introduce kmem_buckets_create(), which behaves like
kmem_cache_create(). Introduce kmem_buckets_alloc(), which behaves like
kmem_cache_alloc(). Introduce kmem_buckets_alloc_track_caller() for
where caller tracking is needed. Introduce kmem_buckets_valloc() for
cases where vmalloc callback is needed.

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 to extend codetag allocation
annotations to implement per-caller allocation cache isolation[1] even
for dynamic allocations.

Memory allocation pinning[2] is still needed to plug the Use-After-Free
cross-allocator weakness, but that is an existing and separate issue
which is complementary to this improvement. Development continues for
that feature via the SLAB_VIRTUAL[3] series (which could also provide
guard pages -- another complementary improvement).

Link: https://lore.kernel.org/lkml/202402211449.401382D2AF@keescook [1]
Link: https://googleprojectzero.blogspot.com/2021/10/how-simple-linux-kernel-memory.html [2]
Link: https://lore.kernel.org/lkml/20230915105933.495735-1-matteorizzo@google.com/ [3]

After the core implementation are 2 patches that cover the most heavily
abused "repeat offenders" used in exploits. Repeating those details 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]
Link: https://syst3mfailure.io/wall-of-perdition/ [7]

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.

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]

Thanks!

-Kees

Kees Cook (6):
mm/slab: Introduce kmem_buckets typedef
mm/slab: Plumb kmem_buckets into __do_kmalloc_node()
mm/slab: Introduce __kvmalloc_node() that can take kmem_buckets
argument
mm/slab: Introduce kmem_buckets_create() and family
ipc, msg: Use dedicated slab buckets for alloc_msg()
mm/util: Use dedicated slab buckets for memdup_user()

include/linux/slab.h | 44 ++++++++++++++++--------
ipc/msgutil.c | 13 +++++++-
lib/fortify_kunit.c | 2 +-
lib/rhashtable.c | 2 +-
mm/slab.h | 6 ++--
mm/slab_common.c | 79 +++++++++++++++++++++++++++++++++++++++++---
mm/slub.c | 14 ++++----
mm/util.c | 21 +++++++++---
8 files changed, 146 insertions(+), 35 deletions(-)

--
2.34.1