[0/4] slab: Introduce dedicated bucket allocator

Message

Kees Cook March 4, 2024, 6:49 p.m. UTC

Hi,

Repeating the commit logs for patch 1 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() and kmem_buckets_alloc(),
    which behave like kmem_cache_create() and like kmem_cache_alloc() 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 3 example patch 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 2 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.

    After a fresh boot under Ubuntu 23.10, we can see the caches are already
    in use:

     # grep ^msg_msg /proc/slabinfo
     msg_msg-8k             0      0   8192    4    8 : ...
     msg_msg-4k            96    128   4096    8    8 : ...
     msg_msg-2k            64     64   2048   16    8 : ...
     msg_msg-1k            64     64   1024   16    4 : ...
     msg_msg-16          1024   1024     16  256    1 : ...
     msg_msg-8              0      0      8  512    1 : ...

    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

Kees Cook (4):
  slab: Introduce dedicated bucket allocator
  ipc, msg: Use dedicated slab buckets for alloc_msg()
  xattr: Use dedicated slab buckets for setxattr()
  mm/util: Use dedicated slab buckets for memdup_user()

 fs/xattr.c           | 12 ++++++++-
 include/linux/slab.h | 26 ++++++++++++++++++
 ipc/msgutil.c        | 11 +++++++-
 mm/slab_common.c     | 64 ++++++++++++++++++++++++++++++++++++++++++++
 mm/util.c            | 12 ++++++++-
 5 files changed, 122 insertions(+), 3 deletions(-)