[PATCH RFC 0/2] Break heap spraying needed for exploiting use-after-free

[Alexander Popov]

Hello everyone! Requesting for your comments.

Use-after-free vulnerabilities in the Linux kernel are very popular for
exploitation. A few examples:
 https://googleprojectzero.blogspot.com/2018/09/a-cache-invalidation-bug-in-linux.html
 https://googleprojectzero.blogspot.com/2019/11/bad-binder-android-in-wild-exploit.html?m=1
 https://a13xp0p0v.github.io/2020/02/15/CVE-2019-18683.html

Use-after-free exploits usually employ heap spraying technique.
Generally it aims to put controlled bytes at a predetermined memory
location on the heap. Heap spraying for exploiting use-after-free in
the Linux kernel relies on the fact that on kmalloc(), the slab allocator
returns the address of the memory that was recently freed. So allocating
a kernel object with the same size and controlled contents allows
overwriting the vulnerable freed object.

I've found an easy way to break heap spraying for use-after-free
exploitation. I simply extracted slab freelist quarantine from KASAN
functionality and called it CONFIG_SLAB_QUARANTINE. Please see patch 1.

If this feature is enabled, freed allocations are stored in the quarantine
and can't be instantly reallocated and overwritten by the exploit
performing heap spraying.

In patch 2 you can see the lkdtm test showing how CONFIG_SLAB_QUARANTINE
prevents immediate reallocation of a freed heap object.

I tested this patch series both for CONFIG_SLUB and CONFIG_SLAB.

CONFIG_SLAB_QUARANTINE disabled:
  # echo HEAP_SPRAY > /sys/kernel/debug/provoke-crash/DIRECT
  lkdtm: Performing direct entry HEAP_SPRAY
  lkdtm: Performing heap spraying...
  lkdtm: attempt 0: spray alloc addr 00000000f8699c7d vs freed addr 00000000f8699c7d
  lkdtm: freed addr is reallocated!
  lkdtm: FAIL! Heap spraying succeed :(

CONFIG_SLAB_QUARANTINE enabled:
  # echo HEAP_SPRAY > /sys/kernel/debug/provoke-crash/DIRECT
  lkdtm: Performing direct entry HEAP_SPRAY
  lkdtm: Performing heap spraying...
  lkdtm: attempt 0: spray alloc addr 000000009cafb63f vs freed addr 00000000173cce94
  lkdtm: attempt 1: spray alloc addr 000000003096911f vs freed addr 00000000173cce94
  lkdtm: attempt 2: spray alloc addr 00000000da60d755 vs freed addr 00000000173cce94
  lkdtm: attempt 3: spray alloc addr 000000000b415070 vs freed addr 00000000173cce94
  ...
  lkdtm: attempt 126: spray alloc addr 00000000e80ef807 vs freed addr 00000000173cce94
  lkdtm: attempt 127: spray alloc addr 00000000398fe535 vs freed addr 00000000173cce94
  lkdtm: OK! Heap spraying hasn't succeed :)

I did a brief performance evaluation of this feature.

1. Memory consumption. KASAN quarantine uses 1/32 of the memory.
CONFIG_SLAB_QUARANTINE disabled:
  # free -m
                total        used        free      shared  buff/cache   available
  Mem:           1987          39        1862          10          86        1907
  Swap:             0           0           0
CONFIG_SLAB_QUARANTINE enabled:
  # free -m
                total        used        free      shared  buff/cache   available
  Mem:           1987         140        1760          10          87        1805
  Swap:             0           0           0

2. Performance penalty. I used `hackbench -s 256 -l 200 -g 15 -f 25 -P`.
CONFIG_SLAB_QUARANTINE disabled (x86_64, CONFIG_SLUB):
  Times: 3.088, 3.103, 3.068, 3.103, 3.107
  Mean: 3.0938
  Standard deviation: 0.0144
CONFIG_SLAB_QUARANTINE enabled (x86_64, CONFIG_SLUB):
  Times: 3.303, 3.329, 3.356, 3.314, 3.292
  Mean: 3.3188 (+7.3%)
  Standard deviation: 0.0223

I would appreciate your feedback!

Best regards,
Alexander

Alexander Popov (2):
  mm: Extract SLAB_QUARANTINE from KASAN
  lkdtm: Add heap spraying test

 drivers/misc/lkdtm/core.c  |   1 +
 drivers/misc/lkdtm/heap.c  |  40 ++++++++++++++
 drivers/misc/lkdtm/lkdtm.h |   1 +
 include/linux/kasan.h      | 107 ++++++++++++++++++++-----------------
 include/linux/slab_def.h   |   2 +-
 include/linux/slub_def.h   |   2 +-
 init/Kconfig               |  11 ++++
 mm/Makefile                |   3 +-
 mm/kasan/Makefile          |   2 +
 mm/kasan/kasan.h           |  75 +++++++++++++-------------
 mm/kasan/quarantine.c      |   2 +
 mm/kasan/slab_quarantine.c |  99 ++++++++++++++++++++++++++++++++++
 mm/slub.c                  |   2 +-
 13 files changed, 258 insertions(+), 89 deletions(-)
 create mode 100644 mm/kasan/slab_quarantine.c

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2.26.2