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

[Alexander Popov <alex.popov@linux.com>]

On 13.08.2020 18:19, Alexander Popov wrote:
> 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!

While waiting for the feedback on these RFC patches, I compiled a list of topics
for further research:

 - Possible ways to overwrite a quarantined heap object by making a large amount
of allocations (with/without freeing them)

 - How init_on_free=1 affects heap spraying on a system with the heap quarantine

 - How releasing batches of quarantine objects right away affects heap spraying
reliability

 - Heap spraying on multi-core systems with the heap quarantine

 - More precise performance evaluation

 - Possible ways to improve the security properties and performance results
(KASAN quarantine has some interesting settings)

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
>