[v6,0/2] allow KASAN to detect UAF in SLAB_TYPESAFE_BY_RCU slabs

Message

Jann Horn Aug. 2, 2024, 8:31 p.m. UTC

Hi!

The purpose of the series is to allow KASAN to detect use-after-free
access in SLAB_TYPESAFE_BY_RCU slab caches, by essentially making them
behave as if the cache was not SLAB_TYPESAFE_BY_RCU but instead every
kfree() in the cache was a kfree_rcu().
This is gated behind a config flag that is supposed to only be enabled
in fuzzing/testing builds where the performance impact doesn't matter.

Output of the new kunit testcase I added to the KASAN test suite:
==================================================================
BUG: KASAN: slab-use-after-free in kmem_cache_rcu_uaf+0x3ae/0x4d0
Read of size 1 at addr ffff888106224000 by task kunit_try_catch/224

CPU: 7 PID: 224 Comm: kunit_try_catch Tainted: G    B            N 6.10.0-00003-g065427d4b87f #430
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
Call Trace:
 <TASK>
 dump_stack_lvl+0x53/0x70
 print_report+0xce/0x670
[...]
 kasan_report+0xa5/0xe0
[...]
 kmem_cache_rcu_uaf+0x3ae/0x4d0
[...]
 kunit_try_run_case+0x1b3/0x490
[...]
 kunit_generic_run_threadfn_adapter+0x80/0xe0
 kthread+0x2a5/0x370
[...]
 ret_from_fork+0x34/0x70
[...]
 ret_from_fork_asm+0x1a/0x30
 </TASK>

Allocated by task 224:
 kasan_save_stack+0x33/0x60
 kasan_save_track+0x14/0x30
 __kasan_slab_alloc+0x6e/0x70
 kmem_cache_alloc_noprof+0xef/0x2b0
 kmem_cache_rcu_uaf+0x10d/0x4d0
 kunit_try_run_case+0x1b3/0x490
 kunit_generic_run_threadfn_adapter+0x80/0xe0
 kthread+0x2a5/0x370
 ret_from_fork+0x34/0x70
 ret_from_fork_asm+0x1a/0x30

Freed by task 0:
 kasan_save_stack+0x33/0x60
 kasan_save_track+0x14/0x30
 kasan_save_free_info+0x3b/0x60
 __kasan_slab_free+0x57/0x80
 slab_free_after_rcu_debug+0xe3/0x220
 rcu_core+0x676/0x15b0
 handle_softirqs+0x22f/0x690
 irq_exit_rcu+0x84/0xb0
 sysvec_apic_timer_interrupt+0x6a/0x80
 asm_sysvec_apic_timer_interrupt+0x1a/0x20

Last potentially related work creation:
 kasan_save_stack+0x33/0x60
 __kasan_record_aux_stack+0x8e/0xa0
 kmem_cache_free+0x10c/0x420
 kmem_cache_rcu_uaf+0x16e/0x4d0
 kunit_try_run_case+0x1b3/0x490
 kunit_generic_run_threadfn_adapter+0x80/0xe0
 kthread+0x2a5/0x370
 ret_from_fork+0x34/0x70
 ret_from_fork_asm+0x1a/0x30

The buggy address belongs to the object at ffff888106224000
 which belongs to the cache test_cache of size 200
The buggy address is located 0 bytes inside of
 freed 200-byte region [ffff888106224000, ffff8881062240c8)

The buggy address belongs to the physical page:
page: refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x106224
head: order:1 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
flags: 0x200000000000040(head|node=0|zone=2)
page_type: 0xffffefff(slab)
raw: 0200000000000040 ffff88810621c140 dead000000000122 0000000000000000
raw: 0000000000000000 00000000801f001f 00000001ffffefff 0000000000000000
head: 0200000000000040 ffff88810621c140 dead000000000122 0000000000000000
head: 0000000000000000 00000000801f001f 00000001ffffefff 0000000000000000
head: 0200000000000001 ffffea0004188901 ffffffffffffffff 0000000000000000
head: 0000000000000002 0000000000000000 00000000ffffffff 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff888106223f00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff888106223f80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff888106224000: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                   ^
 ffff888106224080: fb fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc
 ffff888106224100: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
==================================================================
    ok 38 kmem_cache_rcu_uaf

Signed-off-by: Jann Horn <[e-mail]>
---
Changes in v6:
- in patch 1/2:
  - fix commit message (Andrey)
  - change comments (Andrey)
  - fix mempool handling of kfence objects (Andrey)
- in patch 2/2:
  - fix is_kfence_address argument (syzbot and Marco)
  - refactor slab_free_hook() to create "still_accessible" variable
  - change kasan_slab_free() hook argument to "still_accessible"
  - add documentation to kasan_slab_free() hook
- Link to v5: https://lore.kernel.org/r/[e-mail]

Changes in v5:
- rebase to latest origin/master (akpm), no other changes from v4
- Link to v4: https://lore.kernel.org/r/[e-mail]

Changes in v4:
- note I kept vbabka's ack for the SLUB changes in patch 1/2 since the
  SLUB part didn't change, even though I refactored a bunch of the
  KASAN parts
- in patch 1/2 (major rework):
  - fix commit message (Andrey)
  - add doc comments in header (Andrey)
  - remove "ip" argument from __kasan_slab_free()
  - rework the whole check_slab_free() thing and move code around (Andrey)
- in patch 2/2:
  - kconfig description and dependency changes (Andrey)
  - remove useless linebreak (Andrey)
  - fix comment style (Andrey)
  - fix do_slab_free() invocation (kernel test robot)
- Link to v3: https://lore.kernel.org/r/[e-mail]

Changes in v3:
- in patch 1/2, integrate akpm's fix for !CONFIG_KASAN build failure
- in patch 2/2, as suggested by vbabka, use dynamically allocated
  rcu_head to avoid having to add slab metadata
- in patch 2/2, add a warning in the kconfig help text that objects can
  be recycled immediately under memory pressure
- Link to v2: https://lore.kernel.org/r/[e-mail]

Changes in v2:
Patch 1/2 is new; it's some necessary prep work for the main patch to
work, though the KASAN integration maybe is a bit ugly.
Patch 2/2 is a rebased version of the old patch, with some changes to
how the config is wired up, with poison/unpoison logic added as
suggested by dvyukov@ back then, with cache destruction fixed using
rcu_barrier() as pointed out by dvyukov@ and the test robot, and a test
added as suggested by elver@.

---
Jann Horn (2):
      kasan: catch invalid free before SLUB reinitializes the object
      slub: Introduce CONFIG_SLUB_RCU_DEBUG

 include/linux/kasan.h | 63 ++++++++++++++++++++++++++++++++++---
 mm/Kconfig.debug      | 30 ++++++++++++++++++
 mm/kasan/common.c     | 62 ++++++++++++++++++++++---------------
 mm/kasan/kasan_test.c | 46 +++++++++++++++++++++++++++
 mm/slab_common.c      | 12 +++++++
 mm/slub.c             | 86 ++++++++++++++++++++++++++++++++++++++++++++++-----
 6 files changed, 261 insertions(+), 38 deletions(-)
---
base-commit: 94ede2a3e9135764736221c080ac7c0ad993dc2d
change-id: 20240723-kasan-tsbrcu-b715a901f776