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Subjectarm64/v4.16-rc1: KASAN: use-after-free Read in finish_task_switch

As a heads-up, I hit the splat below when fuzzing v4.16-rc1 on arm64.

Evidently, we get to finish_task_switch() with rq->prev_mm != NULL,
despite rq->prev_mm having been freed. KASAN spots the dereference of
mm->membarrier_state in membarrier_mm_sync_core_before_usermode(mm), but
AFAICT the underlying issue is independent of the membarrier code, and
we could get a splat on the subsequent mmdrop(mm).

I've seen this once in ~2500 CPU hours of fuzzing, so it looks pretty
difficult to hit, and I have no reproducer so far.

Syzkaller report below, mirrored with Syzkaller log at [1]. If I hit
this again, I'll upload new info there.



BUG: KASAN: use-after-free in __read_once_size include/linux/compiler.h:183 [inline]
BUG: KASAN: use-after-free in membarrier_mm_sync_core_before_usermode include/linux/sched/mm.h:216 [inline]
BUG: KASAN: use-after-free in finish_task_switch+0x590/0x5e8 kernel/sched/core.c:2720
Read of size 4 at addr ffff800073dd4980 by task swapper/1/0

CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.16.0-rc1-00001-g3fffa6166965-dirty #13
Hardware name: linux,dummy-virt (DT)
Call trace:
dump_backtrace+0x0/0x388 arch/arm64/kernel/time.c:52
show_stack+0x20/0x30 arch/arm64/kernel/traps.c:151
__dump_stack lib/dump_stack.c:17 [inline]
dump_stack+0xd0/0x120 lib/dump_stack.c:53
print_address_description+0x5c/0x2a8 mm/kasan/report.c:262
kasan_report_error mm/kasan/report.c:362 [inline]
kasan_report+0x210/0x340 mm/kasan/report.c:420
__asan_report_load4_noabort+0x18/0x20 mm/kasan/report.c:440
__read_once_size include/linux/compiler.h:183 [inline]
membarrier_mm_sync_core_before_usermode include/linux/sched/mm.h:216 [inline]
finish_task_switch+0x590/0x5e8 kernel/sched/core.c:2720
context_switch kernel/sched/core.c:2860 [inline]
__schedule+0x4f0/0x1558 kernel/sched/core.c:3435
schedule_idle+0x40/0x80 kernel/sched/core.c:3521
do_idle+0x114/0x2e0 kernel/sched/idle.c:269
cpu_startup_entry+0x24/0x28 kernel/sched/idle.c:351
secondary_start_kernel+0x290/0x318 arch/arm64/kernel/smp.c:283

Allocated by task 12964:
save_stack mm/kasan/kasan.c:447 [inline]
set_track mm/kasan/kasan.c:459 [inline]
kasan_kmalloc+0xd0/0x180 mm/kasan/kasan.c:552
kasan_slab_alloc+0x14/0x20 mm/kasan/kasan.c:489
slab_post_alloc_hook mm/slab.h:443 [inline]
slab_alloc_node mm/slub.c:2725 [inline]
slab_alloc mm/slub.c:2733 [inline]
kmem_cache_alloc+0x150/0x240 mm/slub.c:2738
dup_mm kernel/fork.c:1235 [inline]
copy_mm kernel/fork.c:1298 [inline]
copy_process.isra.7.part.8+0x1a10/0x5018 kernel/fork.c:1804
copy_process kernel/fork.c:1617 [inline]
_do_fork+0x178/0x998 kernel/fork.c:2098
SYSC_clone kernel/fork.c:2205 [inline]
SyS_clone+0x48/0x60 kernel/fork.c:2183

Freed by task 10882:
save_stack mm/kasan/kasan.c:447 [inline]
set_track mm/kasan/kasan.c:459 [inline]
__kasan_slab_free+0x114/0x220 mm/kasan/kasan.c:520
kasan_slab_free+0x10/0x18 mm/kasan/kasan.c:527
slab_free_hook mm/slub.c:1393 [inline]
slab_free_freelist_hook mm/slub.c:1414 [inline]
slab_free mm/slub.c:2968 [inline]
kmem_cache_free+0x88/0x270 mm/slub.c:2990
__mmdrop+0x164/0x248 kernel/fork.c:604
mmdrop+0x50/0x60 kernel/fork.c:615
__mmput kernel/fork.c:981 [inline]
mmput+0x270/0x338 kernel/fork.c:992
exit_mm kernel/exit.c:544 [inline]
do_exit+0x640/0x2100 kernel/exit.c:852
do_group_exit+0xdc/0x260 kernel/exit.c:968
get_signal+0x2ec/0xfc8 kernel/signal.c:2469
do_signal+0x270/0x4f8 arch/arm64/kernel/signal.c:869
do_notify_resume+0x150/0x1c0 arch/arm64/kernel/signal.c:927

The buggy address belongs to the object at ffff800073dd4600
which belongs to the cache mm_struct of size 1104
The buggy address is located 896 bytes inside of
1104-byte region [ffff800073dd4600, ffff800073dd4a50)
The buggy address belongs to the page:
page:ffff7e0001cf7400 count:1 mapcount:0 mapping: (null) index:0x0 compound_mapcount: 0
flags: 0x4fffc00000008100(slab|head)
raw: 4fffc00000008100 0000000000000000 0000000000000000 0000000180190019
raw: ffff7e0001d2b000 0000000700000007 ffff800075803000 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff800073dd4880: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff800073dd4900: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff800073dd4980: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff800073dd4a00: fb fb fb fb fb fb fb fb fb fb fc fc fc fc fc fc
ffff800073dd4a80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc

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