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SubjectRe: [PATCH -next] mm/kmemleak: annotate a data race in checksum
On Tue, 17 Mar 2020 at 14:22, Qian Cai <cai@lca.pw> wrote:
>
> Even if KCSAN is disabled for kmemleak, update_checksum() could still
> call crc32() (which is outside of kmemleak.c) to dereference
> object->pointer. Thus, the value of object->pointer could be accessed
> concurrently as noticed by KCSAN,
>
> BUG: KCSAN: data-race in crc32_le_base / do_raw_spin_lock
>
> write to 0xffffb0ea683a7d50 of 4 bytes by task 23575 on cpu 12:
> do_raw_spin_lock+0x114/0x200
> debug_spin_lock_after at kernel/locking/spinlock_debug.c:91
> (inlined by) do_raw_spin_lock at kernel/locking/spinlock_debug.c:115
> _raw_spin_lock+0x40/0x50
> __handle_mm_fault+0xa9e/0xd00
> handle_mm_fault+0xfc/0x2f0
> do_page_fault+0x263/0x6f9
> page_fault+0x34/0x40
>
> read to 0xffffb0ea683a7d50 of 4 bytes by task 839 on cpu 60:
> crc32_le_base+0x67/0x350
> crc32_le_base+0x67/0x350:
> crc32_body at lib/crc32.c:106
> (inlined by) crc32_le_generic at lib/crc32.c:179
> (inlined by) crc32_le at lib/crc32.c:197
> kmemleak_scan+0x528/0xd90
> update_checksum at mm/kmemleak.c:1172
> (inlined by) kmemleak_scan at mm/kmemleak.c:1497
> kmemleak_scan_thread+0xcc/0xfa
> kthread+0x1e0/0x200
> ret_from_fork+0x27/0x50
>
> If a shattered value was returned due to a data race, it will be
> corrected in the next scan. Thus, annotate it as an intentional data
> race using the data_race() macro.
>
> Signed-off-by: Qian Cai <cai@lca.pw>
> ---
> mm/kmemleak.c | 7 ++++++-
> 1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/mm/kmemleak.c b/mm/kmemleak.c
> index e362dc3d2028..d3327756c3a4 100644
> --- a/mm/kmemleak.c
> +++ b/mm/kmemleak.c
> @@ -1169,7 +1169,12 @@ static bool update_checksum(struct kmemleak_object *object)
> u32 old_csum = object->checksum;
>
> kasan_disable_current();

Suggested:
+ kcsan_disable_current();

> - object->checksum = crc32(0, (void *)object->pointer, object->size);
> + /*
> + * crc32() will dereference object->pointer. If an unstable value was
> + * returned due to a data race, it will be corrected in the next scan.
> + */
> + object->checksum = data_race(crc32(0, (void *)object->pointer,
> + object->size));

This will work with the default config, because for word-sized-aligned
writes no marking is enforced. But this will still cause a data race
if the write is e.g. due to a memcpy.

There are already markers for KASAN around, so the most reliable thing
is to just disable KCSAN in this region.

> kasan_enable_current();

Suggested:
+ kcsan_enable_current();

Thanks,
-- Marco

> return object->checksum != old_csum;
> --
> 2.21.0 (Apple Git-122.2)
>

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