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From:   Qian Cai <cai@lca.pw>
To:     akpm@linux-foundation.org
Cc:     elver@google.com, catalin.marinas@arm.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Qian Cai <cai@lca.pw>
Subject: [PATCH -next] mm/kmemleak: annotate a data race in checksum
Date:   Tue, 17 Mar 2020 09:21:57 -0400
Message-Id: <20200317132157.1272-1-cai@lca.pw>
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Sender: linux-kernel-owner@vger.kernel.org
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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();
-	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));
 	kasan_enable_current();
 
 	return object->checksum != old_csum;
-- 
2.21.0 (Apple Git-122.2)