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mm, kcsan: Instrument SLAB/SLUB free with "ASSERT_EXCLUSIVE_ACCESS"

PATCH Applied Marco Elver 1 participant 1 message Jun 23, 2020

Subsystems (2)
Memory management Slab allocator
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Marco Elver

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PATCH v2 1/1 mm, kcsan: Instrument SLAB/SLUB free with "ASSERT_EXCLUSIVE_ACCESS"

Provide the necessary KCSAN checks to assist with debugging racy use-after-frees. While KASAN is more reliable at generally catching such use-after-frees (due to its use of a quarantine), it can be difficult to debug racy use-after-frees. If a reliable reproducer exists, KCSAN can assist in debugging such issues.

Note: ASSERT_EXCLUSIVE_ACCESS is a convenience wrapper if the size is simply sizeof(var). Instead, here we just use __kcsan_check_access() explicitly to pass the correct size.

Signed-off-by Marco Elver <[email protected]>

---
v2:
* SLAB_TYPESAFE_BY_RCU allows racy use after free within RCU grace period. If slab is SLAB_TYPESAFE_BY_RCU do not check access.

---
 mm/slab.c | 5 +++++
 mm/slub.c | 5 +++++
 2 files changed, 10 insertions(+)
mm/slab.c +5 -0
index 9350062ffc1a..cba71d88e89c 100644
@@ -3426,6 +3426,11 @@ static __always_inline void __cache_free(struct kmem_cache *cachep, void *objp,
 	if (kasan_slab_free(cachep, objp, _RET_IP_))
 		return;
 
+	/* Use KCSAN to help debug racy use-after-free. */
+	if (!(cachep->flags & SLAB_TYPESAFE_BY_RCU))
+		__kcsan_check_access(objp, cachep->object_size,
+				     KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT);
+
 	___cache_free(cachep, objp, caller);
 }
mm/slub.c +5 -0
index b8f798b50d44..4a9d43fda669 100644
@@ -1470,6 +1470,11 @@ static __always_inline bool slab_free_hook(struct kmem_cache *s, void *x)
 	if (!(s->flags & SLAB_DEBUG_OBJECTS))
 		debug_check_no_obj_freed(x, s->object_size);
 
+	/* Use KCSAN to help debug racy use-after-free. */
+	if (!(s->flags & SLAB_TYPESAFE_BY_RCU))
+		__kcsan_check_access(x, s->object_size,
+				     KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT);
+
 	/* KASAN might put x into memory quarantine, delaying its reuse */
 	return kasan_slab_free(s, x, _RET_IP_);
 }
Signature · 1 line
2.27.0.111.gc72c7da667-goog