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

[Marco Elver]

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 <elver@xxxxxxxxxx>
---
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(+)

diff --git a/mm/slab.c b/mm/slab.c
index 9350062ffc1a..cba71d88e89c 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -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);
 }
 
diff --git a/mm/slub.c b/mm/slub.c
index b8f798b50d44..4a9d43fda669 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -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_);
 }
-- 
2.27.0.111.gc72c7da667-goog