[PATCH] 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>
---
 mm/slab.c | 4 ++++
 mm/slub.c | 4 ++++
 2 files changed, 8 insertions(+)

diff --git a/mm/slab.c b/mm/slab.c
index 9350062ffc1a..4c7013eeacd9 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -3426,6 +3426,10 @@ 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. */
+	__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..57db6ca2e0dc 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1470,6 +1470,10 @@ 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. */
+	__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_);
 }
-- 
2.27.0.290.gba653c62da-goog