[patch 039/163] mm, kcsan: instrument SLAB/SLUB free with "ASSERT_EXCLUSIVE_ACCESS"

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From: Marco Elver <elver@xxxxxxxxxx>
Subject: 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.

Link: http://lkml.kernel.org/r/20200623072653.114563-1-elver@xxxxxxxxxx
Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
Cc: Alexander Potapenko <glider@xxxxxxxxxx>
Cc: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: Pekka Enberg <penberg@xxxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 mm/slab.c |    5 +++++
 mm/slub.c |    5 +++++
 2 files changed, 10 insertions(+)

--- a/mm/slab.c~mm-kcsan-instrument-slab-slub-free-with-assert_exclusive_access
+++ a/mm/slab.c
@@ -3432,6 +3432,11 @@ static __always_inline void __cache_free
 	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);
 }
 
--- a/mm/slub.c~mm-kcsan-instrument-slab-slub-free-with-assert_exclusive_access
+++ a/mm/slub.c
@@ -1549,6 +1549,11 @@ static __always_inline bool slab_free_ho
 	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_);
 }
_



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