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

Andrew Morton
2020-08-07T06:19:16Z

From: Marco Elver <elver@google.com>
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@google.com
Signed-off-by: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Konovalov <andreyknvl@google.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

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_);
}
_