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_); } _
