kmsan: support SLAB_POISON

Avoid false KMSAN negatives with SLUB_DEBUG by allowing kmsan_slab_free()
to poison the freed memory, and by preventing init_object() from
unpoisoning new allocations by using __memset().

There are two alternatives to this approach.  First, init_object() can be
marked with __no_sanitize_memory.  This annotation should be used with
great care, because it drops all instrumentation from the function, and
any shadow writes will be lost.  Even though this is not a concern with
the current init_object() implementation, this may change in the future.

Second, kmsan_poison_memory() calls may be added after memset() calls. 
The downside is that init_object() is called from free_debug_processing(),
in which case poisoning will erase the distinction between simply
uninitialized memory and UAF.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Ilya Leoshkevich <[email protected]>
Reviewed-by: Alexander Potapenko <[email protected]>
Cc: Alexander Gordeev <[email protected]>
Cc: Christian Borntraeger <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Cc: Heiko Carstens <[email protected]>
Cc: Hyeonggon Yoo <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: <[email protected]>
Cc: Marco Elver <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Masami Hiramatsu (Google) <[email protected]>
Cc: Pekka Enberg <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Steven Rostedt (Google) <[email protected]>
Cc: Sven Schnelle <[email protected]>
Cc: Vasily Gorbik <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

2 files changed, 12 insertions, 5 deletions

diff --git a/mm/kmsan/hooks.c b/mm/kmsan/hooks.c
index 267d0afa2e8b..26d86dfdc819 100644
--- a/mm/kmsan/hooks.c
+++ b/mm/kmsan/hooks.c
@@ -74,7 +74,7 @@ void kmsan_slab_free(struct kmem_cache *s, void *object)
 		return;
 
 	/* RCU slabs could be legally used after free within the RCU period */
-	if (unlikely(s->flags & (SLAB_TYPESAFE_BY_RCU | SLAB_POISON)))
+	if (unlikely(s->flags & SLAB_TYPESAFE_BY_RCU))
 		return;
 	/*
 	 * If there's a constructor, freed memory must remain in the same state
diff --git a/mm/slub.c b/mm/slub.c
index 4927edec6a8c..dbe8a6303937 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1139,7 +1139,13 @@ static void init_object(struct kmem_cache *s, void *object, u8 val)
 	unsigned int poison_size = s->object_size;
 
 	if (s->flags & SLAB_RED_ZONE) {
-		memset(p - s->red_left_pad, val, s->red_left_pad);
+		/*
+		 * Here and below, avoid overwriting the KMSAN shadow. Keeping
+		 * the shadow makes it possible to distinguish uninit-value
+		 * from use-after-free.
+		 */
+		memset_no_sanitize_memory(p - s->red_left_pad, val,
+					  s->red_left_pad);
 
 		if (slub_debug_orig_size(s) && val == SLUB_RED_ACTIVE) {
 			/*
@@ -1152,12 +1158,13 @@ static void init_object(struct kmem_cache *s, void *object, u8 val)
 	}
 
 	if (s->flags & __OBJECT_POISON) {
-		memset(p, POISON_FREE, poison_size - 1);
-		p[poison_size - 1] = POISON_END;
+		memset_no_sanitize_memory(p, POISON_FREE, poison_size - 1);
+		memset_no_sanitize_memory(p + poison_size - 1, POISON_END, 1);
 	}
 
 	if (s->flags & SLAB_RED_ZONE)
-		memset(p + poison_size, val, s->inuse - poison_size);
+		memset_no_sanitize_memory(p + poison_size, val,
+					  s->inuse - poison_size);
 }
 
 static void restore_bytes(struct kmem_cache *s, char *message, u8 data,