kasan, slub: fix more conflicts with CONFIG_SLAB_FREELIST_HARDENED

When CONFIG_KASAN_SW_TAGS is enabled, ptr_addr might be tagged.  Normally,
this doesn't cause any issues, as both set_freepointer() and
get_freepointer() are called with a pointer with the same tag.  However,
there are some issues with CONFIG_SLUB_DEBUG code.  For example, when
__free_slub() iterates over objects in a cache, it passes untagged
pointers to check_object().  check_object() in turns calls
get_freepointer() with an untagged pointer, which causes the freepointer
to be restored incorrectly.

Add kasan_reset_tag to freelist_ptr(). Also add a detailed comment.

Link: http://lkml.kernel.org/r/bf858f26ef32eb7bd24c665755b3aee4bc58d0e4.1550103861.git.andreyknvl@google.com
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Reported-by: Qian Cai <cai@lca.pw>
Tested-by: Qian Cai <cai@lca.pw>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/slub.c

@@ -249,7 +249,18 @@ static inline void *freelist_ptr(const struct kmem_cache *s, void *ptr,
 				 unsigned long ptr_addr)
 {
 #ifdef CONFIG_SLAB_FREELIST_HARDENED
-	return (void *)((unsigned long)ptr ^ s->random ^ ptr_addr);
+	/*
+	 * When CONFIG_KASAN_SW_TAGS is enabled, ptr_addr might be tagged.
+	 * Normally, this doesn't cause any issues, as both set_freepointer()
+	 * and get_freepointer() are called with a pointer with the same tag.
+	 * However, there are some issues with CONFIG_SLUB_DEBUG code. For
+	 * example, when __free_slub() iterates over objects in a cache, it
+	 * passes untagged pointers to check_object(). check_object() in turns
+	 * calls get_freepointer() with an untagged pointer, which causes the
+	 * freepointer to be restored incorrectly.
+	 */
+	return (void *)((unsigned long)ptr ^ s->random ^
+			(unsigned long)kasan_reset_tag((void *)ptr_addr));
 #else
 	return ptr;
 #endif