[PATCH] kasan, slub: fix more conflicts with CONFIG_SLAB_FREELIST_HARDENED

[Andrey Konovalov <andreyknvl@google.com>]

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.

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
---
 mm/slub.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/mm/slub.c b/mm/slub.c
index 80da3a40b74d..c80e6699357c 100644
--- a/mm/slub.c
+++ b/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
-- 
2.20.1.791.gb4d0f1c61a-goog