kasan: fix tag for large allocations when using CONFIG_SLAB

If an object is allocated on a tail page of a multi-page slab, kasan
will get the wrong tag because page->s_mem is NULL for tail pages.  I'm
not quite sure what the user-visible effect of this might be.

Link: https://lkml.kernel.org/r/20211001024105.3217339-1-willy@infradead.org
Fixes: 7f94ffbc4c6a ("kasan: add hooks implementation for tag-based mode")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Marco Elver <elver@google.com>
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.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>

diff --git a/mm/kasan/common.c b/mm/kasan/common.c
index 3e09998..8428da2 100644 (file)
--- a/mm/kasan/common.c
+++ b/mm/kasan/common.c
@@ -298,7 +298,7 @@ static inline u8 assign_tag(struct kmem_cache *cache,
        /* For caches that either have a constructor or SLAB_TYPESAFE_BY_RCU: */
 #ifdef CONFIG_SLAB
        /* For SLAB assign tags based on the object index in the freelist. */
-       return (u8)obj_to_index(cache, virt_to_page(object), (void *)object);
+       return (u8)obj_to_index(cache, virt_to_head_page(object), (void *)object);
 #else
        /*
         * For SLUB assign a random tag during slab creation, otherwise reuse