mm/slub: remove redundant kasan_reset_tag() from freelist_ptr calculations

Commit d36a63a943e3 ("kasan, slub: fix more conflicts with
CONFIG_SLAB_FREELIST_HARDENED") has introduced kasan_reset_tags() to
freelist_ptr() encoding/decoding when CONFIG_SLAB_FREELIST_HARDENED is
enabled to resolve issues when passing tagged or untagged pointers
inconsistently would lead to incorrect calculations.

Later, commit aa1ef4d7b3f6 ("kasan, mm: reset tags when accessing
metadata") made sure all pointers have tags reset regardless of
CONFIG_SLAB_FREELIST_HARDENED, because there was no other way to access
the freepointer metadata safely with hw tag-based KASAN.

Therefore the kasan_reset_tag() usage in freelist_ptr_encode()/decode()
is now redundant, as all callers use kasan_reset_tag() unconditionally
when constructing ptr_addr. Remove the redundant calls and simplify the
code and remove obsolete comments.

Also in freelist_ptr_encode() introduce an 'encoded' variable to make
the lines shorter and make it similar to the _decode() one.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Acked-by: Kees Cook <keescook@chromium.org>

diff --git a/mm/slub.c b/mm/slub.c
index f8cc47eff7420dff78742e9db99b52d361151e79..07edad305512fdbf329c05370f7fcea8df1b0468 100644 (file)
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -374,22 +374,14 @@ typedef struct { unsigned long v; } freeptr_t;
 static inline freeptr_t freelist_ptr_encode(const struct kmem_cache *s,
                                            void *ptr, unsigned long ptr_addr)
 {
+       unsigned long encoded;
+
 #ifdef CONFIG_SLAB_FREELIST_HARDENED
-       /*
-        * When CONFIG_KASAN_SW/HW_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 (freeptr_t){.v = (unsigned long)ptr ^ s->random ^
-                       swab((unsigned long)kasan_reset_tag((void *)ptr_addr))};
+       encoded = (unsigned long)ptr ^ s->random ^ swab(ptr_addr);
 #else
-       return (freeptr_t){.v = (unsigned long)ptr};
+       encoded = (unsigned long)ptr;
 #endif
+       return (freeptr_t){.v = encoded};
 }
 
 static inline void *freelist_ptr_decode(const struct kmem_cache *s,
@@ -398,9 +390,7 @@ static inline void *freelist_ptr_decode(const struct kmem_cache *s,
        void *decoded;
 
 #ifdef CONFIG_SLAB_FREELIST_HARDENED
-       /* See the comment in freelist_ptr_encode */
-       decoded = (void *)(ptr.v ^ s->random ^
-               swab((unsigned long)kasan_reset_tag((void *)ptr_addr)));
+       decoded = (void *)(ptr.v ^ s->random ^ swab(ptr_addr));
 #else
        decoded = (void *)ptr.v;
 #endif