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

Commit d36a63a943 ("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 aa1ef4d7b3 ("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>
This commit is contained in:
Vlastimil Babka 2023-07-11 12:24:59 +02:00
parent 44f6a42d49
commit b06952cdbc
1 changed files with 6 additions and 16 deletions
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22
mm/slub.c
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@ -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