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mm/slub: refactor freelist to use custom type 

Currently the SLUB code represents encoded freelist entries as "void*".
That's misleading, those things are encoded under
CONFIG_SLAB_FREELIST_HARDENED so that they're not actually dereferencable.

Give them their own type, and split freelist_ptr() into one function per
direction (one for encoding, one for decoding).

Signed-off-by: Jann Horn <jannh@google.com>
Co-developed-by: Matteo Rizzo <matteorizzo@google.com>
Signed-off-by: Matteo Rizzo <matteorizzo@google.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
This commit is contained in:
Jann Horn 2023-07-04 13:58:34 +00:00 committed by Vlastimil Babka
parent 06c2afb862
commit 44f6a42d49
1 changed files with 32 additions and 11 deletions
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43
mm/slub.c
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@ -360,13 +360,19 @@ static struct workqueue_struct *flushwq;
* Core slab cache functions
*******************************************************************/
/*
* freeptr_t represents a SLUB freelist pointer, which might be encoded
* and not dereferenceable if CONFIG_SLAB_FREELIST_HARDENED is enabled.
*/
typedef struct { unsigned long v; } freeptr_t;
/*
* Returns freelist pointer (ptr). With hardening, this is obfuscated
* with an XOR of the address where the pointer is held and a per-cache
* random number.
*/
static inline void *freelist_ptr(const struct kmem_cache *s, void *ptr,
unsigned long ptr_addr)
static inline freeptr_t freelist_ptr_encode(const struct kmem_cache *s,
void *ptr, unsigned long ptr_addr)
{
#ifdef CONFIG_SLAB_FREELIST_HARDENED
/*
@ -379,25 +385,40 @@ static inline void *freelist_ptr(const struct kmem_cache *s, void *ptr,
* calls get_freepointer() with an untagged pointer, which causes the
* freepointer to be restored incorrectly.
*/
return (void *)((unsigned long)ptr ^ s->random ^
swab((unsigned long)kasan_reset_tag((void *)ptr_addr)));
return (freeptr_t){.v = (unsigned long)ptr ^ s->random ^
swab((unsigned long)kasan_reset_tag((void *)ptr_addr))};
#else
return ptr;
return (freeptr_t){.v = (unsigned long)ptr};
#endif
}
static inline void *freelist_ptr_decode(const struct kmem_cache *s,
freeptr_t ptr, unsigned long ptr_addr)
{
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)));
#else
decoded = (void *)ptr.v;
#endif
return decoded;
}
/* Returns the freelist pointer recorded at location ptr_addr. */
static inline void *freelist_dereference(const struct kmem_cache *s,
void *ptr_addr)
{
return freelist_ptr(s, (void *)*(unsigned long *)(ptr_addr),
return freelist_ptr_decode(s, *(freeptr_t *)(ptr_addr),
(unsigned long)ptr_addr);
}
static inline void *get_freepointer(struct kmem_cache *s, void *object)
{
object = kasan_reset_tag(object);
return freelist_dereference(s, object + s->offset);
return freelist_dereference(s, (freeptr_t *)(object + s->offset));
}
#ifndef CONFIG_SLUB_TINY
@ -421,15 +442,15 @@ __no_kmsan_checks
static inline void *get_freepointer_safe(struct kmem_cache *s, void *object)
{
unsigned long freepointer_addr;
void *p;
freeptr_t p;
if (!debug_pagealloc_enabled_static())
return get_freepointer(s, object);
object = kasan_reset_tag(object);
freepointer_addr = (unsigned long)object + s->offset;
copy_from_kernel_nofault(&p, (void **)freepointer_addr, sizeof(p));
return freelist_ptr(s, p, freepointer_addr);
copy_from_kernel_nofault(&p, (freeptr_t *)freepointer_addr, sizeof(p));
return freelist_ptr_decode(s, p, freepointer_addr);
}
static inline void set_freepointer(struct kmem_cache *s, void *object, void *fp)
@ -441,7 +462,7 @@ static inline void set_freepointer(struct kmem_cache *s, void *object, void *fp)
#endif
freeptr_addr = (unsigned long)kasan_reset_tag((void *)freeptr_addr);
*(void **)freeptr_addr = freelist_ptr(s, fp, freeptr_addr);
*(freeptr_t *)freeptr_addr = freelist_ptr_encode(s, fp, freeptr_addr);
}
/* Loop over all objects in a slab */