From: Jann Horn <[email protected]> Sanity-check that: - non-NULL freepointers point into the slab - freepointers look plausibly aligned
Signed-off-by: Jann Horn <[email protected]> Co-developed-by: Matteo Rizzo <[email protected]> Signed-off-by: Matteo Rizzo <[email protected]> --- lib/slub_kunit.c | 4 ++++ mm/slab.h | 8 +++++++ mm/slub.c | 57 ++++++++++++++++++++++++++++++++++++++++++++++++ 3 files changed, 69 insertions(+) diff --git a/lib/slub_kunit.c b/lib/slub_kunit.c index d4a3730b08fa..acf8600bd1fd 100644 --- a/lib/slub_kunit.c +++ b/lib/slub_kunit.c @@ -45,6 +45,10 @@ static void test_clobber_zone(struct kunit *test) #ifndef CONFIG_KASAN static void test_next_pointer(struct kunit *test) { + if (IS_ENABLED(CONFIG_SLAB_VIRTUAL)) + kunit_skip(test, + "incompatible with freepointer corruption detection in CONFIG_SLAB_VIRTUAL"); + struct kmem_cache *s = test_kmem_cache_create("TestSlub_next_ptr_free", 64, SLAB_POISON); u8 *p = kmem_cache_alloc(s, GFP_KERNEL); diff --git a/mm/slab.h b/mm/slab.h index 460c802924bd..8d10a011bdf0 100644 --- a/mm/slab.h +++ b/mm/slab.h @@ -79,6 +79,14 @@ struct slab { struct list_head flush_list_elem; + /* + * Not in kmem_cache because it depends on whether the allocation is + * normal order or fallback order. + * an alternative might be to over-allocate virtual memory for + * fallback-order pages. + */ + unsigned long align_mask; + /* Replaces the page lock */ spinlock_t slab_lock; diff --git a/mm/slub.c b/mm/slub.c index 0f7f5bf0b174..57474c8a6569 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -392,6 +392,44 @@ static inline freeptr_t freelist_ptr_encode(const struct kmem_cache *s, return (freeptr_t){.v = encoded}; } +/* + * Does some validation of freelist pointers. Without SLAB_VIRTUAL this is + * currently a no-op. + */ +static inline bool freelist_pointer_corrupted(struct slab *slab, freeptr_t ptr, + void *decoded) +{ +#ifdef CONFIG_SLAB_VIRTUAL + /* + * If the freepointer decodes to 0, use 0 as the slab_base so that + * the check below always passes (0 & slab->align_mask == 0). + */ + unsigned long slab_base = decoded ? (unsigned long)slab_to_virt(slab) + : 0; + + /* + * This verifies that the SLUB freepointer does not point outside the + * slab. Since at that point we can basically do it for free, it also + * checks that the pointer alignment looks vaguely sane. + * However, we probably don't want the cost of a proper division here, + * so instead we just do a cheap check whether the bottom bits that are + * clear in the size are also clear in the pointer. + * So for kmalloc-32, it does a perfect alignment check, but for + * kmalloc-192, it just checks that the pointer is a multiple of 32. + * This should probably be reconsidered - is this a good tradeoff, or + * should that part be thrown out, or do we want a proper accurate + * alignment check (and can we make it work with acceptable performance + * cost compared to the security improvement - probably not)? + */ + return CHECK_DATA_CORRUPTION( + ((unsigned long)decoded & slab->align_mask) != slab_base, + "bad freeptr (encoded %lx, ptr %p, base %lx, mask %lx", + ptr.v, decoded, slab_base, slab->align_mask); +#else + return false; +#endif +} + static inline void *freelist_ptr_decode(const struct kmem_cache *s, freeptr_t ptr, unsigned long ptr_addr, struct slab *slab) @@ -403,6 +441,10 @@ static inline void *freelist_ptr_decode(const struct kmem_cache *s, #else decoded = (void *)ptr.v; #endif + + if (unlikely(freelist_pointer_corrupted(slab, ptr, decoded))) + return NULL; + return decoded; } @@ -2122,6 +2164,21 @@ static struct slab *get_free_slab(struct kmem_cache *s, if (slab == NULL) return NULL; + /* + * Bits that must be equal to start-of-slab address for all + * objects inside the slab. + * For compatibility with pointer tagging (like in HWASAN), this would + * need to clear the pointer tag bits from the mask. + */ + slab->align_mask = ~((PAGE_SIZE << oo_order(oo)) - 1); + + /* + * Object alignment bits (must be zero, which is equal to the bits in + * the start-of-slab address) + */ + if (s->red_left_pad == 0) + slab->align_mask |= (1 << (ffs(s->size) - 1)) - 1; + return slab; } -- 2.42.0.459.ge4e396fd5e-goog
