On Fri, Sep 15, 2023 at 10:59:32AM +0000, Matteo Rizzo wrote: > 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)?
Is it really that much more expensive to check the alignment exactly? > + */ > + 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 > We can improve the sanity checking in the future, so as-is, sure: Reviewed-by: Kees Cook <[email protected]> -- Kees Cook
