On Fri, Sep 15, 2023 at 10:59:32AM +0000, Matteo Rizzo wrote:
> From: Jann Horn <jannh@xxxxxxxxxx>
>
> Sanity-check that:
> - non-NULL freepointers point into the slab
> - freepointers look plausibly aligned
>
> Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
> Co-developed-by: Matteo Rizzo <matteorizzo@xxxxxxxxxx>
> Signed-off-by: Matteo Rizzo <matteorizzo@xxxxxxxxxx>
> ---
> 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 <keescook@xxxxxxxxxxxx>
--
Kees Cook
