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    Subject[PATCH v4 2/3] mm/slub: Fix redzoning for small allocations
    Date
    The redzone area for SLUB exists between s->object_size and s->inuse
    (which is at least the word-aligned object_size). If a cache were created
    with an object_size smaller than sizeof(void *), the in-object stored
    freelist pointer would overwrite the redzone (e.g. with boot param
    "slub_debug=ZF"):

    BUG test (Tainted: G B ): Right Redzone overwritten
    -----------------------------------------------------------------------------

    INFO: 0xffff957ead1c05de-0xffff957ead1c05df @offset=1502. First byte 0x1a instead of 0xbb
    INFO: Slab 0xffffef3950b47000 objects=170 used=170 fp=0x0000000000000000 flags=0x8000000000000200
    INFO: Object 0xffff957ead1c05d8 @offset=1496 fp=0xffff957ead1c0620

    Redzone (____ptrval____): bb bb bb bb bb bb bb bb ........
    Object (____ptrval____): f6 f4 a5 40 1d e8 ...@..
    Redzone (____ptrval____): 1a aa ..
    Padding (____ptrval____): 00 00 00 00 00 00 00 00 ........

    Store the freelist pointer out of line when object_size is smaller than
    sizeof(void *) and redzoning is enabled.

    Additionally remove the "smaller than sizeof(void *)" check under
    CONFIG_DEBUG_VM in kmem_cache_sanity_check() as it is now redundant:
    SLAB and SLOB both handle small sizes.

    (Note that no caches within this size range are known to exist in the
    kernel currently.)

    Fixes: 81819f0fc828 ("SLUB core")
    Cc: stable@vger.kernel.org
    Signed-off-by: Kees Cook <keescook@chromium.org>
    ---
    mm/slab_common.c | 3 +--
    mm/slub.c | 8 +++++---
    2 files changed, 6 insertions(+), 5 deletions(-)

    diff --git a/mm/slab_common.c b/mm/slab_common.c
    index a4a571428c51..7cab77655f11 100644
    --- a/mm/slab_common.c
    +++ b/mm/slab_common.c
    @@ -97,8 +97,7 @@ EXPORT_SYMBOL(kmem_cache_size);
    #ifdef CONFIG_DEBUG_VM
    static int kmem_cache_sanity_check(const char *name, unsigned int size)
    {
    - if (!name || in_interrupt() || size < sizeof(void *) ||
    - size > KMALLOC_MAX_SIZE) {
    + if (!name || in_interrupt() || size > KMALLOC_MAX_SIZE) {
    pr_err("kmem_cache_create(%s) integrity check failed\n", name);
    return -EINVAL;
    }
    diff --git a/mm/slub.c b/mm/slub.c
    index f91d9fe7d0d8..f58cfd456548 100644
    --- a/mm/slub.c
    +++ b/mm/slub.c
    @@ -3734,15 +3734,17 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
    */
    s->inuse = size;

    - if (((flags & (SLAB_TYPESAFE_BY_RCU | SLAB_POISON)) ||
    - s->ctor)) {
    + if ((flags & (SLAB_TYPESAFE_BY_RCU | SLAB_POISON)) ||
    + ((flags & SLAB_RED_ZONE) && s->object_size < sizeof(void *)) ||
    + s->ctor) {
    /*
    * Relocate free pointer after the object if it is not
    * permitted to overwrite the first word of the object on
    * kmem_cache_free.
    *
    * This is the case if we do RCU, have a constructor or
    - * destructor or are poisoning the objects.
    + * destructor, are poisoning the objects, or are
    + * redzoning an object smaller than sizeof(void *).
    *
    * The assumption that s->offset >= s->inuse means free
    * pointer is outside of the object is used in the
    --
    2.25.1
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