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    Subject[PATCH 4.14 031/199] slub: improve bit diffusion for freelist ptr obfuscation
    Date
    From: Kees Cook <keescook@chromium.org>

    commit 1ad53d9fa3f6168ebcf48a50e08b170432da2257 upstream.

    Under CONFIG_SLAB_FREELIST_HARDENED=y, the obfuscation was relatively weak
    in that the ptr and ptr address were usually so close that the first XOR
    would result in an almost entirely 0-byte value[1], leaving most of the
    "secret" number ultimately being stored after the third XOR. A single
    blind memory content exposure of the freelist was generally sufficient to
    learn the secret.

    Add a swab() call to mix bits a little more. This is a cheap way (1
    cycle) to make attacks need more than a single exposure to learn the
    secret (or to know _where_ the exposure is in memory).

    kmalloc-32 freelist walk, before:

    ptr ptr_addr stored value secret
    ffff90c22e019020@ffff90c22e019000 is 86528eb656b3b5bd (86528eb656b3b59d)
    ffff90c22e019040@ffff90c22e019020 is 86528eb656b3b5fd (86528eb656b3b59d)
    ffff90c22e019060@ffff90c22e019040 is 86528eb656b3b5bd (86528eb656b3b59d)
    ffff90c22e019080@ffff90c22e019060 is 86528eb656b3b57d (86528eb656b3b59d)
    ffff90c22e0190a0@ffff90c22e019080 is 86528eb656b3b5bd (86528eb656b3b59d)
    ...

    after:

    ptr ptr_addr stored value secret
    ffff9eed6e019020@ffff9eed6e019000 is 793d1135d52cda42 (86528eb656b3b59d)
    ffff9eed6e019040@ffff9eed6e019020 is 593d1135d52cda22 (86528eb656b3b59d)
    ffff9eed6e019060@ffff9eed6e019040 is 393d1135d52cda02 (86528eb656b3b59d)
    ffff9eed6e019080@ffff9eed6e019060 is 193d1135d52cdae2 (86528eb656b3b59d)
    ffff9eed6e0190a0@ffff9eed6e019080 is f93d1135d52cdac2 (86528eb656b3b59d)

    [1] https://blog.infosectcbr.com.au/2020/03/weaknesses-in-linux-kernel-heap.html

    Fixes: 2482ddec670f ("mm: add SLUB free list pointer obfuscation")
    Reported-by: Silvio Cesare <silvio.cesare@gmail.com>
    Signed-off-by: Kees Cook <keescook@chromium.org>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Cc: Christoph Lameter <cl@linux.com>
    Cc: Pekka Enberg <penberg@kernel.org>
    Cc: David Rientjes <rientjes@google.com>
    Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
    Cc: <stable@vger.kernel.org>
    Link: http://lkml.kernel.org/r/202003051623.AF4F8CB@keescook
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    [kees: Backport to v4.19 which doesn't call kasan_reset_untag()]
    Signed-off-by: Kees Cook <keescook@chromium.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    mm/slub.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    --- a/mm/slub.c
    +++ b/mm/slub.c
    @@ -248,7 +248,7 @@ static inline void *freelist_ptr(const s
    unsigned long ptr_addr)
    {
    #ifdef CONFIG_SLAB_FREELIST_HARDENED
    - return (void *)((unsigned long)ptr ^ s->random ^ ptr_addr);
    + return (void *)((unsigned long)ptr ^ s->random ^ swab(ptr_addr));
    #else
    return ptr;
    #endif

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