Re: [PATCH] slub: choose the right freelist pointer location when creating small caches

["Lin, Zhenpeng" <zplin@psu.edu>]

There do exist objects whose size is smaller than 2*sizeof(void*). E.g. struct ccid in DCCP module.

On 6/8/21, 2:26 PM, "Kees Cook" <keescook@chromium.org> wrote:

    On Sat, Jun 05, 2021 at 01:58:13AM +0000, Lin, Zhenpeng wrote:
    > When enabling CONFIG_SLUB_DEBUG and booting with "slub_debug=Z", the
    > kernel crashes at creating caches if the object size is smaller
    > than 2*sizeof(void*). The problem is due to the wrong calculation
    > of freepointer_area. The freelist pointer can be stored in the
    > middle of object only if the object size is not smaller than
    > 2*sizeof(void*). Otherwise, the freelist pointer will be corrupted by
    > SLUB_RED_ZONE.
    > 
    > Fixes: 3202fa62fb43 ("slub: relocate freelist pointer to middle of object")
    > Fixes: 89b83f282d8b ("slub: avoid redzone when choosing freepointer location")
    > Signed-off-by: Zhenpeng Lin <zplin@psu.edu>
    > ---
    > mm/slub.c | 4 ++--
    > 1 file changed, 2 insertions(+), 2 deletions(-)
    > 
    > diff --git a/mm/slub.c b/mm/slub.c
    > index 3f96e099817a..cb23233ee683 100644
    > --- a/mm/slub.c
    > +++ b/mm/slub.c
    > @@ -3704,7 +3704,7 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
    > 	 * can't use that portion for writing the freepointer, so
    > 	 * s->offset must be limited within this for the general case.
    > 	 */
    > -	freepointer_area = size;
    > +	freepointer_area = s->object_size;
    > 
    > #ifdef CONFIG_SLUB_DEBUG
    > 	/*
    > @@ -3751,7 +3751,7 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
    > 		 */
    > 		s->offset = size;
    > 		size += sizeof(void *);
    > -	} else if (freepointer_area > sizeof(void *)) {
    > +	} else if (freepointer_area >= 2 * sizeof(void *)) {
    > 		/*
    > 		 * Store freelist pointer near middle of object to keep
    > 		 * it away from the edges of the object to avoid small
    > --
    > 2.17.1

    NAK, I'd prefer this get cleaned up more completely, especially since
    there are no objects of that size in the kernel currently:

    https://nam10.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Flkml%2F20201015033712.1491731-1-keescook%40chromium.org%2F&amp;data=04%7C01%7Czplin%40psu.edu%7C28b6f3c5a3b149be56e808d92aaafd26%7C7cf48d453ddb4389a9c1c115526eb52e%7C0%7C0%7C637587736155493816%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&amp;sdata=%2F8CXVkqlhA7RnfX%2BDP07%2F4t1NIw1CHsUpuuWrsLyU9o%3D&amp;reserved=0

    -- 
    Kees Cook