[ Participants list changed - syzbot thought this was networking and
p9, but it really looks entirely like a slub internal bug. I left the
innocent people on the list just to let them know they are innocent ]

On Wed, Jan 27, 2021 at 6:27 AM syzbot
[off-list ref] wrote:

The issue was bisected to:

commit dde3c6b72a16c2db826f54b2d49bdea26c3534a2
Author: Wang Hai [off-list ref]
Date:   Wed Jun 3 22:56:21 2020 +0000

    mm/slub: fix a memory leak in sysfs_slab_add()

BUG: KASAN: double-free or invalid-free in slab_free mm/slub.c:3142 [inline]
BUG: KASAN: double-free or invalid-free in kmem_cache_free+0x82/0x350 mm/slub.c:3158

The p9 part of this bug report seems to be a red herring.

The problem looks like it's simply the kmem_cache failure case, ie:

 - mm/slab_common.c: create_cache(): if the __kmem_cache_create()
fails, it does:

        out_free_cache:
                kmem_cache_free(kmem_cache, s);

 - but __kmem_cache_create() - at least for slub() - will have done

        sysfs_slab_add(s) .. fails ..
            -> kobject_del(&s->kobj); .. which frees s ...

so the networking and p9 are fine, and the only reason p9 shows up in
the trace is that apparently it causes that failure in
kobject_init_and_add() for whatever reason, and that then exposes the
problem.

So the added kobject_put() really looks buggy in this situation, and
the memory leak that that commit dde3c6b72a16 ("mm/slub: fix a memory
leak in sysfs_slab_add()") fixes is now a double free.

And no, I don't think you can just remove the kmem_cache_free() in
create_cache(), because _other_ error cases of __kmem_cache_create()
do not free this.

Wang Hai - comments? I'm inclined to revert that commit for now unless
somebody can come up with a proper fix..

              Linus