On 4/29/24 22:22, Vlastimil Babka wrote:
On 4/29/24 6:16 PM, Nicolas Bouchinet wrote:
On 4/29/24 16:52, Chengming Zhou wrote:
On 2024/4/29 22:32, Nicolas Bouchinet wrote:
On 4/29/24 15:35, Chengming Zhou wrote:
On 2024/4/29 20:59, Nicolas Bouchinet wrote:
I help maintaining the Linux-Hardened patchset in which we have a slab object canary feature that helps detecting overflows. It is located just after the object freepointer.
I've tried a patch where the freepointer is avoided but it results in the same bug. It seems that the commit 0f181f9fbea8bc7ea ("mm/slub.c: init_on_free=1 should wipe freelist ptr for bulk allocations") inits the freepointer on allocation if init_on_free is set in order to return a clean initialized object to the caller.
Good catch! You may need to change maybe_wipe_obj_freeptr() too,
I haven't tested this, not sure whether it works for you. :)
diff --git a/mm/slub.c b/mm/slub.c
index 3e33ff900d35..3f250a167cb5 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3796,7 +3796,8 @@ static void *__slab_alloc_node(struct kmem_cache *s,
static __always_inline void maybe_wipe_obj_freeptr(struct kmem_cache *s,
void *obj)
{
- if (unlikely(slab_want_init_on_free(s)) && obj)
+ if (unlikely(slab_want_init_on_free(s)) && obj &&
+ !freeptr_outside_object(s))
memset((void *)((char *)kasan_reset_tag(obj) + s->offset),
0, sizeof(void *));
}
Thanks!
Indeed since check_object() avoids objects for which freepointer is in the object and since val is equal to SLUB_RED_ACTIVE in our specific case it should work. Do you want me to add you as Co-authored ?
Ok, it's great. Thanks!
Now I think of it, doesn't it seems a bit odd to only properly
init_on_free object's freepointer only if it's inside the object ? IMHO
it is equally necessary to avoid information leaking about the
freepointer whether it is inside or outside the object.
I think it break the semantic of the commit 0f181f9fbea8bc7ea
("mm/slub.c: init_on_free=1 should wipe freelist ptr for bulk
allocations") ?
Hm, AFAIU, wiping inside object prevents misuse of some buggy kernel code
that would allocate and accidentally leak prior content (including the
in-object freepointer) somewhere the attacker can read. Now for wiping the
freepointer outside the object to be useful it would have assume said
leak-prone code to additionally be reading past the allocated object size,
i.e. a read buffer overflow. That to me seems to be a much more rare
combination, and also in that case such code could also likely read even
further past the object, i.e. leak the next object's data? IOW I don't think
it buys us much additional security protection in practice?
Moreover, with CONFIG_SLAB_FREELIST_HARDENED activated, freepointers are
encoded and harder to exploit.
Thanks.
