Re: [PATCH] slub: Fixes freepointer encoding for single free

From: Nicolas Bouchinet
Date: Mon Apr 29 2024 - 09:13:07 EST


On 4/29/24 11:09, Nicolas Bouchinet wrote:
Hi Vlastimil,

thanks for your review and your proposal.

On 4/29/24 10:52, Vlastimil Babka wrote:
On 4/25/24 5:14 PM, Chengming Zhou wrote:
On 2024/4/25 23:02, Nicolas Bouchinet wrote:
Thanks for finding the bug and the fix!

Hy,

First of all, thanks a lot for your time.

On 4/25/24 10:36, Chengming Zhou wrote:
On 2024/4/24 20:47, Nicolas Bouchinet wrote:
From: Nicolas Bouchinet<nicolas.bouchinet@xxxxxxxxxxx>

Commit 284f17ac13fe ("mm/slub: handle bulk and single object freeing
separately") splits single and bulk object freeing in two functions
slab_free() and slab_free_bulk() which leads slab_free() to call
slab_free_hook() directly instead of slab_free_freelist_hook().
Right.
y not suitable for a stable-candidate fix we need
If `init_on_free` is set, slab_free_hook() zeroes the object.
Afterward, if `slub_debug=F` and `CONFIG_SLAB_FREELIST_HARDENED` are
set, the do_slab_free() slowpath executes freelist consistency
checks and try to decode a zeroed freepointer which leads to a
"Freepointer corrupt" detection in check_object().
IIUC, the "freepointer" can be checked on the free path only when
it's outside the object memory. Here slab_free_hook() zeroed the
freepointer and caused the problem.

But why we should zero the memory outside the object_size? It seems
more reasonable to only zero the object_size when init_on_free is set?
The original purpose was to avoid leaking information through the object and its metadata / tracking information as described in init_on_free initial Commit 6471384af2a6 ("mm: security: introduce init_on_alloc=1 and init_on_free=1 boot options").

I have to admit I didn't read the entire lore about the original patchset yet, though it could be interesting to know a bit more the threat models, specifically regarding the object metadata init.
Thank you for the reference! I also don't get why it needs to zero
the metadata and tracking information.
Hmm taking a step back, it seems really suboptimal to initialize the
outside-object freepointer as part of init_on_free:

- the freeing itself will always set it one way or another, in this case
free_to_partial_list() will do set_freepointer() after free_debug_processing()

- we lose the ability to detect if the allocated slab object's user wrote to
it, which is a buffer overflow

So the best option to me would be to adjust the init in slab_free_hook() to
avoid the outside-object freepointer similarly to how it avoids the red zone.

We'll still not have the buffer overflow detection ability for bulk free
where slab_free_freelist_hook() will set the free pointer before we reach
the checks, but changing that is most likely not worth the trouble, and
especially not suitable for a stable-candidate fix we need here.

It seems like a good alternative to me, I'll push a V2 patch with those changes.

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.




The patch could also be optimized a bit by restricting set_freepointer() call to the `CONFIG_SLAB_FREELIST_HARDENED` option value.

Yeah. Maybe memcg_alloc_abort_single() needs this too.

Thanks.

Thanks again, Nicolas

Thanks.

Object's freepointer thus needs to be properly set using
set_freepointer() after init_on_free.

To reproduce, set `slub_debug=FU init_on_free=1 log_level=7` on the
command line of a kernel build with `CONFIG_SLAB_FREELIST_HARDENED=y`.

dmesg sample log:
[   10.708715] =============================================================================
[   10.710323] BUG kmalloc-rnd-05-32 (Tainted: G B           T ): Freepointer corrupt
[   10.712695] -----------------------------------------------------------------------------
[   10.712695]
[   10.712695] Slab 0xffffd8bdc400d580 objects=32 used=4 fp=0xffff9d9a80356f80 flags=0x200000000000a00(workingset|slab|node=0|zone=2)
[   10.716698] Object 0xffff9d9a80356600 @offset=1536 fp=0x7ee4f480ce0ecd7c
[   10.716698]
[   10.716698] Bytes b4 ffff9d9a803565f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
[   10.720703] Object   ffff9d9a80356600: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
[   10.720703] Object   ffff9d9a80356610: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
[   10.724696] Padding  ffff9d9a8035666c: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
[   10.724696] Padding  ffff9d9a8035667c: 00 00 00 00                                      ....
[   10.724696] FIX kmalloc-rnd-05-32: Object at 0xffff9d9a80356600 not freed

Signed-off-by: Nicolas Bouchinet<nicolas.bouchinet@xxxxxxxxxxx>
---
   mm/slub.c | 8 +++++++-
   1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/mm/slub.c b/mm/slub.c
index 3aa12b9b323d9..71dbff9ad8f17 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -4342,10 +4342,16 @@ static __fastpath_inline
   void slab_free(struct kmem_cache *s, struct slab *slab, void *object,
              unsigned long addr)
   {
+    bool init = false;
+
       memcg_slab_free_hook(s, slab, &object, 1);
+    init = slab_want_init_on_free(s);
   -    if (likely(slab_free_hook(s, object, slab_want_init_on_free(s))))
+    if (likely(slab_free_hook(s, object, init))) {
+        if (init)
+            set_freepointer(s, object, NULL);
           do_slab_free(s, slab, object, object, 1, addr);
+    }
   }
     static __fastpath_inline
Thanks again for your review,

Nicolas