On 6/5/26 4:12 AM, Pedro Falcato wrote:

On Thu, Jun 04, 2026 at 02:30:34PM +0900, Harry Yoo wrote:

quoted

On 6/3/26 3:31 AM, Pedro Falcato wrote:

quoted

SKB data area allocations (as done from alloc_skb()) use kmalloc().
These allocations can be variably sized and their contents can be more
or less controlled from userspace, which makes them useful for attackers
that want to overwrite a use-after-free'd object from the same kmalloc slab
(which often just requires the sizes to roughly match into the same kmalloc
bucket). [0] is an easy example of an exploit that uses netlink skb
allocation to target another similarly-sized accidentally freed object.

While other mitigations like CONFIG_RANDOM_KMALLOC_CACHES exist, these are
probabilistic. Use the existing kmem buckets API to further isolate these
allocations in a guaranteed fashion, when CONFIG_SLAB_BUCKETS=y.

Link: https://github.com/google/security-research/blob/master/pocs/linux/kernelctf/CVE-2023-4207_lts_cos_mitigation_2/docs/exploit.md [0]
Signed-off-by: Pedro Falcato <pfalcato@suse.de>
---
 net/core/skbuff.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 44a7f8401468..1f6c6b531ece 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c

@@ -594,6 +594,8 @@ static void *kmalloc_pfmemalloc(size_t obj_size, gfp_t flags, int node)
 	return kmalloc_node_track_caller(obj_size, flags, node);
 }
 
+static kmem_buckets *skb_data_buckets __ro_after_init;
+
 /*
  * kmalloc_reserve is a wrapper around kmalloc_node_track_caller that tells
  * the caller if emergency pfmemalloc reserves are being used. If it is and

@@ -632,7 +634,7 @@ static void *kmalloc_reserve(unsigned int *size, gfp_t flags, int node,
 	 * Try a regular allocation, when that fails and we're not entitled
 	 * to the reserves, fail.
 	 */
-	obj = kmalloc_node_track_caller(obj_size,
+	obj = kmem_buckets_alloc_node_track_caller(skb_data_buckets, obj_size,
 					flags | __GFP_NOMEMALLOC | __GFP_NOWARN,
 					node);
 	if (likely(obj))

What about kmalloc_pfmemalloc()?

Good point, that looks free as well.

Sidenote: isolating kmem_cache_alloc for possibly-aliasing caches could also
be useful. skb allocation has net_hotdata.skb_small_head_cache. It doesn't merge
with anything for $raisins (odd size, plus I don't think usercopy caches are
getting merged?) but it feels too... accidental?

Right, we never merge caches with useroffset/usersize.

Hmm...

/* SKB_SMALL_HEAD_CACHE_SIZE is the size used for the skbuff_small_head
 * kmem_cache. The non-power-of-2 padding is kept for historical reasons and
 * to avoid potential collisions with generic kmalloc bucket sizes.
 */
#define SKB_SMALL_HEAD_CACHE_SIZE					\
	(is_power_of_2(SKB_SMALL_HEAD_SIZE) ?			\
		(SKB_SMALL_HEAD_SIZE + L1_CACHE_BYTES) :	\
		SKB_SMALL_HEAD_SIZE)


What are "historical reasons" other than avoiding collisions with
kmalloc caches?

Maybe passing something like SLAB_NO_MERGE and making the size
standard-looking would be nice. I have a size of 704 bytes per object, and
this probably causes some weird wastage for each slab.

Yes, unless the "historical reasons" do not make it infeasible to do that.

And I wonder if net/core/skbuff.c intends to always prevent merging, or
only with hardening configs like SLAB_BUCKETS.

-- 
Cheers,
Harry / Hyeonggon