Re: [PATCH net v2] net: skb: fix cross-cache free of KFENCE-allocated skb head

[Eric Dumazet]

On Thu, Apr 2, 2026 at 6:45 PM Jiayuan Chen <jiayuan.chen@xxxxxxxxx> wrote:
>
> SKB_SMALL_HEAD_CACHE_SIZE is intentionally set to a non-power-of-2
> value (e.g. 704 on x86_64) to avoid collisions with generic kmalloc
> bucket sizes. This ensures that skb_kfree_head() can reliably use
> skb_end_offset to distinguish skb heads allocated from
> skb_small_head_cache vs. generic kmalloc caches.
>
> However, when KFENCE is enabled, kfence_ksize() returns the exact
> requested allocation size instead of the slab bucket size. If a caller
> (e.g. bpf_test_init) allocates skb head data via kzalloc() and the
> requested size happens to equal SKB_SMALL_HEAD_CACHE_SIZE, then
> slab_build_skb() -> ksize() returns that exact value. After subtracting
> skb_shared_info overhead, skb_end_offset ends up matching
> SKB_SMALL_HEAD_HEADROOM, causing skb_kfree_head() to incorrectly free
> the object to skb_small_head_cache instead of back to the original
> kmalloc cache, resulting in a slab cross-cache free:
>
>   kmem_cache_free(skbuff_small_head): Wrong slab cache. Expected
>   skbuff_small_head but got kmalloc-1k
>
> Fix this by always calling kfree(head) in skb_kfree_head(). This keeps
> the free path generic and avoids allocator-specific misclassification
> for KFENCE objects.
>
> Fixes: bf9f1baa279f ("net: add dedicated kmem_cache for typical/small skb->head")
> Reported-by: Antonius <antonius@xxxxxxxxxxxxxxxxx>
> Closes: https://lore.kernel.org/netdev/CAK8a0jxC5L5N7hq-DT2_NhUyjBxrPocoiDazzsBk4TGgT1r4-A@xxxxxxxxxxxxxx/
> Signed-off-by: Jiayuan Chen <jiayuan.chen@xxxxxxxxx>

Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>