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

[Eric Dumazet]

On Thu, Apr 2, 2026 at 7:23 PM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
>
> 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>

Note: we can later in net-next remove skb_kfree_head() completely.