Re: [PATCH net v1] net: skb: fix cross-cache free of KFENCE-allocated skb head

[Eric Dumazet]

On Wed, Apr 1, 2026 at 8:32 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 adding an is_kfence_address() check in skb_kfree_head().
> When the head is a KFENCE object, we skip the kmem_cache_free() path
> and fall through to kfree(), which correctly handles KFENCE objects
> via kfence_free(). The check compiles away when CONFIG_KFENCE is
> disabled.
>
> 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>
> ---
>  net/core/skbuff.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 0e217041958a..87cecd40381b 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -91,6 +91,7 @@
>  #include <linux/user_namespace.h>
>  #include <linux/indirect_call_wrapper.h>
>  #include <linux/textsearch.h>
> +#include <linux/kfence.h>
>
>  #include "dev.h"
>  #include "devmem.h"
> @@ -1083,7 +1084,8 @@ static int skb_pp_frag_ref(struct sk_buff *skb)
>
>  static void skb_kfree_head(void *head, unsigned int end_offset)
>  {
> -       if (end_offset == SKB_SMALL_HEAD_HEADROOM)
> +       if (end_offset == SKB_SMALL_HEAD_HEADROOM &&
> +           !is_kfence_address(head))
>                 kmem_cache_free(net_hotdata.skb_small_head_cache, head);
>         else
>                 kfree(head);

Oh well, time to simply call kfree(head) ?

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 3d6978dd0aa83f63984b994359d0c914c6427a00..6b35aed23b8936409f6b0d4ee8d378f726c569c6
100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -1072,10 +1072,7 @@ static int skb_pp_frag_ref(struct sk_buff *skb)

 static void skb_kfree_head(void *head, unsigned int end_offset)
 {
-       if (end_offset == SKB_SMALL_HEAD_HEADROOM)
-               kmem_cache_free(net_hotdata.skb_small_head_cache, head);
-       else
-               kfree(head);
+       kfree(head);
 }

 static void skb_free_head(struct sk_buff *skb)