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

[Jiayuan Chen]

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);
-- 
2.43.0