[PATCH 6.6.y] net: skb: fix cross-cache free of KFENCE-allocated skb head

[Sasha Levin <sashal@kernel.org>]

From: Jiayuan Chen <jiayuan.chen@linux.dev>

[ Upstream commit 0f42e3f4fe2a58394e37241d02d9ca6ab7b7d516 ]

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@bluedragonsec.com>
Closes: https://lore.kernel.org/netdev/CAK8a0jxC5L5N7hq-DT2_NhUyjBxrPocoiDazzsBk4TGgT1r4-A@mail.gmail.com/
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260403014517.142550-1-jiayuan.chen@linux.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
[ adapted variable names ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 net/core/skbuff.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 4c28954f915fa..c81ef99d39b04 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -943,10 +943,7 @@ static bool skb_pp_recycle(struct sk_buff *skb, void *data, bool napi_safe)
 
 static void skb_kfree_head(void *head, unsigned int end_offset)
 {
-	if (end_offset == SKB_SMALL_HEAD_HEADROOM)
-		kmem_cache_free(skb_small_head_cache, head);
-	else
-		kfree(head);
+	kfree(head);
 }
 
 static void skb_free_head(struct sk_buff *skb, bool napi_safe)
-- 
2.53.0