[PATCH net-next v2 1/8] net: skbuff: don't BUG() on bad arguments to pskb_expand_head()

[Josef Bacik <josef@toxicpanda.com>]

pskb_expand_head() BUG()s if it is handed a negative nhead or a shared
skb.  Both are bugs in the caller, and both keep getting hit: commit
2cbb259ec4f8 ("bpf: Reject negative head_room in __bpf_skb_change_head")
and commit 64e6a754d33d ("llc: do not use skb_get() before
dev_queue_xmit()") each fixed a crash here that took down the whole box.

The function already returns an error that every caller has to handle,
and at this point it hasn't touched the skb.  Return -EINVAL instead of
crashing.  The warning is DEBUG_NET_WARN_ON_ONCE(), so debug kernels
still point at the caller, while production kernels, including ones
running with panic_on_warn, just fail the call.

Assisted-by: LLM
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 net/core/skbuff.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 43ebe61c7fc4..d226fe77d484 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -2303,9 +2303,15 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
 	u8 *data;
 	int i;
 
-	BUG_ON(nhead < 0);
+	if (unlikely(nhead < 0)) {
+		DEBUG_NET_WARN_ON_ONCE(1);
+		return -EINVAL;
+	}
 
-	BUG_ON(skb_shared(skb));
+	if (unlikely(skb_shared(skb))) {
+		DEBUG_NET_WARN_ON_ONCE(1);
+		return -EINVAL;
+	}
 
 	skb_zcopy_downgrade_managed(skb);
 

-- 
2.55.0