[PATCH net] net: skbuff: reject invalid pull bounds

skb_maybe_pull_tail() subtracts skb_headlen(skb) from the unsigned max
argument and passes the result to __pskb_pull_tail() as a signed int.  The
function does not ensure that max is at least skb_headlen(skb).

This can happen while parsing IPv6 extension headers when an skb already
has a linear area larger than MAX_IPV6_HDR_LEN.  Once the parser needs data
beyond the linear area, max - skb_headlen(skb) wraps and is converted to a
negative delta.  __pskb_pull_tail() then passes that negative length to
skb_copy_bits(), where it can become a very large copy length.

Reject this inconsistent bound before doing the subtraction.  In this case
pulling up to max cannot satisfy len because the initial check established
that skb_headlen(skb) is smaller than len.

Fixes: 1431fb31ecba ("xen-netback: fix fragment detection in checksum setup")
Cc: [email protected]
Assisted-by: LLM
Signed-off-by: Shihuang Liu <[email protected]>
---
 net/core/skbuff.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index c82a147..33022f7 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -5838,6 +5838,9 @@ static int skb_maybe_pull_tail(struct sk_buff *skb, unsigned int len,
 	if (max > skb->len)
 		max = skb->len;
 
+	if (max < skb_headlen(skb))
+		return -EPROTO;
+
 	if (__pskb_pull_tail(skb, max - skb_headlen(skb)) == NULL)
 		return -ENOMEM;
 
-- 
2.43.0