[PATCH net v2] net: skbuff: fix pull-bound underflow in skb_checksum_setup_ipv6()

[Shihuang Liu <shlomojune6@gmail.com>]

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.

Pass the requested length itself as the pull bound at the three
extension-header call sites, so the delta can no longer go negative.

Fixes: 1431fb31ecba ("xen-netback: fix fragment detection in checksum setup")
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Shihuang Liu <shlomojune6@gmail.com>
---
Changes in v2:
- fix the three extension-header call sites in
  skb_checksum_setup_ipv6() to pass the requested length as the pull
  bound, keeping the helper's fast path untouched.

v1:
https://lore.kernel.org/r/20260823142620.126424-1-shlomojune6@gmail.com/

 net/core/skbuff.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index cc3b4b70288b4..a2d7b2fbc006d 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -5977,7 +5977,8 @@ static int skb_checksum_setup_ipv6(struct sk_buff *skb, bool recalculate)
 			err = skb_maybe_pull_tail(skb,
 						  off +
 						  sizeof(struct ipv6_opt_hdr),
-						  MAX_IPV6_HDR_LEN);
+						  off +
+						  sizeof(struct ipv6_opt_hdr));
 			if (err < 0)
 				goto out;
 
@@ -5992,7 +5993,8 @@ static int skb_checksum_setup_ipv6(struct sk_buff *skb, bool recalculate)
 			err = skb_maybe_pull_tail(skb,
 						  off +
 						  sizeof(struct ip_auth_hdr),
-						  MAX_IPV6_HDR_LEN);
+						  off +
+						  sizeof(struct ip_auth_hdr));
 			if (err < 0)
 				goto out;
 
@@ -6007,7 +6009,8 @@ static int skb_checksum_setup_ipv6(struct sk_buff *skb, bool recalculate)
 			err = skb_maybe_pull_tail(skb,
 						  off +
 						  sizeof(struct frag_hdr),
-						  MAX_IPV6_HDR_LEN);
+						  off +
+						  sizeof(struct frag_hdr));
 			if (err < 0)
 				goto out;
 
-- 
2.43.0