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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 Signed-off-by: Shihuang Liu --- 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