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net: skbuff: fix pull-bound underflow in skb_checksum_setup_ipv6()

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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: 1431fb31ec ("xen-netback: fix fragment detection in checksum setup")
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Shihuang Liu <shlomojune6@gmail.com>
Link: https://patch.msgid.link/20260919133604.50948-1-shlomojune6@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Shihuang Liu 2026-09-19 21:36:04 +08:00 committed by Jakub Kicinski
parent 7104a37071
commit 3b4e0b0c00
1 changed files with 6 additions and 3 deletions
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9
net/core/skbuff.c
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@ -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;