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| author | Eric Dumazet <edumazet@google.com> | 2026-06-12 19:25:17 +0300 |
|---|---|---|
| committer | Jakub Kicinski <kuba@kernel.org> | 2026-06-15 22:51:04 +0300 |
| commit | 2bf43d0e2e6a27d52a7d624e2d6b9116972e8a22 (patch) | |
| tree | a57d8226510288943d3caf939e624bf5f5f356c1 | |
| parent | f4c3d89fc986b0da196ddfc6cfe0ea5d5d08bec6 (diff) | |
| download | linux-2bf43d0e2e6a27d52a7d624e2d6b9116972e8a22.tar.xz | |
tcp: ipv6: clamp default adverting MSS to avoid GSO_BY_FRAGS (0xFFFF)
When MTU is large, ip6_default_advmss() can return IPV6_MAXPLEN (65535).
This is interpreted by TCP as mss_clamp, allowing the MSS to reach 65535.
However, 0xFFFF is also used as a magic value GSO_BY_FRAGS in the kernel.
If a TCP packet with gso_size=0xFFFF is passed to skb_segment(), it will
be mistakenly treated as GSO_BY_FRAGS, leading to a NULL pointer
dereference because local TCP packets do not use frag_list.
Fix this by returning min(IPV6_MAXPLEN, GSO_BY_FRAGS - 1) (65534) from
ip6_default_advmss() when MTU is large.
Also update the stale comment in ip6_default_advmss() which suggested
that IPV6_MAXPLEN is returned to mean "any MSS".
Fixes: 3953c46c3ac7 ("sk_buff: allow segmenting based on frag sizes")
Reported-by: syzbot+ebdb22d461c904fc3cb2@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/6a2c3193.8812e0fc.3c3fa4.0001.GAE@google.com/T/#u
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260612162517.83394-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
| -rw-r--r-- | net/ipv6/route.c | 6 |
1 files changed, 3 insertions, 3 deletions
diff --git a/net/ipv6/route.c b/net/ipv6/route.c index 636f0120d7e3..3c97ba01297a 100644 --- a/net/ipv6/route.c +++ b/net/ipv6/route.c @@ -3275,11 +3275,11 @@ static unsigned int ip6_default_advmss(const struct dst_entry *dst) /* * Maximal non-jumbo IPv6 payload is IPV6_MAXPLEN and * corresponding MSS is IPV6_MAXPLEN - tcp_header_size. - * IPV6_MAXPLEN is also valid and means: "any MSS, - * rely only on pmtu discovery" + * Limit the default MSS to GSO_BY_FRAGS - 1 to avoid + * collision with the GSO_BY_FRAGS magic value (0xFFFF). */ if (mtu > IPV6_MAXPLEN - sizeof(struct tcphdr)) - mtu = IPV6_MAXPLEN; + mtu = min_t(unsigned int, IPV6_MAXPLEN, GSO_BY_FRAGS - 1); return mtu; } |