From: Eric Dumazet Date: Fri, 12 Jun 2026 16:25:17 +0000 (+0000) Subject: tcp: ipv6: clamp default adverting MSS to avoid GSO_BY_FRAGS (0xFFFF) X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=2bf43d0e2e6a27d52a7d624e2d6b9116972e8a22;p=thirdparty%2Fkernel%2Flinux.git 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 Reviewed-by: Kuniyuki Iwashima Link: https://patch.msgid.link/20260612162517.83394-1-edumazet@google.com Signed-off-by: Jakub Kicinski --- diff --git a/net/ipv6/route.c b/net/ipv6/route.c index 636f0120d7e38..3c97ba01297aa 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; }