tcp: ipv6: clamp default adverting MSS to avoid GSO_BY_FRAGS (0xFFFF)

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>