github-mirror/linux
1
0
Fork 0
mirror of https://github.com/torvalds/linux.git synced 2026-07-27 17:47:41 +02:00
Code Issues Actions Packages Projects Releases Wiki Activity Graph

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

Browse Source 
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: 3953c46c3a ("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>
This commit is contained in:
Eric Dumazet 2026-06-12 16:25:17 +00:00 committed by Jakub Kicinski
parent f4c3d89fc9
commit 2bf43d0e2e
1 changed files with 3 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
net/ipv6/route.c
View File

@ -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;
}