Affected: Linux

In the Linux kernel, the following vulnerability has been resolved:

tcp: fix tcp_mtup_probe_success vs wrong snd_cwnd

syzbot got a new report [1] finally pointing to a very old bug, added in initial support for MTU probing.

tcp_mtu_probe() has checks about starting an MTU probe if tcp_snd_cwnd(tp) >= 11.

But nothing prevents tcp_snd_cwnd(tp) to be reduced later and before the MTU probe succeeds.

This bug would lead to potential zero-divides.

Debugging added in commit 40570375356c ("tcp: add accessors to read/set tp->snd_cwnd") has paid off :)

While we are at it, address potential overflows in this code.

[1] WARNING: CPU: 1 PID: 14132 at include/net/tcp.h:1219 tcp_mtup_probe_success+0x366/0x570 net/ipv4/tcp_input.c:2712 Modules linked in: CPU: 1 PID: 14132 Comm: syz-executor.2 Not tainted 5.18.0-syzkaller-07857-gbabf0bb978e3 #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011 RIP: 0010:tcp_snd_cwnd_set include/net/tcp.h:1219 [inline] RIP: 0010:tcp_mtup_probe_success+0x366/0x570 net/ipv4/tcp_input.c:2712 Code: 74 08 48 89 ef e8 da 80 17 f9 48 8b 45 00 65 48 ff 80 80 03 00 00 48 83 c4 30 5b 41 5c 41 5d 41 5e 41 5f 5d c3 e8 aa b0 c5 f8 <0f> 0b e9 16 fe ff ff 48 8b 4c 24 08 80 e1 07 38 c1 0f 8c c7 fc ff RSP: 0018:ffffc900079e70f8 EFLAGS: 00010287 RAX: ffffffff88c0f7f6 RBX: ffff8880756e7a80 RCX: 0000000000040000 RDX: ffffc9000c6c4000 RSI: 0000000000031f9e RDI: 0000000000031f9f RBP: 0000000000000000 R08: ffffffff88c0f606 R09: ffffc90…