CVE-2024-41007: tcp: avoid too many retransmit packets

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

tcp: avoid too many retransmit packets

If a TCP socket is using TCPUSERTIMEOUT, and the other peer retracted its window to zero, tcpretransmittimer() can retransmit a packet every two jiffies (2 ms for HZ=1000), for about 4 minutes after TCPUSERTIMEOUT has 'expired'.

The fix is to make sure tcprtxprobe0timedout() takes icsk->icskusertimeout into account.

Before blamed commit, the socket would not timeout after icsk->icskusertimeout, but would use standard exponential backoff for the retransmits.

Also worth noting that before commit e89688e3e978 ("net: tcp: fix unexcepted socket die when sndwnd is 0"), the issue would last 2 minutes instead of 4.