CVE-2025-39955: tcp: Clear tcp_sk(sk)->fastopen_rsk in tcp_disconnect().

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

tcp: Clear tcpsk(sk)->fastopenrsk in tcpdisconnect().

syzbot reported the splat below where a socket had tcpsk(sk)->fastopenrsk in the TCPESTABLISHED state. [0]

syzbot reused the server-side TCP Fast Open socket as a new client before the TFO socket completes 3WHS:

1. accept() 2. connect(AFUNSPEC) 3. connect() to another destination

As of accept(), sk->skstate is TCPSYNRECV, and tcpdisconnect() changes it to TCPCLOSE and makes connect() possible, which restarts timers.

Since tcpdisconnect() forgot to clear tcpsk(sk)->fastopenrsk, the retransmit timer triggered the warning and the intended packet was not retransmitted.

Let's call reqskfastopenremove() in tcpdisconnect().

[0]: WARNING: CPU: 2 PID: 0 at net/ipv4/tcptimer.c:542 tcpretransmittimer (net/ipv4/tcptimer.c:542 (discriminator 7)) Modules linked in: CPU: 2 UID: 0 PID: 0 Comm: swapper/2 Not tainted 6.17.0-rc5-g201825fb4278 #62 PREEMPT(voluntary) Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014 RIP: 0010:tcpretransmittimer (net/ipv4/tcptimer.c:542 (discriminator 7)) Code: 41 55 41 54 55 53 48 8b af b8 08 00 00 48 89 fb 48 85 ed 0f 84 55 01 00 00 0f b6 47 12 3c 03 74 0c 0f b6 47 12 3c 04 74 04 90 <0f> 0b 90 48 8b 85 c0 00 00 00 48 89 ef 48 8b 40 30 e8 6a 4f 06 3e RSP: 0018:ffffc900002f8d40 EFLAGS: 00010293 RAX: 0000000000000002 RBX: ffff888106911400 RCX: 0000000000000017 RDX: 0000000002517619 RSI: ffffffff83764080 RDI: ffff888106911400 RBP: ffff888106d5c000 R08: 0000000000000001 R09: ffffc900002f8de8 R10: 00000000000000c2 R11: ffffc900002f8ff8 R12: ffff888106911540 R13: ffff888106911480 R14: ffff888106911840 R15: ffffc900002f8de0 FS: 0000000000000000(0000) GS:ffff88907b768000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007f8044d69d90 CR3: 0000000002c30003 CR4: 0000000000370ef0 Call Trace: <IRQ> tcpwritetimer (net/ipv4/tcptimer.c:738) calltimerfn (kernel/time/timer.c:1747) runtimers (kernel/time/timer.c:1799 kernel/time/timer.c:2372) timerexpireremote (kernel/time/timer.c:2385 kernel/time/timer.c:2376 kernel/time/timer.c:2135) tmigrhandleremoteup (kernel/time/timermigration.c:944 kernel/time/timermigration.c:1035) walkgroups.isra.0 (kernel/time/timermigration.c:533 (discriminator 1)) tmigrhandleremote (kernel/time/timermigration.c:1096) handlesoftirqs (./arch/x86/include/asm/jumplabel.h:36 ./include/trace/events/irq.h:142 kernel/softirq.c:580) irqexitrcu (kernel/softirq.c:614 kernel/softirq.c:453 kernel/softirq.c:680 kernel/softirq.c:696) sysvecapictimerinterrupt (arch/x86/kernel/apic/apic.c:1050 (discriminator 35) arch/x86/kernel/apic/apic.c:1050 (discriminator 35)) </IRQ>