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

JIRA: https://issues.redhat.com/browse/RHEL-115628
CVE: CVE-2025-39955

Upstream commit:
commit 45c8a6c
Author: Kuniyuki Iwashima <kuniyu@google.com>
Date:   Mon Sep 15 17:56:46 2025 +0000

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

    syzbot reported the splat below where a socket had tcp_sk(sk)->fastopen_rsk
    in the TCP_ESTABLISHED 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(AF_UNSPEC)
      3. connect() to another destination

    As of accept(), sk->sk_state is TCP_SYN_RECV, and tcp_disconnect() changes
    it to TCP_CLOSE and makes connect() possible, which restarts timers.

    Since tcp_disconnect() forgot to clear tcp_sk(sk)->fastopen_rsk, the
    retransmit timer triggered the warning and the intended packet was not
    retransmitted.

    Let's call reqsk_fastopen_remove() in tcp_disconnect().

    [0]:
    WARNING: CPU: 2 PID: 0 at net/ipv4/tcp_timer.c:542 tcp_retransmit_timer (net/ipv4/tcp_timer.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:tcp_retransmit_timer (net/ipv4/tcp_timer.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>
     tcp_write_timer (net/ipv4/tcp_timer.c:738)
     call_timer_fn (kernel/time/timer.c:1747)
     __run_timers (kernel/time/timer.c:1799 kernel/time/timer.c:2372)
     timer_expire_remote (kernel/time/timer.c:2385 kernel/time/timer.c:2376 kernel/time/timer.c:2135)
     tmigr_handle_remote_up (kernel/time/timer_migration.c:944 kernel/time/timer_migration.c:1035)
     __walk_groups.isra.0 (kernel/time/timer_migration.c:533 (discriminator 1))
     tmigr_handle_remote (kernel/time/timer_migration.c:1096)
     handle_softirqs (./arch/x86/include/asm/jump_label.h:36 ./include/trace/events/irq.h:142 kernel/softirq.c:580)
     irq_exit_rcu (kernel/softirq.c:614 kernel/softirq.c:453 kernel/softirq.c:680 kernel/softirq.c:696)
     sysvec_apic_timer_interrupt (arch/x86/kernel/apic/apic.c:1050 (discriminator 35) arch/x86/kernel/apic/apic.c:1050 (discriminator 35))
     </IRQ>

    Fixes: 8336886 ("tcp: TCP Fast Open Server - support TFO listeners")
    Reported-by: syzkaller <syzkaller@googlegroups.com>
    Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
    Link: https://patch.msgid.link/20250915175800.118793-2-kuniyu@google.com
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: Paolo Abeni

net/ipv4/tcp.c

@@ -3162,6 +3162,7 @@ int tcp_disconnect(struct sock *sk, int flags)
 	struct inet_connection_sock *icsk = inet_csk(sk);
 	struct tcp_sock *tp = tcp_sk(sk);
 	int old_state = sk->sk_state;
+	struct request_sock *req;
 	u32 seq;
 
 	if (old_state != TCP_CLOSE)
@@ -3272,6 +3273,10 @@ int tcp_disconnect(struct sock *sk, int flags)
 
 
 	/* Clean up fastopen related fields */
+	req = rcu_dereference_protected(tp->fastopen_rsk,
+					lockdep_sock_is_held(sk));
+	if (req)
+		reqsk_fastopen_remove(sk, req, false);
 	tcp_free_fastopen_req(tp);
 	inet->defer_connect = 0;
 	tp->fastopen_client_fail = 0;