syzkaller found a reliable way to crash the host, hitting a BUG()
in __tcp_retransmit_skb()

Malicous MSG_FASTOPEN is the root cause. We need to purge write queue
in tcp_connect_init() at the point we init snd_una/write_seq.

This patch also replaces the BUG() by a less intrusive WARN_ON_ONCE()

kernel BUG at net/ipv4/tcp_output.c:2837!
invalid opcode: 0000 [#1] SMP KASAN
Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in:
CPU: 0 PID: 5276 Comm: syz-executor0 Not tainted 4.17.0-rc3+ #51
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:__tcp_retransmit_skb+0x2992/0x2eb0 net/ipv4/tcp_output.c:2837
RSP: 0000:ffff8801dae06ff8 EFLAGS: 00010206
RAX: ffff8801b9fe61c0 RBX: 00000000ffc18a16 RCX: ffffffff864e1a49
RDX: 0000000000000100 RSI: ffffffff864e2e12 RDI: 0000000000000005
RBP: ffff8801dae073a0 R08: ffff8801b9fe61c0 R09: ffffed0039c40dd2
R10: ffffed0039c40dd2 R11: ffff8801ce206e93 R12: 00000000421eeaad
R13: ffff8801ce206d4e R14: ffff8801ce206cc0 R15: ffff8801cd4f4a80
FS:  0000000000000000(0000) GS:ffff8801dae00000(0063) knlGS:00000000096bc900
CS:  0010 DS: 002b ES: 002b CR0: 0000000080050033
CR2: 0000000020000000 CR3: 00000001c47b6000 CR4: 00000000001406f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <IRQ>
 tcp_retransmit_skb+0x2e/0x250 net/ipv4/tcp_output.c:2923
 tcp_retransmit_timer+0xc50/0x3060 net/ipv4/tcp_timer.c:488
 tcp_write_timer_handler+0x339/0x960 net/ipv4/tcp_timer.c:573
 tcp_write_timer+0x111/0x1d0 net/ipv4/tcp_timer.c:593
 call_timer_fn+0x230/0x940 kernel/time/timer.c:1326
 expire_timers kernel/time/timer.c:1363 [inline]
 __run_timers+0x79e/0xc50 kernel/time/timer.c:1666
 run_timer_softirq+0x4c/0x70 kernel/time/timer.c:1692
 __do_softirq+0x2e0/0xaf5 kernel/softirq.c:285
 invoke_softirq kernel/softirq.c:365 [inline]
 irq_exit+0x1d1/0x200 kernel/softirq.c:405
 exiting_irq arch/x86/include/asm/apic.h:525 [inline]
 smp_apic_timer_interrupt+0x17e/0x710 arch/x86/kernel/apic/apic.c:1052
 apic_timer_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:863

Fixes: cf60af03 ("net-tcp: Fast Open client - sendmsg(MSG_FASTOPEN)")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>