tcp: Don't dequeue SYN/FIN-segments from write-queue

If a SYN/FIN-segment is on the write-queue, skb->len is 0, but the
segment actually has been transmitted. end_seq and seq of the tcp_skb_cb
in that case will indicate this difference.

We should not remove such segments from the write-queue as we might be
in SYN_SENT-state and a retransmission-timer is running. When that one
fires, packets_out will be 1, but the write-queue would be empty,
resulting in:

[   61.280214] ------------[ cut here ]------------
[   61.281307] WARNING: CPU: 0 PID: 0 at net/ipv4/tcp_timer.c:429 tcp_retransmit_timer+0x18f9/0x2660
[   61.283498] Modules linked in:
[   61.284084] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.14.142 #58
[   61.285214] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 0.5.1 01/01/2011
[   61.286644] task: ffffffff8401e1c0 task.stack: ffffffff84000000
[   61.287758] RIP: 0010:tcp_retransmit_timer+0x18f9/0x2660
[   61.288715] RSP: 0018:ffff88806ce07cb8 EFLAGS: 00010206
[   61.289669] RAX: ffffffff8401e1c0 RBX: ffff88805c998b00 RCX: 0000000000000006
[   61.290968] RDX: 0000000000000100 RSI: 0000000000000000 RDI: ffff88805c9994d8
[   61.292314] RBP: ffff88805c99919a R08: ffff88807fff901c R09: ffff88807fff9008
[   61.293547] R10: ffff88807fff9017 R11: ffff88807fff9010 R12: ffff88805c998b30
[   61.294834] R13: ffffffff844b9380 R14: 0000000000000000 R15: ffff88805c99930c
[   61.296086] FS:  0000000000000000(0000) GS:ffff88806ce00000(0000) knlGS:0000000000000000
[   61.297523] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   61.298646] CR2: 00007f721da50ff8 CR3: 0000000004014002 CR4: 00000000001606f0
[   61.299944] Call Trace:
[   61.300403]  <IRQ>
[   61.300806]  ? kvm_sched_clock_read+0x21/0x30
[   61.301689]  ? sched_clock+0x5/0x10
[   61.302433]  ? sched_clock_cpu+0x18/0x170
[   61.303173]  tcp_write_timer_handler+0x2c1/0x7a0
[   61.304038]  tcp_write_timer+0x13e/0x160
[   61.304794]  call_timer_fn+0x14a/0x5f0
[   61.305480]  ? tcp_write_timer_handler+0x7a0/0x7a0
[   61.306364]  ? __next_timer_interrupt+0x140/0x140
[   61.307229]  ? _raw_spin_unlock_irq+0x24/0x40
[   61.308033]  ? tcp_write_timer_handler+0x7a0/0x7a0
[   61.308887]  ? tcp_write_timer_handler+0x7a0/0x7a0
[   61.309760]  run_timer_softirq+0xc41/0x1080
[   61.310539]  ? trigger_dyntick_cpu.isra.33+0x180/0x180
[   61.311506]  ? ktime_get+0x13f/0x1c0
[   61.312232]  ? clockevents_program_event+0x10d/0x2f0
[   61.313158]  __do_softirq+0x20b/0x96b
[   61.313889]  irq_exit+0x1a7/0x1e0
[   61.314513]  smp_apic_timer_interrupt+0xfc/0x4d0
[   61.315386]  apic_timer_interrupt+0x8f/0xa0
[   61.316129]  </IRQ>

Followed by a panic.

So, before removing an skb with skb->len == 0, let's make sure that the
skb is really empty by checking the end_seq and seq.

This patch needs to be backported only to 4.14 and older (among those
that applied the backport of fdfc5c8594).

Fixes: fdfc5c8594 ("tcp: remove empty skb from write queue in error cases")
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jason Baron <jbaron@akamai.com>
Cc: Vladimir Rutsky <rutsky@google.com>
Cc: Soheil Hassas Yeganeh <soheil@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Christoph Paasch <cpaasch@apple.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

net/ipv4/tcp.c

@@ -922,7 +922,8 @@ static int tcp_send_mss(struct sock *sk, int *size_goal, int flags)
  */
 static void tcp_remove_empty_skb(struct sock *sk, struct sk_buff *skb)
 {
-	if (skb && !skb->len) {
+	if (skb && !skb->len &&
+	    TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq) {
 		tcp_unlink_write_queue(skb, sk);
 		tcp_check_send_head(sk, skb);
 		sk_wmem_free_skb(sk, skb);