Re: [PATCH 4.14] tcp: fix tcp_rtx_queue_tail in case of empty retransmit queue

[Sasha Levin]

On Sat, Aug 24, 2019 at 08:03:51AM +0200, Tim Froidcoeur wrote:
> Commit 8c3088f895a0 ("tcp: be more careful in tcp_fragment()")
> triggers following stack trace:
>
> [25244.848046] kernel BUG at ./include/linux/skbuff.h:1406!
> [25244.859335] RIP: 0010:skb_queue_prev+0x9/0xc
> [25244.888167] Call Trace:
> [25244.889182]  <IRQ>
> [25244.890001]  tcp_fragment+0x9c/0x2cf
> [25244.891295]  tcp_write_xmit+0x68f/0x988
> [25244.892732]  __tcp_push_pending_frames+0x3b/0xa0
> [25244.894347]  tcp_data_snd_check+0x2a/0xc8
> [25244.895775]  tcp_rcv_established+0x2a8/0x30d
> [25244.897282]  tcp_v4_do_rcv+0xb2/0x158
> [25244.898666]  tcp_v4_rcv+0x692/0x956
> [25244.899959]  ip_local_deliver_finish+0xeb/0x169
> [25244.901547]  __netif_receive_skb_core+0x51c/0x582
> [25244.903193]  ? inet_gro_receive+0x239/0x247
> [25244.904756]  netif_receive_skb_internal+0xab/0xc6
> [25244.906395]  napi_gro_receive+0x8a/0xc0
> [25244.907760]  receive_buf+0x9a1/0x9cd
> [25244.909160]  ? load_balance+0x17a/0x7b7
> [25244.910536]  ? vring_unmap_one+0x18/0x61
> [25244.911932]  ? detach_buf+0x60/0xfa
> [25244.913234]  virtnet_poll+0x128/0x1e1
> [25244.914607]  net_rx_action+0x12a/0x2b1
> [25244.915953]  __do_softirq+0x11c/0x26b
> [25244.917269]  ? handle_irq_event+0x44/0x56
> [25244.918695]  irq_exit+0x61/0xa0
> [25244.919947]  do_IRQ+0x9d/0xbb
> [25244.921065]  common_interrupt+0x85/0x85
> [25244.922479]  </IRQ>
>
> tcp_rtx_queue_tail() (called by tcp_fragment()) can call
> tcp_write_queue_prev() on the first packet in the queue, which will trigger
> the BUG in tcp_write_queue_prev(), because there is no previous packet.
>
> This happens when the retransmit queue is empty, for example in case of a
> zero window.
>
> Patch is needed for 4.4, 4.9 and 4.14 stable branches.

There needs to be a better explanation of why it's not needed
upstream...

--
Thanks,
Sasha