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SubjectRe: [PATCH 4.14] tcp: fix tcp_rtx_queue_tail in case of empty retransmit queue
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Hi Sasha,

Thank you for your reply!

On 31/08/2019 14:20, Sasha Levin wrote:
> 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...

Commit 8c3088f895a0 ("tcp: be more careful in tcp_fragment()") was not a
simple cherry-pick of the original one from master (b617158dc096)
because there is a specific TCP rtx queue only since v4.15. For more
details, please see the commit message of b617158dc096 ("tcp: be more
careful in tcp_fragment()").

The BUG() is hit due to the specific code added to versions older than
v4.15. The comment in skb_queue_prev() (include/linux/skbuff.h:1406),
just before the BUG_ON() somehow suggests to add a check before using
it, what Tim did.

In master, this code path causing the issue will not be taken because
the implementation of tcp_rtx_queue_tail() is different:

tcp_fragment() → tcp_rtx_queue_tail() → tcp_write_queue_prev() →
skb_queue_prev() → BUG_ON()

Because this patch is specific to versions older than the two last
stable ones but still linked to the network architecture, who can review
and approve it? :)

Cheers,
Matt
--
Matthieu Baerts | R&D Engineer
matthieu.baerts@tessares.net
Tessares SA | Hybrid Access Solutions
www.tessares.net
1 Avenue Jean Monnet, 1348 Louvain-la-Neuve, Belgium

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