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.
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()
Fixes: 8c3088f895a0 ("tcp: be more careful in tcp_fragment()") Signed-off-by: Tim Froidcoeur tim.froidcoeur@tessares.net Signed-off-by: Matthieu Baerts matthieu.baerts@tessares.net Reviewed-by: Christoph Paasch cpaasch@apple.com Signed-off-by: Sasha Levin sashal@kernel.org --- include/net/tcp.h | 4 ++++ 1 file changed, 4 insertions(+)
diff --git a/include/net/tcp.h b/include/net/tcp.h index 7994e569644e0..785c4ef4e1bf8 100644 --- a/include/net/tcp.h +++ b/include/net/tcp.h @@ -1702,6 +1702,10 @@ static inline struct sk_buff *tcp_rtx_queue_tail(const struct sock *sk) { struct sk_buff *skb = tcp_send_head(sk);
+ /* empty retransmit queue, for example due to zero window */ + if (skb == tcp_write_queue_head(sk)) + return NULL; + return skb ? tcp_write_queue_prev(sk, skb) : tcp_write_queue_tail(sk); }