Patch "net: Remove acked SYN flag from packet in the transmit queue correctly" has been added to the 6.6-stable tree

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This is a note to let you know that I've just added the patch titled

    net: Remove acked SYN flag from packet in the transmit queue correctly

to the 6.6-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     net-remove-acked-syn-flag-from-packet-in-the-transmi.patch
and it can be found in the queue-6.6 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit f3f5b17e5fc1ea851d3abb63dc7b50dcfd548561
Author: Dong Chenchen <dongchenchen2@xxxxxxxxxx>
Date:   Sun Dec 10 10:02:00 2023 +0800

    net: Remove acked SYN flag from packet in the transmit queue correctly
    
    [ Upstream commit f99cd56230f56c8b6b33713c5be4da5d6766be1f ]
    
    syzkaller report:
    
     kernel BUG at net/core/skbuff.c:3452!
     invalid opcode: 0000 [#1] PREEMPT SMP KASAN PTI
     CPU: 0 PID: 0 Comm: swapper/0 Not tainted 6.7.0-rc4-00009-gbee0e7762ad2-dirty #135
     RIP: 0010:skb_copy_and_csum_bits (net/core/skbuff.c:3452)
     Call Trace:
     icmp_glue_bits (net/ipv4/icmp.c:357)
     __ip_append_data.isra.0 (net/ipv4/ip_output.c:1165)
     ip_append_data (net/ipv4/ip_output.c:1362 net/ipv4/ip_output.c:1341)
     icmp_push_reply (net/ipv4/icmp.c:370)
     __icmp_send (./include/net/route.h:252 net/ipv4/icmp.c:772)
     ip_fragment.constprop.0 (./include/linux/skbuff.h:1234 net/ipv4/ip_output.c:592 net/ipv4/ip_output.c:577)
     __ip_finish_output (net/ipv4/ip_output.c:311 net/ipv4/ip_output.c:295)
     ip_output (net/ipv4/ip_output.c:427)
     __ip_queue_xmit (net/ipv4/ip_output.c:535)
     __tcp_transmit_skb (net/ipv4/tcp_output.c:1462)
     __tcp_retransmit_skb (net/ipv4/tcp_output.c:3387)
     tcp_retransmit_skb (net/ipv4/tcp_output.c:3404)
     tcp_retransmit_timer (net/ipv4/tcp_timer.c:604)
     tcp_write_timer (./include/linux/spinlock.h:391 net/ipv4/tcp_timer.c:716)
    
    The panic issue was trigered by tcp simultaneous initiation.
    The initiation process is as follows:
    
          TCP A                                            TCP B
    
      1.  CLOSED                                           CLOSED
    
      2.  SYN-SENT     --> <SEQ=100><CTL=SYN>              ...
    
      3.  SYN-RECEIVED <-- <SEQ=300><CTL=SYN>              <-- SYN-SENT
    
      4.               ... <SEQ=100><CTL=SYN>              --> SYN-RECEIVED
    
      5.  SYN-RECEIVED --> <SEQ=100><ACK=301><CTL=SYN,ACK> ...
    
      // TCP B: not send challenge ack for ack limit or packet loss
      // TCP A: close
            tcp_close
               tcp_send_fin
                  if (!tskb && tcp_under_memory_pressure(sk))
                      tskb = skb_rb_last(&sk->tcp_rtx_queue); //pick SYN_ACK packet
               TCP_SKB_CB(tskb)->tcp_flags |= TCPHDR_FIN;  // set FIN flag
    
      6.  FIN_WAIT_1  --> <SEQ=100><ACK=301><END_SEQ=102><CTL=SYN,FIN,ACK> ...
    
      // TCP B: send challenge ack to SYN_FIN_ACK
    
      7.               ... <SEQ=301><ACK=101><CTL=ACK>   <-- SYN-RECEIVED //challenge ack
    
      // TCP A:  <SND.UNA=101>
    
      8.  FIN_WAIT_1 --> <SEQ=101><ACK=301><END_SEQ=102><CTL=SYN,FIN,ACK> ... // retransmit panic
    
            __tcp_retransmit_skb  //skb->len=0
                tcp_trim_head
                    len = tp->snd_una - TCP_SKB_CB(skb)->seq // len=101-100
                        __pskb_trim_head
                            skb->data_len -= len // skb->len=-1, wrap around
                ... ...
                ip_fragment
                    icmp_glue_bits //BUG_ON
    
    If we use tcp_trim_head() to remove acked SYN from packet that contains data
    or other flags, skb->len will be incorrectly decremented. We can remove SYN
    flag that has been acked from rtx_queue earlier than tcp_trim_head(), which
    can fix the problem mentioned above.
    
    Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
    Co-developed-by: Eric Dumazet <edumazet@xxxxxxxxxx>
    Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
    Signed-off-by: Dong Chenchen <dongchenchen2@xxxxxxxxxx>
    Link: https://lore.kernel.org/r/20231210020200.1539875-1-dongchenchen2@xxxxxxxxxx
    Signed-off-by: Jakub Kicinski <kuba@xxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 9ccfdc825004d..cab3c1162c3a6 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -3263,7 +3263,13 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
 	if (skb_still_in_host_queue(sk, skb))
 		return -EBUSY;
 
+start:
 	if (before(TCP_SKB_CB(skb)->seq, tp->snd_una)) {
+		if (unlikely(TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN)) {
+			TCP_SKB_CB(skb)->tcp_flags &= ~TCPHDR_SYN;
+			TCP_SKB_CB(skb)->seq++;
+			goto start;
+		}
 		if (unlikely(before(TCP_SKB_CB(skb)->end_seq, tp->snd_una))) {
 			WARN_ON_ONCE(1);
 			return -EINVAL;




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