Re: [syzbot] BUG: "hc->tx_t_ipi == NUM" holds (exception!) at net/dccp/ccids/ccid3.c:LINE/ccid3_upda

Hi,

On 1/10/22 08:16, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    82192cb497f9 Merge branch 'ena-capabilities-field-and-cosm..
> git tree:       net-next
> console output: https://syzkaller.appspot.com/x/log.txt?xec95c7b00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x22a0a6462d9ff7d
> dashboard link: https://syzkaller.appspot.com/bug?extid”641ba6c1d768b1e35e
> compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x0ea4e3b00000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x977b73b00000
> 
> Bisection is inconclusive: the issue happens on the oldest tested release.

Regarding commit 793734b587a6:
Author: Gerrit Renker <[email protected]>
Date:   Mon Feb 27 12:29:44 2012 -0700

    dccp ccid-3: replace incorrect BUG_ON


Is this an algorithm (RFC) problem or a Linux implementation problem?

Could we round a calculated 0 (unbounded) t_ipi up to 1?

Or is just something "nasty" that the reproducer program is doing?


I ran the C reproducer for about 15 seconds and got 71 occurrences
of this
	DCCP_BUG_ON(hc->tx_t_ipi = 0);

> 
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x698c63b00000
> final oops:     https://syzkaller.appspot.com/x/report.txt?xe98c63b00000
> console output: https://syzkaller.appspot.com/x/log.txt?x698c63b00000
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: [email protected]
> 
> BUG: "hc->tx_t_ipi = 0" holds (exception!) at net/dccp/ccids/ccid3.c:90/ccid3_update_send_interval()
> CPU: 0 PID: 29976 Comm: syz-executor890 Not tainted 5.16.0-rc8-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> Call Trace:
>  <TASK>
>  __dump_stack lib/dump_stack.c:88 [inline]
>  dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
>  ccid3_update_send_interval net/dccp/ccids/ccid3.c:90 [inline]
>  ccid3_update_send_interval.cold+0x87/0x93 net/dccp/ccids/ccid3.c:86
>  ccid3_hc_tx_update_s net/dccp/ccids/ccid3.c:169 [inline]
>  ccid3_hc_tx_packet_sent+0x12e/0x160 net/dccp/ccids/ccid3.c:353
>  ccid_hc_tx_packet_sent net/dccp/ccid.h:175 [inline]
>  dccp_xmit_packet+0x2f2/0x750 net/dccp/output.c:289
>  dccp_write_xmit+0x16d/0x1d0 net/dccp/output.c:366
>  dccp_sendmsg+0x922/0xc90 net/dccp/proto.c:783
>  inet_sendmsg+0x99/0xe0 net/ipv4/af_inet.c:819
>  sock_sendmsg_nosec net/socket.c:705 [inline]
>  sock_sendmsg+0xcf/0x120 net/socket.c:725
>  ____sys_sendmsg+0x331/0x810 net/socket.c:2413
>  ___sys_sendmsg+0xf3/0x170 net/socket.c:2467
>  __sys_sendmmsg+0x195/0x470 net/socket.c:2553
>  __do_sys_sendmmsg net/socket.c:2582 [inline]
>  __se_sys_sendmmsg net/socket.c:2579 [inline]
>  __x64_sys_sendmmsg+0x99/0x100 net/socket.c:2579
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
> RIP: 0033:0x7f2cf5f36da9
> Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 e1 15 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007f2cf5ee4308 EFLAGS: 00000246 ORIG_RAX: 0000000000000133
> RAX: ffffffffffffffda RBX: 0000000000000002 RCX: 00007f2cf5f36da9
> RDX: 0000000000000001 RSI: 000000002000bf40 RDI: 0000000000000004
> RBP: 00007f2cf5fbf4c8 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 00007f2cf5fbf4c0
> R13: 00007f2cf5f8c5dc R14: 00007f2cf5ee4400 R15: 0000000000022000
>  </TASK>
> 
> 
> ---
> This report is generated by a bot. It may contain errors.
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> syzbot engineers can be reached at [email protected].
> 
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> For information about bisection process see: https://goo.gl/tpsmEJ#bisection
> syzbot can test patches for this issue, for details see:
> https://goo.gl/tpsmEJ#testing-patches

thanks.

-- 
~Randy