Re: [syzbot] WARNING in mptcp_sendmsg_frag

[Paolo Abeni <pabeni@redhat.com>]

On Tue, 2021-09-14 at 21:05 -0700, syzbot wrote:
> HEAD commit:    f306b90c69ce Merge tag 'smp-urgent-2021-09-12' of git://gi..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=10694371300000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=2bfb13fa4527da4e
> dashboard link: https://syzkaller.appspot.com/bug?extid=263a248eec3e875baa7b
> compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.1
> 
> Unfortunately, I don't have any reproducer for this issue yet.
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+263a248eec3e875baa7b@syzkaller.appspotmail.com
> 
> ------------[ cut here ]------------
> WARNING: CPU: 1 PID: 810 at net/mptcp/protocol.c:1366 mptcp_sendmsg_frag+0x1362/0x1bc0 net/mptcp/protocol.c:1366
> Modules linked in:
> CPU: 1 PID: 810 Comm: syz-executor.4 Not tainted 5.14.0-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> RIP: 0010:mptcp_sendmsg_frag+0x1362/0x1bc0 net/mptcp/protocol.c:1366
> Code: ff 4c 8b 74 24 50 48 8b 5c 24 58 e9 0f fb ff ff e8 13 44 8b f8 4c 89 e7 45 31 ed e8 98 57 2e fe e9 81 f4 ff ff e8 fe 43 8b f8 <0f> 0b 41 bd ea ff ff ff e9 6f f4 ff ff 4c 89 e7 e8 b9 8e d2 f8 e9
> RSP: 0018:ffffc9000531f6a0 EFLAGS: 00010216
> RAX: 000000000000697f RBX: 0000000000000000 RCX: ffffc90012107000
> RDX: 0000000000040000 RSI: ffffffff88eac9e2 RDI: 0000000000000003
> RBP: ffff888078b15780 R08: 0000000000000000 R09: 0000000000000000
> R10: ffffffff88eac017 R11: 0000000000000000 R12: ffff88801de0a280
> R13: 0000000000006b58 R14: ffff888066278280 R15: ffff88803c2fe9c0
> FS:  00007fd9f866e700(0000) GS:ffff8880b9d00000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007faebcb2f718 CR3: 00000000267cb000 CR4: 00000000001506e0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  __mptcp_push_pending+0x1fb/0x6b0 net/mptcp/protocol.c:1547
>  mptcp_release_cb+0xfe/0x210 net/mptcp/protocol.c:3003
>  release_sock+0xb4/0x1b0 net/core/sock.c:3206
>  sk_stream_wait_memory+0x604/0xed0 net/core/stream.c:145
>  mptcp_sendmsg+0xc39/0x1bc0 net/mptcp/protocol.c:1749
>  inet6_sendmsg+0x99/0xe0 net/ipv6/af_inet6.c:643
>  sock_sendmsg_nosec net/socket.c:704 [inline]
>  sock_sendmsg+0xcf/0x120 net/socket.c:724
>  sock_write_iter+0x2a0/0x3e0 net/socket.c:1057
>  call_write_iter include/linux/fs.h:2163 [inline]
>  new_sync_write+0x40b/0x640 fs/read_write.c:507
>  vfs_write+0x7cf/0xae0 fs/read_write.c:594
>  ksys_write+0x1ee/0x250 fs/read_write.c:647
>  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: 0033:0x4665f9
> Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 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 bc ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007fd9f866e188 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
> RAX: ffffffffffffffda RBX: 000000000056c038 RCX: 00000000004665f9
> RDX: 00000000000e7b78 RSI: 0000000020000000 RDI: 0000000000000003
> RBP: 00000000004bfcc4 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 000000000056c038
> R13: 0000000000a9fb1f R14: 00007fd9f866e300 R15: 0000000000022000
> 
> 
> ---
> This report is generated by a bot. It may contain errors.
> See https://goo.gl/tpsmEJ for more information about syzbot.
> syzbot engineers can be reached at syzkaller@googlegroups.com.
> 
> syzbot will keep track of this issue. See:
> https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

I think (mostly wild guess), this is caused by syzbot enabling tcp skb
tx recycling, so that in mptcp_sendmsg_frag() we end up with:

ssk->sk_tx_skb_cache != NULL

but:

skb_ext_find(ssk->sk_tx_skb_cache, SKB_EXT_MPTCP) == NULL.

Hard to say given the lack of reproducer. For -net we could do
something alike the following (some more testing needed), while for
net-next we have the sk_tx_skb_cache removal pending which should
address the issue.

/P
---
diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 2602f1386160..f0673541a764 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -1325,7 +1325,7 @@ static int mptcp_sendmsg_frag(struct sock *sk, struct sock *ssk,
        }
 
 alloc_skb:
-       if (!must_collapse && !ssk->sk_tx_skb_cache &&
+       if (!must_collapse &&
            !mptcp_alloc_tx_skb(sk, ssk, info->data_lock_held))
                return 0;