netdev - Re: [PATCH net v4] net: ax25: Fix deadlock caused by skb_recv_datagram in ax25

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Date:   Tue, 14 Jun 2022 10:59:35 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Duoming Zhou <duoming@....edu.cn>, linux-kernel@...r.kernel.org
Cc:     jreuter@...na.de, ralf@...ux-mips.org, davem@...emloft.net,
        edumazet@...gle.com, kuba@...nel.org, netdev@...r.kernel.org,
        linux-hams@...r.kernel.org, thomas@...erried.de
Subject: Re: [PATCH net v4] net: ax25: Fix deadlock caused by
 skb_recv_datagram in ax25_recvmsg

On Fri, 2022-06-10 at 20:33 +0800, Duoming Zhou wrote:
> The skb_recv_datagram() in ax25_recvmsg() will hold lock_sock
> and block until it receives a packet from the remote. If the client
> doesn`t connect to server and calls read() directly, it will not
> receive any packets forever. As a result, the deadlock will happen.
> 
> The fail log caused by deadlock is shown below:
> 
> [  369.606973] INFO: task ax25_deadlock:157 blocked for more than 245 seconds.
> [  369.608919] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [  369.613058] Call Trace:
> [  369.613315]  <TASK>
> [  369.614072]  __schedule+0x2f9/0xb20
> [  369.615029]  schedule+0x49/0xb0
> [  369.615734]  __lock_sock+0x92/0x100
> [  369.616763]  ? destroy_sched_domains_rcu+0x20/0x20
> [  369.617941]  lock_sock_nested+0x6e/0x70
> [  369.618809]  ax25_bind+0xaa/0x210
> [  369.619736]  __sys_bind+0xca/0xf0
> [  369.620039]  ? do_futex+0xae/0x1b0
> [  369.620387]  ? __x64_sys_futex+0x7c/0x1c0
> [  369.620601]  ? fpregs_assert_state_consistent+0x19/0x40
> [  369.620613]  __x64_sys_bind+0x11/0x20
> [  369.621791]  do_syscall_64+0x3b/0x90
> [  369.622423]  entry_SYSCALL_64_after_hwframe+0x46/0xb0
> [  369.623319] RIP: 0033:0x7f43c8aa8af7
> [  369.624301] RSP: 002b:00007f43c8197ef8 EFLAGS: 00000246 ORIG_RAX: 0000000000000031
> [  369.625756] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f43c8aa8af7
> [  369.626724] RDX: 0000000000000010 RSI: 000055768e2021d0 RDI: 0000000000000005
> [  369.628569] RBP: 00007f43c8197f00 R08: 0000000000000011 R09: 00007f43c8198700
> [  369.630208] R10: 0000000000000000 R11: 0000000000000246 R12: 00007fff845e6afe
> [  369.632240] R13: 00007fff845e6aff R14: 00007f43c8197fc0 R15: 00007f43c8198700
> 
> This patch replaces skb_recv_datagram() with an open-coded variant of it
> releasing the socket lock before the __skb_wait_for_more_packets() call
> and re-acquiring it after such call in order that other functions that
> need socket lock could be executed.
> 
> what's more, the socket lock will be released only when recvmsg() will
> block and that should produce nicer overall behavior.
> 
> Fixes: 40d0a923f55a ("Implement locking of internal data for NET/ROM and ROSE.")

The correct tag is actually: 

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")

We don't pick hashes from historicaly repository, as they would confuse
most/all users. Please re-post with the correct tag. Note that you can
check patchwork yourself for this kind of formal errors.

The patch LGTM, when you repost you can append:

Acked-by: Paolo Abeni <pabeni@...hat.com>

Thanks,

Paolo