Re: [PATCH v3] net: ax25: Fix deadlock caused by skb_recv_datagram in ax25_recvmsg
From: Paolo Abeni <pabeni@redhat.com>
Date: 2022-06-09 08:44:52
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On Wed, 2022-06-08 at 09:29 +0800, Duoming Zhou wrote:
quoted hunk ↗ jump to hunk
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 moves the skb_recv_datagram() before lock_sock() in order that other functions that need lock_sock could be executed. What`s more, we add skb_free_datagram() before goto out in order to mitigate memory leak. Suggested-by: Thomas Osterried <redacted> Signed-off-by: Duoming Zhou <redacted> Reported-by: Thomas Habets <thomas@@habets.se> --- Changes in v3: - Add skb_free_datagram() before goto out in order to mitigate memory leak. net/ax25/af_ax25.c | 12 +++++++----- 1 file changed, 7 insertions(+), 5 deletions(-)diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c index 95393bb2760..62aa5993093 100644 --- a/net/ax25/af_ax25.c +++ b/net/ax25/af_ax25.c@@ -1665,6 +1665,11 @@ static int ax25_recvmsg(struct socket *sock, struct msghdr *msg, size_t size, int copied; int err = 0; + /* Now we can treat all alike */ + skb = skb_recv_datagram(sk, flags, &err); + if (!skb) + goto done; +
Note that this causes a behavior change: before this patch, calling recvmsg() on unconnected seqpacket sockets returned immediatelly with an error (due to the the check below), now it blocks. The change may confuse (== break) user-space applications. I think it would be better replacing 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. Somewhat alike __unix_dgram_recvmsg(). In any case this lacks a 'Fixes' pointing to the commit introducing the issue addressed here. Thanks, Paolo