Re: [PATCH net-next] ax25: Fix deadlock caused by skb_recv_datagram in ax25_recvmsg
From: Eric Dumazet <edumazet@google.com>
Date: 2022-06-06 17:32:12
Also in:
linux-hams, lkml
On Mon, Jun 6, 2022 at 9:21 AM Duoming Zhou [off-list ref] 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: [ 861.122612] INFO: task ax25_deadlock:148 blocked for more than 737 seconds. [ 861.124543] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. [ 861.127764] Call Trace: [ 861.129688] <TASK> [ 861.130743] __schedule+0x2f9/0xb20 [ 861.131526] schedule+0x49/0xb0 [ 861.131640] __lock_sock+0x92/0x100 [ 861.131640] ? destroy_sched_domains_rcu+0x20/0x20 [ 861.131640] lock_sock_nested+0x6e/0x70 [ 861.131640] ax25_sendmsg+0x46/0x420 [ 861.134383] ? ax25_recvmsg+0x1e0/0x1e0 [ 861.135658] sock_sendmsg+0x59/0x60 [ 861.136791] __sys_sendto+0xe9/0x150 [ 861.137212] ? __schedule+0x301/0xb20 [ 861.137710] ? __do_softirq+0x4a2/0x4fd [ 861.139153] __x64_sys_sendto+0x20/0x30 [ 861.140330] do_syscall_64+0x3b/0x90 [ 861.140731] entry_SYSCALL_64_after_hwframe+0x46/0xb0 [ 861.141249] RIP: 0033:0x7fdf05ee4f64 [ 861.141249] RSP: 002b:00007ffe95772fc0 EFLAGS: 00000246 ORIG_RAX: 000000000000002c [ 861.141249] RAX: ffffffffffffffda RBX: 0000565303a013f0 RCX: 00007fdf05ee4f64 [ 861.141249] RDX: 0000000000000005 RSI: 0000565303a01678 RDI: 0000000000000005 [ 861.141249] RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000 [ 861.141249] R10: 0000000000000000 R11: 0000000000000246 R12: 0000565303a00cf0 [ 861.141249] R13: 00007ffe957730e0 R14: 0000000000000000 R15: 0000000000000000 This patch moves the skb_recv_datagram() before lock_sock() in order that other functions that need lock_sock could be executed.
Why is this targeting net-next tree ? 1) A fix should target net tree 2) It should include a Fixes: tag Also: - this patch bypasses tests in ax25_recvmsg() - This might break applications depending on blocking read() operations. I feel a real fix is going to be slightly more difficult than that. Thank you
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Reported-by: Thomas Habets <thomas@@habets.se> Signed-off-by: Duoming Zhou <redacted> --- net/ax25/af_ax25.c | 11 ++++++----- 1 file changed, 6 insertions(+), 5 deletions(-)diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c index 95393bb2760..02cd6087512 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; + lock_sock(sk); /* * This works for seqpacket too. The receiver has ordered the@@ -1675,11 +1680,6 @@ static int ax25_recvmsg(struct socket *sock, struct msghdr *msg, size_t size, goto out; } - /* Now we can treat all alike */ - skb = skb_recv_datagram(sk, flags, &err); - if (skb == NULL) - goto out; - if (!sk_to_ax25(sk)->pidincl) skb_pull(skb, 1); /* Remove PID */@@ -1725,6 +1725,7 @@ static int ax25_recvmsg(struct socket *sock, struct msghdr *msg, size_t size, out: release_sock(sk); +done: return err; } --2.17.1