Date:   Thu, 13 Feb 2020 16:51:39 +0000
From:   "Boeuf, Sebastien" <sebastien.boeuf@...el.com>
To:     "sgarzare@...hat.com" <sgarzare@...hat.com>,
        "stefanha@...hat.com" <stefanha@...hat.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>
Subject: Re: [PATCH] net: virtio_vsock: Fix race condition between bind and
 listen

On Thu, 2020-02-13 at 14:04 +0100, Stefano Garzarella wrote:
> On Thu, Feb 13, 2020 at 11:39:49AM +0000, Stefan Hajnoczi wrote:
> > On Thu, Feb 13, 2020 at 10:44:18AM +0000, Boeuf, Sebastien wrote:
> > > On Thu, 2020-02-13 at 11:22 +0100, Stefano Garzarella wrote:
> > > > On Thu, Feb 13, 2020 at 09:51:36AM +0000, Boeuf, Sebastien
> > > > wrote:
> > > > > Hi Stefano,
> > > > > 
> > > > > On Thu, 2020-02-13 at 10:41 +0100, Stefano Garzarella wrote:
> > > > > > Hi Sebastien,
> > > > > > 
> > > > > > On Thu, Feb 13, 2020 at 09:16:11AM +0000, Boeuf, Sebastien
> > > > > > wrote:
> > > > > > > From 2f1276d02f5a12d85aec5adc11dfe1eab7e160d6 Mon Sep 17
> > > > > > > 00:00:00
> > > > > > > 2001
> > > > > > > From: Sebastien Boeuf <sebastien.boeuf@...el.com>
> > > > > > > Date: Thu, 13 Feb 2020 08:50:38 +0100
> > > > > > > Subject: [PATCH] net: virtio_vsock: Fix race condition
> > > > > > > between
> > > > > > > bind
> > > > > > > and listen
> > > > > > > 
> > > > > > > Whenever the vsock backend on the host sends a packet
> > > > > > > through
> > > > > > > the
> > > > > > > RX
> > > > > > > queue, it expects an answer on the TX queue.
> > > > > > > Unfortunately,
> > > > > > > there
> > > > > > > is one
> > > > > > > case where the host side will hang waiting for the answer
> > > > > > > and
> > > > > > > will
> > > > > > > effectively never recover.
> > > > > > 
> > > > > > Do you have a test case?
> > > > > 
> > > > > Yes I do. This has been a bug we've been investigating on
> > > > > Kata
> > > > > Containers for quite some time now. This was happening when
> > > > > using
> > > > > Kata
> > > > > along with Cloud-Hypervisor (which rely on the hybrid vsock
> > > > > implementation from Firecracker). The thing is, this bug is
> > > > > very
> > > > > hard
> > > > > to reproduce and was happening for Kata because of the
> > > > > connection
> > > > > strategy. The kata-runtime tries to connect a million times
> > > > > after
> > > > > it
> > > > > started the VM, just hoping the kata-agent will start to
> > > > > listen
> > > > > from
> > > > > the guest side at some point.
> > > > 
> > > > Maybe is related to something else. I tried the following which
> > > > should be
> > > > your case simplified (IIUC):
> > > > 
> > > > guest$ python
> > > >     import socket
> > > >     s = socket.socket(socket.AF_VSOCK, socket.SOCK_STREAM)
> > > >     s.bind((socket.VMADDR_CID_ANY, 1234))
> > > > 
> > > > host$ python
> > > >     import socket
> > > >     s = socket.socket(socket.AF_VSOCK, socket.SOCK_STREAM)
> > > >     s.connect((3, 1234))
> > > > 
> > > > Traceback (most recent call last):
> > > >   File "<stdin>", line 1, in <module>
> > > > TimeoutError: [Errno 110] Connection timed out
> > > 
> > > Yes this is exactly the simplified case. But that's the point, I
> > > don't
> > > think the timeout is the best way to go here. Because this means
> > > that
> > > when we run into this case, the host side will wait for quite
> > > some time
> > > before retrying, which can cause a very long delay before the
> > > communication with the guest is established. By simply answering
> > > the
> > > host with a RST packet, we inform it that nobody's listening on
> > > the
> > > guest side yet, therefore the host side will close and try again.
> > 
> > My expectation is that TCP/IP will produce ECONNREFUSED in this
> > case but
> > I haven't checked.  Timing out is weird behavior.
> 
> I just tried and yes, TCP/IP produces ECONNREFUSED. The same error
> returned
> when no one's bound to the port.
> 
> Instead virtio-vsock returns ECONNRESET in the last case.
> I'm not sure it's correct (looking at the man page connect(2) it
> would
> seem not), but if I understood correctly VMCI returns the same
> ECONNRESET in this case.
> 
> > In any case, the reference for virtio-vsock semantics is:
> > 1. How does VMCI (VMware) vsock behave?  We strive to be compatible
> > with
> > the VMCI transport.
> 
> Looking at the code, it looks like VMCI returns ECONNRESET in this
> case,
> so this patch should be okay. (I haven't tried it)
> 
> > 2. If there is no clear VMCI behavior, then we look at TCP/IP
> > because
> > those semantics are expected by most applications.
> > 
> > This bug needs a test case in tools/testings/vsock/ and that test
> > case
> > will run against VMCI, virtio-vsock, and Hyper-V.  Doing that will
> > answer the question of how VMCI handles this case.
> 
> I agree.

I'm trying to write the test but I'm kinda stuck as I have no way to
ensure the proper timing between the test on the server(guest) and the
test on from the client(host).

Basically I was thinking about creating a new test reusing
test_stream_connection_reset() from vsock_test.c. The reused function
would be used for the run_client callback, while I would define a new
function for the run_server. I wanted to basically do a bind only, and
don't go up to the listen/accept calls.
Problem is, the server won't block after the bind is done, and I don't
know how much the test should wait between the bind and the end.
I would like to show that even when the server reaches the point where
the socket is bound, the connect will still return with ECONNRESET. The
same test without the patch would simply hang till we hit the timeout
on the client side.
Does that make sense? And how much time should I wait for after the
bind on the server side?

Thanks,
Sebastien

> 
> Thanks,
> Stefano
> 
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