Date:   Wed, 15 May 2019 16:24:00 +0100
From:   Stefan Hajnoczi <stefanha@...hat.com>
To:     Stefano Garzarella <sgarzare@...hat.com>
Cc:     Jorge Moreira Broche <jemoreira@...gle.com>,
        linux-kernel@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        kernel-team@...roid.com, stable@...r.kernel.org
Subject: Re: [PATCH] vsock/virtio: Initialize core virtio vsock before
 registering the driver

On Tue, May 07, 2019 at 02:25:43PM +0200, Stefano Garzarella wrote:
> Hi Jorge,
> 
> On Mon, May 06, 2019 at 01:19:55PM -0700, Jorge Moreira Broche wrote:
> > > On Wed, May 01, 2019 at 03:08:31PM -0400, Stefan Hajnoczi wrote:
> > > > On Tue, Apr 30, 2019 at 05:30:01PM -0700, Jorge E. Moreira wrote:
> > > > > Avoid a race in which static variables in net/vmw_vsock/af_vsock.c are
> > > > > accessed (while handling interrupts) before they are initialized.
> > > > >
> > > > >
> > > > > [    4.201410] BUG: unable to handle kernel paging request at ffffffffffffffe8
> > > > > [    4.207829] IP: vsock_addr_equals_addr+0x3/0x20
> > > > > [    4.211379] PGD 28210067 P4D 28210067 PUD 28212067 PMD 0
> > > > > [    4.211379] Oops: 0000 [#1] PREEMPT SMP PTI
> > > > > [    4.211379] Modules linked in:
> > > > > [    4.211379] CPU: 1 PID: 30 Comm: kworker/1:1 Not tainted 4.14.106-419297-gd7e28cc1f241 #1
> > > > > [    4.211379] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
> > > > > [    4.211379] Workqueue: virtio_vsock virtio_transport_rx_work
> > > > > [    4.211379] task: ffffa3273d175280 task.stack: ffffaea1800e8000
> > > > > [    4.211379] RIP: 0010:vsock_addr_equals_addr+0x3/0x20
> > > > > [    4.211379] RSP: 0000:ffffaea1800ebd28 EFLAGS: 00010286
> > > > > [    4.211379] RAX: 0000000000000002 RBX: 0000000000000000 RCX: ffffffffb94e42f0
> > > > > [    4.211379] RDX: 0000000000000400 RSI: ffffffffffffffe0 RDI: ffffaea1800ebdd0
> > > > > [    4.211379] RBP: ffffaea1800ebd58 R08: 0000000000000001 R09: 0000000000000001
> > > > > [    4.211379] R10: 0000000000000000 R11: ffffffffb89d5d60 R12: ffffaea1800ebdd0
> > > > > [    4.211379] R13: 00000000828cbfbf R14: 0000000000000000 R15: ffffaea1800ebdc0
> > > > > [    4.211379] FS:  0000000000000000(0000) GS:ffffa3273fd00000(0000) knlGS:0000000000000000
> > > > > [    4.211379] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > > > [    4.211379] CR2: ffffffffffffffe8 CR3: 000000002820e001 CR4: 00000000001606e0
> > > > > [    4.211379] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > > > > [    4.211379] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > > > > [    4.211379] Call Trace:
> > > > > [    4.211379]  ? vsock_find_connected_socket+0x6c/0xe0
> > > > > [    4.211379]  virtio_transport_recv_pkt+0x15f/0x740
> > > > > [    4.211379]  ? detach_buf+0x1b5/0x210
> > > > > [    4.211379]  virtio_transport_rx_work+0xb7/0x140
> > > > > [    4.211379]  process_one_work+0x1ef/0x480
> > > > > [    4.211379]  worker_thread+0x312/0x460
> > > > > [    4.211379]  kthread+0x132/0x140
> > > > > [    4.211379]  ? process_one_work+0x480/0x480
> > > > > [    4.211379]  ? kthread_destroy_worker+0xd0/0xd0
> > > > > [    4.211379]  ret_from_fork+0x35/0x40
> > > > > [    4.211379] Code: c7 47 08 00 00 00 00 66 c7 07 28 00 c7 47 08 ff ff ff ff c7 47 04 ff ff ff ff c3 0f 1f 00 66 2e 0f 1f 84 00 00 00 00 00 8b 47 08 <3b> 46 08 75 0a 8b 47 04 3b 46 04 0f 94 c0 c3 31 c0 c3 90 66 2e
> > > > > [    4.211379] RIP: vsock_addr_equals_addr+0x3/0x20 RSP: ffffaea1800ebd28
> > > > > [    4.211379] CR2: ffffffffffffffe8
> > > > > [    4.211379] ---[ end trace f31cc4a2e6df3689 ]---
> > > > > [    4.211379] Kernel panic - not syncing: Fatal exception in interrupt
> > > > > [    4.211379] Kernel Offset: 0x37000000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff)
> > > > > [    4.211379] Rebooting in 5 seconds..
> > > > >
> > > > > Fixes: 22b5c0b63f32 ("vsock/virtio: fix kernel panic after device hot-unplug")
> > > > > Cc: Stefan Hajnoczi <stefanha@...hat.com>
> > > > > Cc: "David S. Miller" <davem@...emloft.net>
> > > > > Cc: kvm@...r.kernel.org
> > > > > Cc: virtualization@...ts.linux-foundation.org
> > > > > Cc: netdev@...r.kernel.org
> > > > > Cc: kernel-team@...roid.com
> > > > > Cc: stable@...r.kernel.org [4.9+]
> > > > > Signed-off-by: Jorge E. Moreira <jemoreira@...gle.com>
> > > > > ---
> > > > >  net/vmw_vsock/virtio_transport.c | 13 ++++++-------
> > > > >  1 file changed, 6 insertions(+), 7 deletions(-)
> > > > >
> > > > > diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
> > > > > index 15eb5d3d4750..96ab344f17bb 100644
> > > > > --- a/net/vmw_vsock/virtio_transport.c
> > > > > +++ b/net/vmw_vsock/virtio_transport.c
> > > > > @@ -702,28 +702,27 @@ static int __init virtio_vsock_init(void)
> > > > >     if (!virtio_vsock_workqueue)
> > > > >             return -ENOMEM;
> > > > >
> > > > > -   ret = register_virtio_driver(&virtio_vsock_driver);
> > > > > +   ret = vsock_core_init(&virtio_transport.transport);
> > > >
> > > > Have you checked that all transport callbacks are safe even if another
> > > > CPU calls them while virtio_vsock_probe() is executing on another CPU?
> > > >
> > >
> > > I have the same doubt.
> > >
> > > What do you think to take the 'the_virtio_vsock_mutex' in the
> > > virtio_vsock_init(), keeping the previous order?
> > >
> > > This should prevent this issue because the virtio_vsock_probe() remains
> > > blocked in the mutex until the end of vsock_core_init().
> > >
> > > Cheers,
> > > Stefano
> > 
> > Hi Stefan, Stefano,
> > Sorry for the late reply.
> 
> Don't worry :)
> 
> > 
> > @Stefan
> > The order of vsock_core_exit() does not need to be changed to fix the
> > bug I found, but not changing it means the exit function is not
> > symmetric to the init function.
> > 
> > @Stefano
> > Taking the mutex from virtio_vsock_init() could work too (I haven't
> > tried it yet), but it's unnecessary, all that needs to be done is
> > properly initialize vsock_core before attempting to use it.
> > 
> > I would prefer to change the order in virtio_vsock_init, while leaving
> > virtio_vsock_exit unchanged, but I'll leave the final decision to you
> > since I am not very familiar with the inner workings of these modules.
> 
> In order to fix your issue, IMO changing the order in virtio_vsock_init(),
> is enough.
> 
> I think also that is correct to change the order in the virtio_vsock_exit(),
> otherwise, we should have the same issue if an interrupt comes while we
> are removing the module.
> This should not lead to the problem that I tried to solve in 22b5c0b63f32,
> because the vsock_core_exit() should not be called if there are open sockets,
> since the virtio-vsock driver become the owner of AF_VSOCK protocol
> family.
> 
> Not related to this patch, maybe there are some issues in the
> virtio_vsock_probe(). I'd check better if it is correct to set
> 'the_virtio_vsock' before the end of the initialization (e.g. spinlocks
> are initialized later).
> 
> Accordingly,
> 
> Reviewed-by: Stefano Garzarella <sgarzare@...hat.com>

I'm going to review this once more tomorrow and investigate the
thread-safety issues during init and exit.

The core problem is that we have two sides (the virtio device and the
network stack) that can both induce activity as soon as they are
registered.  We need to be sure that one cannot begin activity before
the other has been fully initialized.

Stefan

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