linux-kernel - Re: virtio_console: lost wakeup due to race between port_fops_poll() and vring

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <564165392dfefdbb6e1739102ed80c3aee92881b.camel@kernel.org>
Date: Wed, 10 Dec 2025 16:58:42 +0100
From: Amit Shah <amit@...nel.org>
To: Lorenz Bauer <lmb@...valent.com>, Arnd Bergmann <arnd@...db.de>, Greg
 Kroah-Hartman <gregkh@...uxfoundation.org>, virtualization@...ts.linux.dev,
 LKML <linux-kernel@...r.kernel.org>
Subject: Re: virtio_console: lost wakeup due to race between
 port_fops_poll() and vring_interrupt()

On Fri, 2025-12-05 at 18:06 +0000, Lorenz Bauer wrote:
> Hi,
> 
> I've been chasing a bug when using virtio console to exchange data
> between guest and host. It manifests in a process inside the guest
> getting stuck while writing to the serial port:
> 
>     [<0>] wait_port_writable+0x139/0x2d0
>     [<0>] port_fops_write+0x88/0x130
>     [<0>] vfs_write+0xf3/0x450
>     [<0>] ksys_write+0x6d/0xe0
>     [<0>] do_syscall_64+0x9e/0x1a0
>     [<0>] entry_SYSCALL_64_after_hwframe+0x77/0x7
> 
> I managed to track this down to a race in virtio_console.c. The
> driver
> doesn't properly account for a port that receives blocking writes and
> is polled at the same time. I suspect that a similar problem exists
> in
> port_fops_read().
> 
> Here is how it goes. We need two threads inside the guest, A and B.
> 
> - Thread A writes to the serial port until the virtqueue fills up.
> - Thread A invokes port_fops_write() which calls
> wait_port_writable().
> There are no used buffers to reclaim and the thread is suspended,
> waiting on port->waitqueue.
> - The host side of the device makes some progress, marks some buffers
> as consumed / used and  sends an interrupt to the guest.
> - Before the guest can service the interrupt, thread B executes
> port_fops_poll(). This calls into
>   reclaim_consumed_buffers() via will_write_block(), which removes
> all
> used buffers.
> - The interrupt gets serviced, calling into vring_interrupt(). Here
> the check for more_used() returns false because the poll just went
> through all the work. The handler returns without waking
> port->waitqueue.
> - port_fops_write() never returns.

Both port_fops_write() and port_fops_poll() call will_write_block() for
checking the block condition -- which is whether port->outvq_full is
set.  Even if buffers were consumed via poll, outvq_full should just
return false, right?

Hm, does adding wake_up(port->waitqueue) in case 'ret' is false in
will_write_block() help?

> I'm not exactly sure how to best fix this. Maybe it's enough to only
> check outvq_full in poll? It'd also be nice to fix this on the read
> side as well.
> 
> I only have a reproducer which requires a Go toolchain unfortunately:
> https://github.com/lmb/vimto/issues/29
> 
> Best
> Lorenz

		Amit