Re: [Qemu-devel] Serial: possible hang during intensive interaction over the console

[Kirill Batuzov]

On Wed, 3 Sep 2014, Andrey Korolyov wrote:

> Given 2.1 and isa-serial output, set as ttyS0 for the guest VM with
> 9600 baud rate.
>
> The test case is quite simple - display as much data as possible over
> serial console and do not hang the system. While qemu-1.1 works
> perfectly, with complaining for lost interrupts (known bug for used
> guest kernel), 2.1 just hangs after some seconds, eating up all
> available cpu quota.
> 
> Test case is 'while true; dmesg; done' in serial console. I`d like to
> ask to consider this bug as very serious as VM going completely
> unresponsive in matter of tens of seconds and there are a lot of side
> attacks to produce enough number of printk() to the ttyS0 with serial
> console being set up and default settings for almost any distro in
> such a way that message suppression would not work and VM can be DoSed
> by an unprivileged user.
> 
>

I tried to reproduce the described behaviour with aboriginal linux and
QEMU 2.1.0 but without luck.

The configurations I tried:

qemu-system-i386 -cpu pentium3 -no-reboot -kernel bzImage -hda hda.sqf \
  -append "root=/dev/hda rw init=/sbin/init.sh panic=1 console=ttyS0 HOST=i686"

qemu-system-i386 -cpu pentium3 -no-reboot -kernel bzImage -hda hda.sqf \
  -append "root=/dev/hda rw init=/bin/ash panic=1 console=ttyS0,9600 HOST=i686"

With all output the system did not hang. In particular I alway could
switch to QEMU monitor and stop the VM from there.

Can you give an exact QEMU command line which leads to the bug?

-- 
Kirill