Re: [Qemu-devel] [PATCH 0/2 v3] kvm: notify host when guest panicked

[Gleb Natapov]

On Wed, Mar 14, 2012 at 07:06:50PM +0800, Wen Congyang wrote:
> At 03/14/2012 06:59 PM, Daniel P. Berrange Wrote:
> > On Wed, Mar 14, 2012 at 06:58:47PM +0800, Wen Congyang wrote:
> >> At 03/14/2012 06:52 PM, Avi Kivity Wrote:
> >>> On 03/14/2012 12:52 PM, Wen Congyang wrote:
> >>>>>
> >>>>>> If so, is this channel visible to guest userspace? If the channle is 
> >>>>>> visible to guest
> >>>>>> userspace, the program running in userspace may write the same message 
> >>>>>> to the channel.
> >>>>>
> >>>>> Access control is via permissions.  You can have udev scripts assign
> >>>>> whatever uid and gid to the port of your interest.  By default, all
> >>>>> ports are only accessible to the root user.
> >>>>
> >>>> We should also prevent root user writing message to this channel if it is
> >>>> used for panicked notification.
> >>>>
> >>>
> >>> Why?  root can easily cause a panic.
> >>>
> >>
> >> root user can write the same message to virtio-serial while the guest is 
> >> running...
> > 
> > Unless you are running a MAC policy which strictly confines the root
> > account, root can cause a kernel panic regardless of virtio-serial
> > permissions in the guest:
> > 
> >   echo c > /proc/sysrq-trigger
> 
> Yes, root user can cause a kernel panic. But if he writes the same message to 
> virtio-serial,
> the host will see the guest is panicked while the guest is not panicked. The 
> host is cheated.
> 
And why is this a problem? If root in a guest wants to cheat host like
that there is no way to stop him. He can load kernel module and do
whatever he wants. Management should treat that condition as if guest
panicked.

> If we use vmcall, and the user causes a kernel panic, we can also know the 
> guest is panicked.
> It is the thing what we need. We need to know the guest is panicked, and we 
> donot aware
> why it is panicked. If the guest is not panicked, and the host think the 
> guest is panicked, it
> is not the thing we need.
> 
Then you cannot get the thing you need and you can as well stop trying.

--
                        Gleb.