Re: device hotplug & file handles

[Michal Privoznik]

On 5/7/20 4:49 PM, Gerd Hoffmann wrote:
>    Hi,
>
> For usb device pass-through (aka -device usb-host) it would be very
> useful to pass file handles from libvirt to qemu.  The workflow would
> change from ...
>
>    (1) libvirt enables access to /dev/usb/$bus/$dev
>    (2) libvirt passes $bus + $dev (using hostbus + hostaddr properties)
>        to qemu.
>    (3) qemu opens /dev/usb/$bus/$dev
>
> ... to ...
>
>    (1) libvirt opens /dev/usb/$bus/$dev
>    (2) libvirt passes filehandle to qemu.
>
> Question is how can we pass the file descriptor best?  My idea would be
> to simply add an fd property to usb-host:
>
>   * Coldplug would be "-device usb-host,fd=<nr>" (cmd line).
>   * Hotplug would be "device_add usb-host,fd=<getfd-name>" (monitor).
>
> Will that work from libvirt point of view?
> Or does anyone have an better idea?
>
> thanks,
>    Gerd
>
> PS: background: https://bugzilla.redhat.com/show_bug.cgi?id=1595525

I don't have a better idea, but a little background on why libvirt even 
invented private /dev in the first place. The reason was that 
occasionally, when udev ran its rules it would overwrite the security 
labels on /dev nodes set by libvirt and thus denying access to QEMU. See:

https://bugzilla.redhat.com/show_bug.cgi?id=1354251

Now, I think there is the same risk with what you are proposing. This 
isn't problem for DAC where permissions are checked during open(), but 
it may be a problem for SELinux where each individual operation with the 
FD is inspected.

Having said that, I am not against this approach, in fact I'm in favour 
of it. Let's hope that people learned that having udev overwriting 
seclabels is a bad idea and the bug won't appear again.

Michal