Re: pci device assignment as non-root?

[Michael Tokarev <mjt@tls.msk.ru>]

Avi Kivity wrote:
> Michael Tokarev wrote:
[]
>> After looking at the source I found this in
>> x86/kvm_main.c:assigned_device_update_intx():
>>
>>                 if (!capable(CAP_SYS_RAWIO))
>>                         return -EPERM;
[]
>> So it looks like some other trick is needed here (not cap_sys_rawio
>> but some traditional unix rwx thing), OR kvm binary has to be able
>> to drop privileges after all the init is done.
> 
> Dropping privileges is easy (well, need to account for all threads) but
> will not play well with hotplug.

It's either one or another for sure.  Personally I use kvm as a
sort of security tool, to run various untrusted stuff inside
guests.  For that, hotplug, while sometimes useful to have,
isn't at all required, and if there's a choice between hotplug
and stronger security I'll definitely prefer the latter.  And
sure thing having a choice is good in any case -- now there's
just no choice.

But maybe there's some other option to achieve similar effect,
i.e. to be able to open and initialize some (PCI/USB/network)
at startup without keeping root (or various CAP_*) all the
time?

>> The latter SEEMS to be easy as it only involves userspace (it's ok
>> for me to start the whole thing as root as long as it drops privs,
>> I don't need to give certain PCI devices to arbitrary users), but
>> has its own issues.  Namely, I'd like kvm to open disk image files
>> and stuff like that as non-root too, since it's the only way to
>> force read-only opens currently.
> 
> Looks like we need -drive ...,access=readonly

Yeah, that'd be good too.

Speaking of which, there's still a bug somewhere that causes a guest
to hang in case it tries to write to a virtual drive open in read-only
mode.  It *is* a bug, because it's pretty normal for various real drives
to be read-only (trivial example is write-protected floppy drive; many
scsi drives has read-only flag which can be turned on/off using hdparm
or sdparm), and with real drives on real hardware there's no hangs of
this sort, the system correctly recognizes read-only media without
hanging on an attempt to write.

Thanks!

/mjt