[libvirt] [PATCH v2] qemu: Introduce caching whether /dev/kvm is accessible

[Michal Privoznik]

On 10/30/2018 04:07 PM, Daniel P. Berrangé wrote:
> On Tue, Oct 30, 2018 at 03:45:36PM +0100, Michal Privoznik wrote:
>> On 10/30/2018 02:46 PM, Michal Privoznik wrote:
>>> On 10/30/2018 01:55 PM, Daniel P. Berrangé wrote:
>>>> On Tue, Oct 30, 2018 at 10:32:08AM +0000, Daniel P. Berrangé wrote:
>>>>> On Tue, Oct 30, 2018 at 11:08:45AM +0100, Michal Privoznik wrote:
>>>>>> On 10/30/2018 10:35 AM, Daniel P. Berrangé wrote:
>>>>>>> On Tue, Oct 30, 2018 at 09:13:50AM +0100, Michal Privoznik wrote:
>>>>>>>> On 10/29/2018 06:34 PM, Marc Hartmayer wrote:
>>>>>>>>> Introduce caching whether /dev/kvm is usable as the QEMU user:QEMU
>>>>>>>>> group. This reduces the overhead of the QEMU capabilities cache
>>>>>>>>> lookup. Before this patch there were many fork() calls used for
>>>>>>>>> checking whether /dev/kvm is accessible. Now we store the result
>>>>>>>>> whether /dev/kvm is accessible or not and we only need to re-run the
>>>>>>>>> virFileAccessibleAs check if the ctime of /dev/kvm has changed.
>>>>>>>>>
>>>>>>>>> Suggested-by: Daniel P. Berrangé <berrange at redhat.com>
>>>>>>>>> Signed-off-by: Marc Hartmayer <mhartmay at linux.ibm.com>
>>>>>>>>> ---
>>>>>>>>>  src/qemu/qemu_capabilities.c | 54 ++++++++++++++++++++++++++++++++++--
>>>>>>>>>  1 file changed, 52 insertions(+), 2 deletions(-)
>>>>>>>>>


> Not really. Udev is in use everywhere, so this behaviour makes the
> patch useless in practice, even though it is technically right in
> theory :-(
> 

Well, caching owner + seclabels + ACLs won't help either. What if user
loads some profile into AppArmor or something that denies previously
allowed access to /dev/kvm (or vice versa)? What I am saying is that
there are some security models which base their decisions on something
else than file attributes.

Honestly, I don't have solution.

Michal