[PATCH] qemu: Label vhostuser net device

[Michal Prívozník]

On 8/18/21 12:31 AM, Jim Fehlig wrote:
> On 8/17/21 12:11 PM, Jim Fehlig wrote:
>> On 8/17/21 4:13 AM, Michal Prívozník wrote:
>>> On 8/13/21 11:36 PM, Jim Fehlig wrote:
>>>> Attaching a newly created vhostuser port to a VM fails due to an
>>>> apparmor denial
>>>>
>>>> internal error: unable to execute QEMU command 'chardev-add': Failed
>>>> to bind socket to /run/openvswitch/vhu838c4d29-c9: Permission denied
>>>>
>>>> In the case of a net device type VIR_DOMAIN_NET_TYPE_VHOSTUSER, the
>>>> underlying chardev is not labeled in qemuDomainAttachNetDevice prior
>>>> to calling qemuMonitorAttachCharDev. Label the chardev before calling
>>>> qemuMonitorAttachCharDev, and restore the label when removing the
>>>> net device.
>>>>
>>>> Signed-off-by: Jim Fehlig <jfehlig at suse.com>
>>>> ---
>>>>   src/qemu/qemu_hotplug.c | 9 +++++++++
>>>>   1 file changed, 9 insertions(+)
>>>>
>>>> diff --git a/src/qemu/qemu_hotplug.c b/src/qemu/qemu_hotplug.c
>>>> index c00e8a7852..42e7997112 100644
>>>> --- a/src/qemu/qemu_hotplug.c
>>>> +++ b/src/qemu/qemu_hotplug.c
>>>> @@ -1467,6 +1467,11 @@ qemuDomainAttachNetDevice(virQEMUDriver *driver,
>>>>       }
>>>>       if (actualType == VIR_DOMAIN_NET_TYPE_VHOSTUSER) {
>>>> +        virDomainChrDef chr = { .source = net->data.vhostuser };
>>>> +
>>>> +        if (qemuSecuritySetChardevLabel(driver, vm, &chr) < 0)
>>>> +            goto cleanup;
>>>> +
>>>>           if (qemuMonitorAttachCharDev(priv->mon, charDevAlias,
>>>> net->data.vhostuser) < 0) {
>>>>               ignore_value(qemuDomainObjExitMonitor(driver, vm));
>>>>               virDomainAuditNet(vm, NULL, net, "attach", false);
>>>> @@ -4692,6 +4697,8 @@ qemuDomainRemoveNetDevice(virQEMUDriver *driver,
>>>>       }
>>>>       if (actualType == VIR_DOMAIN_NET_TYPE_VHOSTUSER) {
>>>> +        virDomainChrDef chr = { .source = net->data.vhostuser };
>>>> +
>>>>           /* vhostuser has a chardev too */
>>>>           if (qemuMonitorDetachCharDev(priv->mon, charDevAlias) < 0) {
>>>>               /* well, this is a messy situation. Guest visible PCI
>>>> device has
>>>> @@ -4699,6 +4706,8 @@ qemuDomainRemoveNetDevice(virQEMUDriver *driver,
>>>>                * to just ignore the error and carry on.
>>>>                */
>>>>           }
>>>> +        if (qemuSecurityRestoreChardevLabel(driver, vm, &chr) < 0)
>>>> +            VIR_WARN("Unable to restore security label on vhostuser
>>>> char device");
>>>>       } else if (actualType == VIR_DOMAIN_NET_TYPE_VDPA) {
>>>>           int vdpafdset = -1;
>>>>           g_autoptr(qemuMonitorFdsets) fdsets = NULL;
>>>>
>>>
>>> What an interesting bug. Previously we assumed that the UNIX socket is
>>> created with broad enough permissions so that QEMU can connect to it. I
>>> mean, when a VM is starting up nor DAC nor SELinux drivers care about
>>> VHOSTUSER. It's AppArmor driver that cares. And it makes sense.
>>> But, what's problematic here is that upon attach the socket perms will
>>> be changed (because both DAC and SELinux drivers implement
>>> domainSetSecurityChardevLabel callback). And since sockets can't have
>>> XATTRs libvirt won't remember its original labels and thus subsequent
>>> restore changes owner to root:root.
>>
>> How are existing chardevs with socket backends handled? It seems they
>> would suffer the same problem when restoring the labels. The DAC and
>> selinux callbacks seem to avoid labeling for "server" sockets
>>
>> https://gitlab.com/libvirt/libvirt/-/blob/master/src/security/security_dac.c#L1534
>>
>> https://gitlab.com/libvirt/libvirt/-/blob/master/src/security/security_selinux.c#L2544
>>

Yeah, you're right. But I view chardevs as runtime, fire and go things.
I mean, you start a VM with a chardev, say an UNIX socket and its
lifetime is identical to the VM lifetime. vhostuser on the other hand is
handled by a third party daemon (typically OVS) and thus UNIX socket
lifetime is different to VM lifetime. IOW, I worry that if we changed
the UNIX socket label we might be preventing other VMs from connecting
to it.

And I don't even know if a single socket can be shared between two VMs.
For instance, if OVS created an UNIX socket whether I can attach
vhostuser interface to one domain and then to the other. Because if I
could, then we should not change labels.

Perhaps Laine can shed more light here.

I do understand that apparmor needs to add an entry to the VM's profile,
but I worry that DAC and SELinux might screw things up.

>>
>>> So I think we should address this inconsistency in behavior. But I don't
>>> know how :-(
>>
>> My first attempt at fixing this introduced
>> domain{Set,Restore}SecurityNetdevLabel to the security driver, but it
>> seemed like overkill after I discovered virDomainChrSourceDef embedded
>> in the virDomainNetDef structure. I can revisit that approach if it
>> sounds more promising.

I think it does sound more promising given my assumption above is
correct. This way we can have only AppArmor driver implement the
callback leaving us with consistent behavior.

Michal