[PATCH] qemu: Label vhostuser net device

[Jim Fehlig]

On 8/18/21 2:21 AM, Michal Prívozník wrote:
> 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.

Agreed, particularly when an openvSwitch port is 'vhost-user' type

https://docs.openvswitch.org/en/latest/topics/dpdk/vhost-user/

where openvSwitch acts as the server and qemu is the client.

> 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.

According to the above doc each vhostuser device would have its own socket

"Note that a separate and different chardev path needs to be specified for each 
vhost-user device."

> 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.

Unfortunately I agree. The unfortunate part is the huge diff compared to this 
small patch :-).

Jim