[PATCH] qemu: Label vhostuser net device

[Jim Fehlig]

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 
> 
> 
>> 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.
> 
>> Also, looking at the code itself - please move qemuSecurity* calls
>> outside of the monitor code. I mean, qemuDomainAttachNetDevice() has a
>> section that prepares interfaces before calling
>> qemuDomainObjEnterMonitor(). That looks like a better place to call
>> qemuSecuritySetChardevLabel(). Similarly, the restore in detach path
>> should be done outside of monitor handling code.
> 
> Good point. I'll change it in V2.
> 
>> Oh, a side note - don't forget that hotplug can fail and thus the
>> seclabel should be restored if it was set. If you want to look around
>> for inspiration, we usually use teardownlabel variable for that.
> 
> Thanks for catching that! I'll fix it in V2.
> 
> Before working on V2 I'd like to resolve your first item. Is it safe to use 
> qemuSecuritySetChardevLabel for a vhostuser net device? Is the underlying socket 
> always in listen mode for these devices?

Answering my own question: No. A vhostuser interface can have a 
mode='client|server' setting on the <source> element describing the socket.

Perhaps the big hammer of adding domain{Set,Restore}SecurityNetdevLabel APIs is 
what's needed for this small nail?

Regards,
Jim

> At least when adding an openvswitch 
> vhostuser port to a running VM, the underlying socket has 'server=true'
> 
> 2021-04-22 15:29:40.261+0000: 643: debug : qemuMonitorJSONCheckError:401 : 
> unable to execute QEMU command 
> {"execute":"chardev-add","arguments":{"id":"charnet1","backend":{"type":"socket","data":{"addr":{"type":"unix","data":{"path":"/run/openvswitch/vhu2fca98d5-52"}},"wait":false,"server":true}}},"id":"libvirt-689"}: 
> {"id":"libvirt-689","error":{"class":"GenericError","desc":"Failed to bind 
> socket to /run/openvswitch/vhu2fca98d5-52: Permission denied"}}
> 
> Regards,
> Jim