On Fri, Mar 03, 2023 at 09:56:55AM -0800, Andrea Bolognani wrote:
On Fri, Mar 03, 2023 at 05:15:43PM +0000, Daniel P. Berrangé wrote:
On Fri, Mar 03, 2023 at 09:06:38AM -0800, Andrea Bolognani wrote:
Since we know that we're launching passt and not some other random helper, why can't we simply use passt_t directly here? It feels like that would be less prone to issues caused by accidental (or intentional) misconfigurations.
That ties libvirt's code to a specific policy impl which is not a desirable thing. Same reason we don't hardcode svirt_t as a type for QEMU, but instead query it dynamically from the installed policy.
Do I understand correctly that this happens in virSecuritySELinuxQEMUInitialize(), by parsing the contents of the file located via a call to selinux_virtual_domain_context_path()?
Yes.
Poking around at the other files present in the same directory I see various formats being used, including... XML? It looks like SELinux implements facilities for exposing arbitrary information about the active policy at well-known locations, with (I assume) the explicit purpose of enabling this kind of interaction.
So wouldn't that be the way to go for passt, and other helpers too? Have SELinux expose a virtual_helpers_context file, that we can parse to figure out the appropriate labels to use for passt and friends?
No, I don't think so. The helpers file is a bit of a special case that was needed because there were multiple contexts we needed to cope with for running QEMU.
I don't see any reason not to follow what the kernel already does by relying on the labelled file context.
Right, but wouldn't the idea of poking at the filesystem to retrieve the label from the binary (passt_exec_t) and then applying a text transformation to obtain the runtime label (passt_t) go directly against the idea of not hardcoding information about a specific policy implementation into libvirt?
I'm not suggesting applying a text transformation. The example code using libselinux I described in the other reply actually askes the kernel to tell us what the target type will be when a process labelled passt_exec_t is execd. The answer we get back (passt_t) will come from the SELinux policy that is currently loaded in the kernel. This is doing exactly the same thing that would happen automatically if we had a normal exec() without the MCS stuff involved.
As I understand it, such a policy would allow virtqemud (virtd_t) to execute passt (passt_exec_t) and automatically result in a transition of the process to the desired context (passt_t).
Yes, and I'm saying we must ask the kernel to tell us what that target context will be for the loaded policy, given the source file context. With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|