[PATCH 4/4] qemu_passt: Don't let passt fork off

[Laine Stump]

On 2/14/23 8:02 AM, Stefano Brivio wrote:
> On Tue, 14 Feb 2023 12:51:22 +0100
> Michal Privoznik <mprivozn at redhat.com> wrote:
> 
>> When passt starts it tries to do some security measures to
>> restrict itself. For instance, it creates its own namespaces,
>> umounts basically everything, drops capabilities, forks off to
>> further restrict itself (the child is where all interesting work
>> takes place now). This is sound, except it's causing two
>> problems:
>>
>> 1) the PID file FD, which we leak into the passt process, gets
>>     closed (and thus our virPidFile*() helpers see unlocked PID
>>     file, which makes them think the process is gone),
> 
> I didn't realise this was the case, but giving passt write (unless I'm
> missing something) access to a file created by libvirtd doesn't look
> desirable to me.

> 
>> 2) the PID file no longer reflects true PID of the process.
>>
>> Worse, the child calls setsid() so we can't even kill the whole
>> process group. I mean, we can but it won't be any good.

I think that (incorrect PID in the pidfile) is  happening because Michal 
is using the original version of my patches that were pushed - I had 
mimicked the behavior of slirp, where libvirt deamonizes the new 
process. If that process then daemonizes itself, we have some sort of 
"double daemon"; libvirt has saved off the pid of what it thinks is 
going to be the final process, but then that process further forks and 
exits from the process whose pid libvirt saved. But because passt was 
cleaning up after itself I hadn't noticed the discrepancy in pids when 
testing.

Without going into all the details of the pidfile and locking and etc, I 
just want to say that if we can fork/exec dnsmasq and let it daemonize 
itself and create its own pidfile, then certainly we can do the same 
thing for passt. (and if there's a fundamental problem, then it's a 
fundamental problem for dnsmasq as well).

>>
>> Fortunately, passt has '--foreground' argument, which causes it
>> to undergo the same security measures but without forking off the
>> child.

But if we do --foreground in combination with calling 
virCommandDaemonize(), then won't we still have the problem that libvirt 
won't know whether or not passt has failed to start (not unless we want 
to put in a bunch of gorp to continue grabbing stderr while watching to 
see if the passt process has exited, etc. It's going to take some 
convincing for me to think we should run passt with --foreground rather 
than letting it daemonize itself.

> 
> They're not the same -- unfortunately they can't be, because, on Linux,
> you can't change the PID of an existing process, so there's no way to
> enter a new PID namespace without clone().
> 
> If passt remains in the same PID namespace, it's still able to see PIDs
> of other processes, which is not desirable from a security perspective.
> 
> Again from a security perspective, this is probably a small impact, so
> I guess it's fine if there's no other way around it. But I see a lot of
> ways around it...
>