On Wed, 7 Jun 2023 13:38:12 +0200 Michal Prívozník <mprivozn(a)redhat.com> wrote:On 6/6/23 22:58, Stefano Brivio wrote:Yes -- I think it would be appropriate that the file is not opened as root, because access control (also on behalf of LSMs) logically happens on open(). It's passt using that file, so it should own it and be accounted for it (also in terms of disk quota), not libvirt. On the other hand:Hi, On Tue, 6 Jun 2023 13:41:28 +0200 Michal Privoznik <mprivozn(a)redhat.com> wrote:Well, is it any different to when libvirt would create the file, set perms and then let passt open() it?The idea behind this is so that libvirt can create the log file, set all DAC/SELinux labels and just pass pre-opened FD to passt. You can view my WIP patches for libvirt here: https://gitlab.com/MichalPrivoznik/libvirt/-/commit/5bbe40087d6888a7c4031a2…Thanks for the implementation... and also for taking care of the details! I'm not really enthusiastic about the security implications of this approach, but _if it's the only reasonable way to solve this_, I won't certainly stand in the way of progress. The series looks mostly good to me, I have only a few nits, reported in the single replies. This adds a further interfacing mode between passt and the parent process, which makes me a bit uncomfortable in general. Specifically, if the parent process runs as root, this gives a rogue passt process a way to write potentially unlimited amounts of data to essentially any place (minus _some_ checks implemented by Linux Security Modules). And a rogue passt process doesn't necessarily imply a rogue parent, so this is additional surface.In fact, I find passing FD safer because libvirt doesn't need to set up perms/owner and can leave the log file be owned by root:root with 0600 mode,...also true, even though:or any other user that libvirtd runs under.in that case, the file would be already, naturally, owned by that (non-root) user and created with 0600 permissions anyway. The issue here in some sense is that libvirtd is commonly (?) running as root. To keep it simple, in the Podman integration (for pasta), we only enabled pasta (same binary as passt) to be used if Podman also runs "rootless". But, oops: https://github.com/containers/podman/issues/17840 This makes me realise another point though: if libvirtd runs as root, at least in the current integration, or at least by default (?), passt will run with the same user as QEMU (usually "qemu") -- not "nobody". And at this point I'm not entirely sure that having a log file owned by root is much preferable to having it owned by that "qemu" user.No, no, by "[not] a file" I really meant [not] a regular file (say, a TTY), and not necessarily libvirtd -- or a rogue libvirtd. All I'm saying is that if you have control over the log file descriptor, outside passt, as regular user, you might also get control over the behaviour of passt, without any mediation by the filesystem.Oh, and by the way of LSMs, we kind of bypass stuff like this. For a non-rogue passt process, if the parent runs as root, I don't see any additional attack scenario -- the parent could do anything it wants anyway. But if the parent runs as regular user, there are a few additional ways to cause passt to misbehave by passing in a file descriptor that doesn't correspond to a file, or that's opened by other processes, without any mediation by the filesystem (which is generally speaking not under control of unprivileged users).So an attacker can cause libvirtd to pass an FD that doesn't belong to the log file opened by libvirtd?Interesting, I though that's impossible.Well, strictly speaking it's not, just use a tracer, but then at that point one would gain nothing additional from it.I mean, it's sort of a goal we're working towards with QEMU - libvirt opens FDs and then just pass them to QEMU. So if it's really unsafe, we should re-evaluate our goal.To me that also poses the problem that if a LSM policy or VFS-based access control forbids QEMU to access a resource, you are effectively bypassing that -- and you're also bypassing whatever flag QEMU would normally use on open(). But I don't know in which cases this is being done and if it's actually a problem (or a bigger problem than the solution it offers).On the other hand (and sure, from a user perspective this is different) we don't allow passt to save its PID file or create its socket in whatever place. But I see the usability point you're making and I think it has its value too. Long story short, if you think (you know better) that users would commonly run libvirtd as root and request that passt writes its log file to an arbitrary location, even if we offer a different default (including a relative path) or somehow recommend something else, I think we should go ahead with this. Otherwise I'm mildly against it.I'd rather much prefer the more common approach of defaulting, or suggesting to the user, to write to a temporary filesystem, available under most distributions under /run or /var/run. Is that really unfeasible?It if feasible. I just thought that when users want their logs to reside under some special path that libvirtd has access to, but not passt then we might use FD passing.Why? I think this also reflects on usability, and might have some weight on the previous point.I'm thinking that libvirt already needs a specific directory for passt to use (socket and PID files). What if logFile, other than an absolute path, supported a relative path? This logic might perhaps apply to other helpers or external programs too.That's tangent to this problem.Either way, do you plan to take care of those? I can, but not right away. -- StefanoBy the way, passt ships with AppArmor and SELinux example policies, which are also included in packages. They would need at least a quick review, probably some edits, and some basic tests. Thinking of those, a relative path would also simplify things.Ah, completely forgot about those. Yeah, they might need tweaking even if we decide to go this route.