Re: [PATCH] ntsync: Set the permissions to be 0666

[Elizabeth Figura]

On Friday, 14 February 2025 19:03:33 CST Darrick J. Wong wrote:
> On Fri, Feb 14, 2025 at 04:15:25PM -0600, Elizabeth Figura wrote:
> > On Friday, 14 February 2025 12:45:39 CST Darrick J. Wong wrote:
> > > On Fri, Feb 14, 2025 at 12:13:03PM -0600, Elizabeth Figura wrote:
> > > > On Friday, 14 February 2025 07:06:20 CST Greg Kroah-Hartman wrote:
> > > > > On Fri, Feb 14, 2025 at 12:28:00PM +0000, Mike Lothian wrote:
> > > > > > This allows ntsync to be usuable by non-root processes out of the box
> > > > >
> > > > > Are you sure you need/want that?  If so, why?  How did existing testing
> > > > > not ever catch this?
> > > >
> > > > Hi, sorry, this is of course my fault.
> > > >
> > > > We do need /dev/ntsync to be openable from user space for it to be
> > > > useful. I'm not sure what the most "correct" permissions are to have
> > > > in this case (when we don't specifically need read or write), but I
> > > > don't think I see a reason not to just set to 666 or 444.
> > > >
> > > > I originally assumed that the right way to do this was not to set the
> > > > mode on the kernel file but rather through udev; I believe I was using
> > > > the code for /dev/loop-control or /dev/fuse as an example, which both
> > > > do that. So I (and others who tested) had just manually set up udev
> > > > rules for this, with the eventual intent of adding a default rule to
> > > > systemd like the others. I only recently realized that doing something
> > > > like this patch is possible and precedented.
> > > >
> > > > I don't know what the best way to address this is, but this is
> > > > certainly the simplest.
> > >
> > > Paranoid defaults in the kernel, and then a udev rule to relax the mode
> > > at runtime.  You could also have logind scripts to add add per-user
> > > allow acls to the device file at user session set up time... or however
> > > it is that /dev/sr0 has me on the allow list.  I'm not sure how that
> > > happens exactly, but it works smoothly.
> > >
> > > I get far less complaining about relaxing posture than tightening it
> > > (==breaking things) after the fact.
> >
> > FWIW, it may be worth stressing that this is not a hardware device in
> > any sense, it's a software driver that only lives in a char device
> > (and dedicated module) for the sake of isolating the code. I can't
> > imagine any reason to control access per-user, although my experience
> > may not be enough to grant such imagination.
>
> Oh, I'm aware that ntsync is a driver for a software "device" that
> implements various Windows APIs and isn't real hardware. :)
>
> But, you might want prevent non-root systemd services (e.g. avahi) from
> being able to access /dev/ntsync if, say, someone breaches that, while
> at the same time allowing access to (say) logged-in users who can run
> Wine.

I see the idea, though I don't know if it's applicable in this case—the individual ntsync file descriptions are also supposed to be isolated from one another, so even a rogue avahi would still have another barrier towards compromising a Wine process. Of course you may be aware of that and be advocating for the more barriers the better.

I can't say what systemd will actually prefer in this case, but they do seem to have requested that the kernel change it... [1]

[1] https://github.com/systemd/systemd/pull/36384