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

["Darrick J. Wong" <djwong@kernel.org>]

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.

--D