Re: [RFC] add mount options to sysfs

From: Greg KH
Date: Wed May 18 2011 - 15:27:59 EST

On Wed, May 18, 2011 at 09:05:45PM +0400, Vasiliy Kulikov wrote:
> On Wed, May 18, 2011 at 09:39 -0700, Greg KH wrote:
> > On Wed, May 18, 2011 at 08:31:44PM +0400, Vasiliy Kulikov wrote:
> > > Currently there is no good way to effectively globally restrict an
> > > access to sysfs files. It's possible only to chmod the sysfs'
> > > root/directories to fully deny access to sysfs (sub-)tree to some users
> > > or chmod files after they are created. The latter approach is racy,
> > > however.
> >
> > Why do you want to do this? What is in sysfs files that is not
> > gloabally ok to access? That should be fixed first, if at all, instead
> > of wanting to modify the whole sysfs tree, right?
> I don't hide the goal (I didn't find any other weird permissions, if
> you mean this). It is primary about additional global controlable
> layer of defining permission:
> 1) *IF* another sensitive file with weird permissions is found, mount
> option is IMO the best temporary workaround.

Maybe, but fixing the file would be the obvious solution.

> 2) Somebody might be worried about information leaks via world readable
> files - not strict bugs, but leaks in sense of local policy. See numerous
> discussions about hiding kernel addresses - there is no unified opinion
> about it. Some admins would be happy with denying access to almost all
> system information except some white list.

What file in sysfs "leaks" information like this? Please let us know
and we will be glad to fix that.

> > > The patch introduces sysfs mount options parsing and adds 4 new options:
> > > uid, gid, mode and umask. uid, gid, and umask are classical options,
> > > mode is a global restricting mode mask that defined the most relaxed
> > > possible file mode. E.g. if mode=0750 then "chmod 0664" changes file's
> > > permissions to 0640.
> >
> > What is going to break if you do this? Have you tested it? I'd be very
> > worried about this.
> I've tested it on my laptop (I'm writing booted with this kernel). By
> default nothing has changed (umask=0000, mode=0777). Mounting sysfs
> with mode=0770 leads to the predictable state - no sysfs information is
> available to nonroot, e.g. no ACPI battery state. umask is tested with
> pluging a USB flash.

A usb storage device is hardly a huge consumer of sysfs files, sorry.
This would need a whole lot more testing before I would ever feel
confortable with it, even if I agreed that something like this should be
added (hint, I still don't.)

> mode has one advantage over umask. It is temporary - "mount -o
> remount,mode=0777" restores the initial state.
> I'd want to implement similar options for debugfs.

I wouldn't.

Again, lets fix the root problems here, if any, instead of adding
complexity and probably breaking systems by changing permissions without
anyone knowing about it.


greg k-h
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