Re: [PATCH 2/2] debugfs: only allow root access to debugging interfaces

Greg KH
2011-02-22T20:40:52Z

On Tue, Feb 22, 2011 at 12:28:56PM -0800, Kees Cook wrote:
> On Tue, Feb 22, 2011 at 12:16:10PM -0800, Greg KH wrote:
> > On Tue, Feb 22, 2011 at 11:50:18AM -0800, Kees Cook wrote:
> > > On Tue, Feb 22, 2011 at 07:34:18PM +0000, Alan Cox wrote:
> > > > > What system do you proposed to keep these "stupid mistakes" from
> > > > > continuing to happen? If debugfs had already been mode 0700, we could have
> > > > > avoided all of these CVEs, including the full-blown local root escalation.
> > > >
> > > > And all sorts of features would have put themselves in sysfs instead and
> > > > broken no doubt.
> > > >
> > > > > The "no rules" approach to debugfs is not a good idea, IMO.
> > > >
> > > > It's a debugging fs, it needs to be "no rules" other than the obvious
> > > > "don't mount it on production systems"
> > >
> > > Okay, so the debugfs is not supposed to be mounted on a production system.
> >
> > No, not true at all, the "enterprise" distros all mount debugfs for good
> > reason on their systems.
>
> What reasons are those? Or better yet, why do you and Alan Cox disagree on
> this point?

These distros have made the decision to support the perf interface,
which lives in debugfs, for their customers. I'm not saying that I
disagree with Alan about this, just pointing out the reality of the
situation here.

> > > This seems to be news to a lot of developers trying to use the interfaces
> > > exposed there. It would be nice to say this more loudly. Basically,
> > > a normal system should not depend on anything in the debugfs. I can get
> > > behind that.
> >
> > Again, not true. Mostly all due to the perf interface, fix that to move
> > out of debugfs (patches have been proposed) and this problem will go
> > away.
>
> You can't have "no rules" and "all distros mount debugfs for good reason".
> This is asking for (even more) trouble. If there is something universally
> useful in debugfs (I do not count perf as universally useful -- my parents
> do not use perf), then why is it living in a filesystem with no rules
> (where "no rules" seems to also include "don't break interfaces").

Again, "don't break interfaces" is just me saying "don't break the
interfaces I have created in debugfs as they are to be used by all
users." Don't take that as a set-in-stone rule of debugfs at all, it
isn't.

Again, you are trying to exclude a whole range of useful and valid files
from being used, when there was only a very very very small percentage
created incorrectly. They have now been fixed, and we have the
infrastructure to prevent future ones from being created as well, so I
don't see the issue here anymore.

thanks,

greg k-h