Messages in this thread | | | Date | Tue, 22 Feb 2011 12:54:13 -0800 | From | Kees Cook <> | Subject | Re: [PATCH 2/2] debugfs: only allow root access to debugging interfaces |
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On Tue, Feb 22, 2011 at 12:37:04PM -0800, Greg KH wrote: > 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.
A tool used only by the root user, so the proposed mount mode of 0700 wouldn't break anything.
> > > > 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.
I'm trying to minimize exposure. So far, debugfs has proven itself to be repeatedly dangerous/flawed. I would like to take preventative measures to contain it. Everyone seems to agree that debugfs is useful for debugging, and I don't doubt that. It may also be riddled with potential holes, so why expose an entire tree of debugging interfaces to non-root users?
We have %pK to keep kernel addresses out of the hands of non-root users when reading the debugging interfaces, and we have either my patchset or Dan Carpenter's to keep non-root users from writing to these debugging interfaces. There needs to be a way for system owners to be able to protect themselves proactively from debugfs.
-Kees
-- Kees Cook Ubuntu Security Team
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