Re: [PATCH v2 0/9] procfs: protect /proc//* files with file->f_cred

Djalal Harouni
2013-10-03T15:41:07Z

On Thu, Oct 03, 2013 at 04:15:43PM +0100, Andy Lutomirski wrote:
> On Thu, Oct 3, 2013 at 1:29 PM, Djalal Harouni <tixxdz@opendz.org> wrote:
> > On Thu, Oct 03, 2013 at 08:12:44AM +0200, Ingo Molnar wrote:
> >> Now procfs might be special, as by its nature of a pseudofilesystem it's
> >> far more atomic than other filesystems, but still IMHO it's a lot more
> >
> >
> >> robust security design wise to revoke access to objects you should not
> >> have a reference to when a security boundary is crossed, than letting
> >> stuff leak through and sprinking all the potential cases that may leak
> >> information with permission checks ...
> > I agree, but those access should also be checked at the beginning, IOW
> > during ->open(). revoke will not help if revoke is not involved at all,
> > the task /proc entries may still be valide (no execve).
> >
> > Currently security boundary is crossed for example arbitrary /proc/*/stack
> > (and others).
> > 1) The target task does not do an execve (no revoke).
> > 2) current task will open these files and *want* and *will* pass the fd to a
> > more privileged process to pass the ptrace check which is done only during
> > ->read().
>
> What does this? Or are you saying that this is a bad thing?
I'm not sure to understand you, revoke if implemented correctly is not a
bad thing! In the other hand, here I try to explain what if the target task
did not execve, revoke will never be involved, file descriptors are
still valid!

> (And *please* don't write software that *depends* on different
> processes having different read()/write() permissions on the *same*
> struct file. I've already found multiple privilege escalations based
> on that, and I'm pretty sure I can find some more.)
Sorry, can't follow you here! examples related to what we discuss here ?

> >
> >
> >> It's also probably faster: security boundary crossings are relatively rare
> >> slowpaths, while read()s are much more common...
> > Hmm, These two are related? can't get rid of permission checks
> > especially on this pseudofilesystem!
> >
> >
> >> So please first get consensus on this fundamental design question before
> >> spreading your solution to more areas.
> > Of course, I did clean the patchset to prove that it will work, and I
> > only implemented full protection for /proc/*/{stack,syscall,stat} other
> > files will wait.
> >
> > But Ingo you can't ignore the fact that:
> > /proc/*/{stack,syscall} are 0444 mode
> > /proc/*/{stack,syscall} do not have ptrace_may_access() during open()
> > /proc/*/{stack,syscall} have the ptrace_may_access() during read()
>
> I think everyone agrees that this is broken. We don't agree on the
> fix check. (Also, as described in my other email, your approach may
> be really hard to get right.)
Well, yes we don't agree perhaps on the fix, but currently there are no
other fixes, will be happy to see other propositions! these files have
been vulnerable for years now...

And for the record it's not my approache. Please just read the emails
correctly. It was proposed and suggested by Eric and perhaps Linus.

I did an experiment with it, and found it easy without any extra
overhead: If cred have changed do extra checkes on the original opener.
It will let you pass file descritors if cred did not change.

Where is this other email that says this approach is hard?
It's not hard, very minor change and it works. Perhaps there is a
better solution yes, but currently it's not implemented!

If you see any bug or if it's not right: then please show us an example,
that's it.

Thanks Andy

> --Andy

--
Djalal Harouni