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

Djalal Harouni
2013-10-03T18:37:43Z

(Andy sorry for the delay, real life...)

On Thu, Oct 03, 2013 at 04:50:54PM +0100, Andy Lutomirski wrote:
> On Thu, Oct 3, 2013 at 4:40 PM, Djalal Harouni <tixxdz@opendz.org> wrote:
> > 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!
>
> Ah. You're saying that both revoke and checking permissions at open
> time (or using f_cred) is important. I think I agree. (Except that,
> arguably, /proc/self/stat should always be fully functional even if
> passed to a different process and yama is in use. This seems minor.)
Yes, ok

And I do agree on the /proc/self/stat, it should always work, and it
does with this series. Permissions on f_cred are checked only if
current's cred change between ->open() and ->read(), and this check may
succeed, it depends on f_cred! so /proc/self/stat will work.

> >
> >
> >> (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 ?
>
> There were various bugs (CVE-2013-1959) in /proc/pid/uid_map, etc,
> that were exploitable to obtain uid 0. They happened because write()
> checked its caller's credentials.
Ok, will recheck all of them soon. Thanks Andy.

Oh Andy, take a look at commit 935d8aabd4331f47 by Linus
Add file_ns_capable() helper function for open-time capability checking

Don't you see the same change as from my patches:
file_ns_capable() it uses the file->f_cred ! and yes it uses
security_capable() as with the patches I proposed... but in the code I
touched there is a need for security_capable_noaudit() also, I think.

Same logic! file->f_cred is already beeing/planned to be used.

That also goes for commit 6708075f104c3c9b0 by Eric,
userns: Don't let unprivileged users trick privileged users into setting the id_map

So Andy, what do you think? file->f_cred is already used to fix urgent
vulnerabilities, and now, everyone here knows that /proc/*/{stack,maps}
cab be used to leak ASLR...

[...]
> >> > 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!
>
> I just sent it a couple minutes ago -- it may not have made it yet.
> It's here, though:
>
> http://www.openwall.com/lists/kernel-hardening/2013/10/03/9
Sorry, will respond, thanks.

> --Andy

--
Djalal Harouni