linux-kernel - Re: [PATCH 1/9] procfs: use flags to deny or allow access to /proc/<pid>/$entry

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Date:	Wed, 28 May 2014 12:42:48 +0100
From:	Djalal Harouni <tixxdz@...ndz.org>
To:	Kees Cook <keescook@...omium.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Alexey Dobriyan <adobriyan@...il.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andy Lutomirski <luto@...capital.net>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 1/9] procfs: use flags to deny or allow access to
 /proc/<pid>/$entry

On Tue, May 27, 2014 at 11:38:54AM -0700, Kees Cook wrote:
> On Mon, May 26, 2014 at 6:27 AM, Djalal Harouni <tixxdz@...ndz.org> wrote:
> > Add the deny or allow flags, so we can perform proper permission checks
> > and set the result accordingly. These flags are needed in case we have
> > to cache the result of permission checks that are done during ->open()
> > time. Later during ->read(), we can decide to allow or deny the read().
> >
> > The pid entries that need these flags are:
> > /proc/<pid>/stat
> > /proc/<pid>/wchan
> > /proc/<pid>/maps  (will be handled in next patches).
> >
> > These files are world readable, userspace depend on that. To prevent
> > ASLR leaks and to avoid breaking userspace, we follow this scheme:
> >
> > a) Perform permission checks during ->open()
> > b) Cache the result of a) and return success
> > c) Recheck the cached result during ->read()
> > d) If cached == PID_ENTRY_DENY:
> >    then we replace the sensitive fields with zeros, userspace won't
> >    break and sensitive fields are protected.
> >
> > These flags are internal to /proc/<pid>/*
> 
> Since this complex area of behavior has seen a lot of changes, I think
> I'd really like to see some tests in tools/testsing/selftests/
> somewhere that actually codify what the expected behaviors should be.
Ok, sounds good!

> We have a lot of corner cases, a lot of userspace behaviors to retain,
> and given how fragile this area has been, I'd love to avoid seeing
> regressions. It seems like we need to test file permissions, open/read
> permissions, contents, etc, under many different cases (priv, unpriv,
> passing between priv/unpriv and unpriv/priv, ptrace checks, etc).
Yes, nice.

> If we could do a "make run_tests" in a selftests subdirectory, it'd be
> much easier to a) validate these fixes, and b) avoid regressions.
Ok!

Since I'm working on this on my free time and when time permits, please
give me some days! I'll try to handle the cases I've discussed here.

Now Kees, some of these files are still world readable and affected:
smaps, maps ... I know, it's a matter of suid binary on your distro, and
every one can exploit it. So what to do: make the tests public or write
the tests and fix these entries then at last make the tests public ?


Where should I send the tests ?

Thank you!


> -Kees
> 
> -- 
> Kees Cook
> Chrome OS Security

-- 
Djalal Harouni
http://opendz.org
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