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

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Date:	Tue, 27 May 2014 11:38:54 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Djalal Harouni <tixxdz@...ndz.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 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.

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).

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.

-Kees

-- 
Kees Cook
Chrome OS Security
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