linux-kernel - Re: [PATCH] futex: do not leak robust list to unprivileged process

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Date:	Tue, 20 Mar 2012 10:11:02 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Serge Hallyn <serge.hallyn@...onical.com>,
	linux-kernel@...r.kernel.org, Darren Hart <dvhart@...ux.intel.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jiri Kosina <jkosina@...e.cz>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	David Howells <dhowells@...hat.com>,
	kernel-hardening@...ts.openwall.com, spender@...ecurity.net,
	mingo@...nel.org
Subject: Re: [PATCH] futex: do not leak robust list to unprivileged process

On Tue, Mar 20, 2012 at 10:02 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Tue, 20 Mar 2012, Serge Hallyn wrote:
>
>> Quoting Kees Cook (keescook@...omium.org):
>> > It was possible to extract the robust list head address from a setuid
>> > process if it had used set_robust_list(), allowing an ASLR info leak. This
>> > changes the permission checks to be the same as those used for similar
>> > info that comes out of /proc.
>> >
>> > Running a setuid program that uses robust futexes would have had:
>> >   cred->euid != pcred->euid
>> >   cred->euid == pcred->uid
>> > so the old permissions check would allow it. I'm not aware of any setuid
>> > programs that use robust futexes, so this is just a preventative measure.
>> >
>> > (This patch is based on changes from grsecurity.)
>> >
>> > Signed-off-by: Kees Cook <keescook@...omium.org>
>>
>> I like the change.  Much cleaner.  I'm not 100% sure though that
>> there are no legitimate cases of robust futexes use which would now
>> be forbidden.  (Explicitly cc:ing Ingo)
>
> get_robust_list is not necessary for robust futexes. There is no
> reference to get_robust_list in glibc.
>
> I really wonder why we have this syscall at all.

The documentation I found yesterday while looking at this was:
http://linux.die.net/man/2/get_robust_list

Which says "The system call is only available for debugging purposes
and is not needed for normal operations. Both system calls are not
available to application programs as functions; they can be called
using the syscall(3) function."

Dropping the syscall entirely would certainly make it secure. ;)

-Kees

-- 
Kees Cook
ChromeOS Security
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