linux-kernel - Re: + syscalls-x86-add-__nr_kcmp-syscall-v8.patch added to -mm tree

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Date:	Wed, 15 Feb 2012 19:43:36 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Cyrill Gorcunov <gorcunov@...nvz.org>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Andrey Vagin <avagin@...nvz.org>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Glauber Costa <glommer@...allels.com>,
	Andi Kleen <andi@...stfloor.org>, Tejun Heo <tj@...nel.org>,
	Matt Helsley <matthltc@...ibm.com>,
	Pekka Enberg <penberg@...nel.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Vasiliy Kulikov <segoon@...nwall.com>,
	Alexey Dobriyan <adobriyan@...il.com>, Valdis.Kletnieks@...edu,
	Michal Marek <mmarek@...e.cz>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: + syscalls-x86-add-__nr_kcmp-syscall-v8.patch added to -mm tree

On 02/15, Cyrill Gorcunov wrote:
>
> On Wed, Feb 15, 2012 at 05:22:22PM +0100, Oleg Nesterov wrote:
> >
> > > So I don't understand how it's different from what
> > > is provided in this patch. What I'm missing?
> >
> > environ_read() does
> >
> > 	mm = mm_access(task);
> > 	if (mm)
> > 		do_something(mm);
> >
> > even if it races with, say, execve(setuid_app) we can't read the
> > new ->mm.
>
> Wait, I'm confused
>
> 	process 1 (reader)	process 2 ("task" itself)
> 	mm = mm_access(task);
> 				task changes own credentials
> 				so reader can't access on next
> 				read if it would try, but since
> 				access already granted... it
> 				continues do_something(mm)
> 	if (mm)
> 		do_something(mm);
>
> So in the patch I tried the same, once access is granted it
> belongs to a caller.

See the "execve(setuid_app)", this is what I meant. Even if we
race with execve() and the task raises its privileges we can't
read the new ->mm, we will read the old mm for which we have
(had) the rights to access.

> > while your code (very roughly) does something like
> >
> > 	mm = mm_access(task);
> > 	if (mm)
> > 		do_something(task->mm);
> >
> > while it is quite possible that mm != task->mm.
>
> Oleg, could you please explain me where it happens
> that task->mm (I've got access to) will be changed
> to some new -mm while I'm inspecting it.

Cough... this is question I am trying to ask ;)

Let me try again. To simplify, lets discuss the KCMP_VM case
only.

I do not really understand why do we need ptrace_may_access().
I do not see any security problems with kcmp_ptr(task->mm), but
I am not expert.

However, you added this check so I assume you have some reason.
But this can race with execve(setuid_app) and KCMP_VM can play
with task->mm after this task raises its caps. If this is fine,
then why do we need ptrace_may_access?

OK, please ignore. I sent the initial email just becase KCMP_FILE
is buggy.

> > > +       for (i = 0; i < KCMP_TYPES; i++)
> > > +               cookies[i][1] |= (~(~0UL >>  1) | 1);
> >
> > I am puzzled, help ;) this is equal to
> >
> > 		cookies[i][1] |= -LONG_MAX;
> > or
> > 		cookies[i][1] |= (LONG_MIN | 1);
> >
> > for what? why do we want to set these 2 bits (MSB and LSB) ?
>
> Letme quote hpa@ here :)
>
>  | This code is wrong.  You will have a zero cookie, legitimately, once in
>  | 2^32 or 2^64 attempts, depending on the bitness.
>  |
>  | The other thing is that for the multiplicative cookie you should OR in
>  | the value (~(~0UL >> 1) | 1) in order to make sure that the value is (a)
>  | large and (b) odd.

OK, thanks.

Oleg.

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