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Date:	Sun, 26 Oct 2014 10:36:45 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Richard Weinberger <richard.weinberger@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	linux-arch <linux-arch@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] all arches, signal: Move restart_block to struct task_struct

On Sun, Oct 26, 2014 at 10:18 AM, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Sun, Oct 26, 2014 at 10:02:16AM -0700, Andy Lutomirski wrote:
>> If an attacker can cause a controlled kernel stack overflow,
>> overwriting the restart block is a very juicy exploit target.
>> Moving the restart block to struct task_struct prevents this type of
>> exploit.
>
> The hell it does.  In a _lot_ of cases current is current_thread_info()->task,
> which makes your variant just as vulnerable, in addition to being (slightly)
> more costly.

I never said it was the *only* juicy target, but we can fix the rest,
too.  Also, I suspect that overwriting task could be harder to
exploit.  First, you need to avoid crashing, and second, on systems
with SMAP or similar protection, you need to make task point somewhere
that contains a useful exploit payload.

We could probably get rid of thread_info's task pointer on x86, too --
it's not used by get_current() any more.

--Andy
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