From mboxrd@z Thu Jan  1 00:00:00 1970
From: Al Viro <viro@ZenIV.linux.org.uk>
Subject: Re: [PATCH] all arches, signal: Move restart_block to struct
 task_struct
Date: Sun, 26 Oct 2014 18:09:39 +0000
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To: Andy Lutomirski <luto@amacapital.net>
Cc: Richard Weinberger <richard.weinberger@gmail.com>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, Andrew Morton <akpm@linux-foundation.org>, Kees Cook <keescook@chromium.org>, linux-arch <linux-arch@vger.kernel.org>, Ingo Molnar <mingo@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>

On Sun, Oct 26, 2014 at 10:36:45AM -0700, Andy Lutomirski wrote:

> 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.

Huh?  If you can overwrite that pointer, you can bloody well overwrite
->task itself, making it point into the overwritten part of stack right
next to thread_info.

Again, on most of the architectures the _only_ way to reach task_struct
is via thread_info:
	* everything that uses asm-generic/current.h - arm, arm64, blackfin,
c6x, hexagon, metag, mips, openrisc, sh, um, unicore32
	* everything that should be using it - alpha, avr32, cris, m32r,
parisc, score, tile.  These guys can simply add generic-y += current.h
into their asm/Kbuild and remove asm/current.h.
	* nearly the same situation - xtensa (there's an asm variant of
the same thing + copy of asm-generic/current.h for C)
	* sparc32
	* m68k-noMMU
	* mn10300-SMP

It's a strong majority.  Check arch/*/asm/current.h and see for yourself.