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Date:	Sun, 26 Oct 2014 18:08:21 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	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>
Subject: Re: [PATCH] all arches, signal: Move restart_block to struct task_struct

On Sun, Oct 26, 2014 at 11:11 AM, H. Peter Anvin <hpa@...or.com> wrote:
> On x86 we have discussed to combine thread_info and task_struct into a single allocation with a percpu variable lining pointng at it.

Is that better than just gradually eliminating all the members of thread_info?

Anyway, ftrace is the only user of ti->task on x86, so it's not a very
juicy target.

--Andy

>
> On October 26, 2014 11:09:39 AM PDT, Al Viro <viro@...IV.linux.org.uk> wrote:
>>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.
>
> --
> Sent from my mobile phone.  Please pardon brevity and lack of formatting.



-- 
Andy Lutomirski
AMA Capital Management, LLC
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