linux-kernel - Re: [PATCH v4 3/3] MIPS: set stack/data protection as non-executable

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Date:	Wed, 5 Aug 2015 17:23:55 -0700
From:	Leonid Yegoshin <Leonid.Yegoshin@...tec.com>
To:	David Daney <ddaney@...iumnetworks.com>
CC:	Paul Burton <paul.burton@...tec.com>, <daniel.sanders@...tec.com>,
	<linux-mips@...ux-mips.org>, <cernekee@...il.com>,
	<Zubair.Kakakhel@...tec.com>, <geert+renesas@...der.be>,
	<david.daney@...ium.com>, <peterz@...radead.org>,
	<heiko.carstens@...ibm.com>, <paul.gortmaker@...driver.com>,
	<behanw@...verseincode.com>, <macro@...ux-mips.org>,
	<cl@...ux.com>, <pkarat@...sta.com>, <linux@...ck-us.net>,
	<tkhai@...dex.ru>, <james.hogan@...tec.com>,
	<alexinbeijing@...il.com>, <rusty@...tcorp.com.au>,
	<Steven.Hill@...tec.com>, <lars.persson@...s.com>,
	<aleksey.makarov@...iga.com>, <linux-kernel@...r.kernel.org>,
	<ralf@...ux-mips.org>, <luto@...capital.net>,
	<dahi@...ux.vnet.ibm.com>, <markos.chandras@...tec.com>,
	<eunb.song@...sung.com>, <kumba@...too.org>
Subject: Re: [PATCH v4 3/3] MIPS: set stack/data protection as non-executable

On 08/05/2015 05:14 PM, David Daney wrote:
> On 08/05/2015 05:06 PM, Leonid Yegoshin wrote:
>> On 08/05/2015 04:55 PM, Paul Burton wrote:
>>>
>>>
>>> As was pointed out last time you posted this, it breaks backwards
>>> compatibility with userland & thus cannot be applied.
>>
>> Never observed since first version.
>>
>> In other side, the problem with apps like ssh_keygen is observed in
>> absence of executable stack protection.
>
> You cannot change the default.
>
> If your ssh_keygen is broken, get a working version.

It is actually any application which requests non-executable stack 
protection and needs some emulation BEFORE GLIBC cancels that 
non-executable stack protection due to libraries.

If you build all libraries with PT_GNU_STACK 'non-executable' and use 
application with the same protection then you can't emulate even a 
single instruction - it crashes immediately. So, it is not a bad 
application, it is a bad choice for emulation space in past.

>
> I have never had a problem running ssh_keygen (on platforms requiring 
> emulation).

Create a buildroot FS with PT_GNU_STACK 'non-executable' libraries. Then 
run ssh_keygen on CPU without FPU and look.

You also may try to run MIPS R2 Debian on MIPS R6 CPU, and see a 
spectacular failure of ssh_keygen (it tries to emulate MIPS R2 
instruction before first library is loaded and that fails due to 
non-executable stack protection.

- Leonid.

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