linux-kernel - Re: [tip:x86/asm] x86/i386: Make sure stack-protector segment base is cache aligned

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Fri, 04 Sep 2009 08:59:42 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Arjan van de Ven <arjan@...radead.org>
CC:	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	mingo@...hat.com, linux-kernel@...r.kernel.org,
	jeremy.fitzhardinge@...rix.com, stable@...nel.org,
	tglx@...utronix.de, linux-tip-commits@...r.kernel.org,
	Tejun Heo <tj@...nel.org>
Subject: Re: [tip:x86/asm] x86/i386: Make sure stack-protector segment base
 is cache aligned

On 09/04/09 07:15, Arjan van de Ven wrote:
> On Thu, 3 Sep 2009 23:18:05 +0200
> Ingo Molnar <mingo@...e.hu> wrote:
>
>   
>> * H. Peter Anvin <hpa@...or.com> wrote:
>>
>>     
>>> On 09/03/2009 01:45 PM, Jeremy Fitzhardinge wrote:
>>>       
>>>> Two problems:
>>>>
>>>>     * gcc generates %gs: references for stack-protector, but we
>>>> use %fs for percpu data (because restoring %fs is faster if it's
>>>> a null selector; TLS uses %gs).  I guess we could use %fs if
>>>>       !CONFIG_CC_STACKPROTECTOR, or %gs if we are using it
>>>> (though that has some fiddly ramifications for things like
>>>> ptrace).
>>>>         
>>> Well, by touching two segments we're getting the worst of both 
>>> worlds, so at least assuming some significant number of real-world 
>>> deployments use CC_STACKPROTECTOR, we really don't want to 
>>> pessimize that case too much.
>>>       
>> Fedora has stackprotector enabled so it's used in a widespread way.
>>
>> 	Ingo
>>     
> the other issue is that afaik we want the kernel to use the other
> register than userspace does...
>   

We do for percpu (%fs), but gcc always generates %gs references for
stack-protector.  The difference between "pop %seg" for a null vs
non-null selector was fairly small (a couple of cycles), so using %gs
when stack-protector is enabled isn't a huge deal.  To put it another
way, calling one stack-protected function in kernel mode would probably
make up the difference between using %fs vs %gs.

    J
>
>   

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/