linux-kernel - Re: [PATCH 2/2] x86_64,signal: Remove 'fs' and 'gs' from sigcontext

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Date:	Thu, 17 Jul 2014 18:13:12 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	"the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: [PATCH 2/2] x86_64,signal: Remove 'fs' and 'gs' from sigcontext

On Sat, Jul 12, 2014 at 2:17 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Sat, Jul 12, 2014 at 11:52 AM, Andi Kleen <andi@...stfloor.org> wrote:
>> On Sat, Jul 12, 2014 at 11:40:03AM -0700, H. Peter Anvin wrote:
>>> Because you are doing something weird (like Pin, for example) and take an asynchronous fault?
>>
>> But even for pin that would need executing 16 bit code, or really weird
>> 32bit code. AFAIK for 32bit the only good use case was NX emulation
>> (and old virtualization) which are both completely obsolete.
>
> Nothing particularly weird is needed.  Set a non-default stack segment
> (e.g. any 16-bit ss) and take *any* fault.  This could be something
> asynchronous or even a normal synchronous fault.  Return from the
> signal handler: boom.
>
> We know that people use 16-bit stack segments: it's what prompted the
> whole espfix64 thing.
>
>>
>> I don't think it's worth messing with the signal handlers for 16bit
>> code. If there's any problem with saving/restoring state that emulator
>> can always handle it by itself.
>>
>
> How?
>
> I can think of at least two vaguely feasible ways.  The program could
> ptrace itself, trap sigreturn, and fix ss.  Or it could restore
> registers itself and return using far ret or iret.  Both suck.

You can't even hack around this with ptrace -- ptrace silently fails
to write ss for non-TIF_IA32 tasks.

--Andy
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