linux-kernel - Re: x86-64: Maintain 16-byte stack alignment

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Date:   Thu, 12 Jan 2017 09:03:18 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>
Subject: Re: x86-64: Maintain 16-byte stack alignment

On Wed, Jan 11, 2017 at 11:51:10PM -0800, Andy Lutomirski wrote:
> On Wed, Jan 11, 2017 at 11:05 PM, Herbert Xu
> <herbert@...dor.apana.org.au> wrote:
> > On Tue, Jan 10, 2017 at 09:05:28AM -0800, Linus Torvalds wrote:
> >>
> >> I'm pretty sure we have random asm code that may not maintain a
> >> 16-byte stack alignment when it calls other code (including, in some
> >> cases, calling C code).
> >>
> >> So I'm not at all convinced that this is a good idea. We shouldn't
> >> expect 16-byte alignment to be something trustworthy.
> >
> > So what if we audited all the x86 assembly code to fix this? Would
> > it then be acceptable to do a 16-byte aligned stack?
> >
> > On the face of it it doesn't seem to be a huge amount of code
> > assuming they mostly live under arch/x86.
> 
> The problem is that we have nasties like TRACE_IRQS_OFF.  Performance
> doesn't really matter for these macros, so we could probably rig up a
> helper for forcibly align the stack there.  Maybe
> FRAME_BEGIN_FORCE_ALIGN?  I also think I'd rather not to modify
> pt_regs.  We should just fix the small number of code paths that
> create a pt_regs and then call into C code to align the stack.
> 
> But if we can't do this with automatic verification, then I'm not sure
> I want to do it at all.  The asm is already more precarious than I'd
> like, and having a code path that is misaligned is asking for obscure
> bugs down the road.

For the entry code, could we just replace all calls with CALL_ALIGNED?
That might be less intrusive than trying to adjust all the pt_regs
accesses.

Then to ensure that nobody ever uses 'call' directly:

  '#define call please-use-CALL-ALIGNED-instead-of-call'

I think that would be possible if CALL_ALIGNED were a ".macro".

-- 
Josh