From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Ingo Molnar <mingo@kernel.org>
Cc: Herbert Xu <herbert@gondor.apana.org.au>,
Linus Torvalds <torvalds@linux-foundation.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Andy Lutomirski <luto@kernel.org>,
Ard Biesheuvel <ard.biesheuvel@linaro.org>
Subject: Re: x86-64: Maintain 16-byte stack alignment
Date: Thu, 12 Jan 2017 08:49:14 -0600 [thread overview]
Message-ID: <20170112144914.qmc5lgocp6vfq3b6@treble> (raw)
In-Reply-To: <20170112074601.GB30151@gmail.com>
On Thu, Jan 12, 2017 at 08:46:01AM +0100, Ingo Molnar wrote:
>
> * Herbert Xu <herbert@gondor.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?
>
> Audits for small but deadly details that isn't checked automatically by tooling
> would inevitably bitrot again - and in this particular case there's a 50% chance
> that a new, buggy change would test out to be 'fine' on a kernel developer's own
> box - and break on different configs, different hw or with unrelated (and
> innocent) kernel changes, sometime later - spreading the pain unnecessarily.
>
> So my feeling is that we really need improved tooling for this (and yes, the GCC
> toolchain should have handled this correctly).
>
> But fortunately we have related tooling in the kernel: could objtool handle this?
> My secret hope was always that objtool would grow into a kind of life insurance
> against toolchain bogosities (which is a must for things like livepatching or a
> DWARF unwinder - but I digress).
Are we talking about entry code, or other asm code? Because objtool
audits *most* asm code, but entry code is way too specialized for
objtool to understand.
(I do have a pending objtool rewrite which would make it very easy to
ensure 16-byte stack alignment. But again, objtool can only understand
callable C or asm functions, not entry code.)
Another approach would be to solve this problem with unwinder warnings,
*if* there's enough test coverage.
I recently made some changes to try to standardize the "end" of the
stack, so that the stack pointer is always a certain value before
calling into C code. I also added some warnings to the unwinder to
ensure that it always reaches that point on the stack. So if the "end"
of the stack were adjusted by a word by adding padding to pt_regs, the
unwinder warnings could help preserve that.
We could take that a step further by adding an unwinder check to ensure
that *every* frame is 16-byte aligned if -mpreferred-stack-boundary=3
isn't used.
Yet another step would be to add a debug feature which does stack sanity
checking from a periodic NMI, to flush out these unwinder warnings.
(Though I've found that current 0-day and fuzzing efforts, combined with
lockdep and perf's frequent unwinder usage, are already doing a great
job at flushing out unwinder warnings.)
The only question is if there would be enough test coverage,
particularly with those versions of gcc which don't have
-mpreferred-stack-boundary=3.
--
Josh
next prev parent reply other threads:[~2017-01-12 14:49 UTC|newest]
Thread overview: 54+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-01-10 14:33 x86-64: Maintain 16-byte stack alignment Herbert Xu
2017-01-10 14:39 ` Herbert Xu
2017-01-10 17:05 ` Linus Torvalds
2017-01-10 17:09 ` Andy Lutomirski
2017-01-11 3:11 ` Herbert Xu
2017-01-11 3:30 ` Linus Torvalds
2017-01-11 4:17 ` Linus Torvalds
2017-01-11 4:35 ` Herbert Xu
2017-01-11 6:01 ` Andy Lutomirski
2017-01-12 6:21 ` Andy Lutomirski
2017-01-12 7:40 ` Ingo Molnar
2017-01-12 14:02 ` Josh Poimboeuf
2017-01-12 19:51 ` Linus Torvalds
2017-01-12 20:08 ` Andy Lutomirski
2017-01-12 20:15 ` Josh Poimboeuf
2017-01-12 20:55 ` Josh Poimboeuf
2017-01-12 21:40 ` Linus Torvalds
2017-01-13 8:38 ` Herbert Xu
2017-01-13 1:46 ` Andy Lutomirski
2017-01-13 3:11 ` Josh Poimboeuf
2017-01-13 3:23 ` Andy Lutomirski
2017-01-13 4:27 ` Josh Poimboeuf
[not found] ` <CA+55aFzRrSwGxxfZk-RUEnsz=xhcSmOwE1CenfCPBWtsS9MwDw@mail.gmail.com>
2017-01-13 5:07 ` Josh Poimboeuf
2017-01-13 8:43 ` Herbert Xu
2017-01-13 8:42 ` Herbert Xu
2017-01-13 8:39 ` Herbert Xu
2017-01-13 8:36 ` Herbert Xu
2017-01-13 13:07 ` Josh Poimboeuf
[not found] ` <CA+55aFw+Z_ieo6DzTVB6_-TvQ0jj60s=T0mvXfqkBVFdKFPw_Q@mail.gmail.com>
2017-01-11 8:06 ` Ard Biesheuvel
2017-01-11 8:09 ` Herbert Xu
2017-01-11 18:20 ` Andy Lutomirski
2017-01-12 7:05 ` Herbert Xu
2017-01-12 7:46 ` Ingo Molnar
2017-01-12 14:49 ` Josh Poimboeuf [this message]
2017-01-12 7:51 ` Andy Lutomirski
2017-01-12 8:04 ` Herbert Xu
2017-01-12 8:18 ` Ingo Molnar
2017-01-12 15:03 ` Josh Poimboeuf
2017-01-12 15:06 ` Herbert Xu
2017-01-12 15:18 ` Josh Poimboeuf
2017-01-12 15:10 ` Josh Poimboeuf
2017-01-10 17:30 ` Ard Biesheuvel
2017-01-10 19:00 ` Andy Lutomirski
2017-01-10 19:16 ` Ard Biesheuvel
2017-01-10 19:22 ` Andy Lutomirski
2017-01-10 20:00 ` Ard Biesheuvel
2017-01-10 23:25 ` Andy Lutomirski
2017-01-11 3:26 ` Herbert Xu
2017-01-11 3:26 ` Herbert Xu
2017-01-11 3:16 ` Herbert Xu
2017-01-11 3:15 ` Herbert Xu
2017-01-12 6:12 ` Herbert Xu
2017-01-12 8:01 ` Ard Biesheuvel
2017-01-12 8:06 ` Herbert Xu
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