linux-kernel - Re: [PATCH] x86/crypto: Add missing RETs

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Date:   Thu, 5 Jul 2018 09:58:15 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Alexey Dobriyan <adobriyan@...il.com>,
        Borislav Petkov <bp@...en8.de>, linux-crypto@...r.kernel.org,
        Mike Galbraith <efault@....de>, torvalds@...ux-foundation.org,
        tglx@...utronix.de, luto@...nel.org, peterz@...radead.org,
        brgerst@...il.com, hpa@...or.com, linux-kernel@...r.kernel.org,
        dvlasenk@...hat.com, h.peter.anvin@...el.com,
        linux-tip-commits <linux-tip-commits@...r.kernel.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] x86/crypto: Add missing RETs


* Josh Poimboeuf <jpoimboe@...hat.com> wrote:

> > So that's still incomplete in that doesn't analyze the 32-bit build yet, right?
> 
> We could do INT3s on 64-bit and NOPs on 32-bit.
> 
> Or, possibly even better, we could just keep NOPs everywhere and instead
> make objtool smart enough to detect function fallthroughs.  That should
> be pretty easy, actually.  It already does it for C files.
> 
> Something like the below should work, though it's still got a few
> issues:
> 
>   a) objtool is currently disabled for crypto code because it doesn't
>      yet understand crypto stack re-alignments (which really needs
>      fixing anyway); and
> 
>   b) it complains about the blank xen hypercalls falling through.  Those
>      aren't actual functions anyway, so we should probably annotate
>      those somehow so that objtool ignores them anyway.
> 
> I'm a bit swamped at the moment but I can fix those once I get a little
> more bandwidth.  I at least verified that this patch caught the crypto
> missing RETs.

Great, I'd be perfectly fine with such an approach.

Also, if we have that then we could re-apply Alexey's patch and switch to INT3 
(only on 64-bit kernels) without any trouble, because objtool should detect any 
execution flow bugs before the INT3 could trigger, right?

I.e. any INT3 fault would show a combination of *both* an objtool bug and a 
probable code flow bug - which I suspect would warrant crashing the box ...

Thanks,

	Ingo