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Date:   Thu, 5 Jul 2018 09:58:15 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     Josh Poimboeuf <jpoimboe@redhat.com>
Cc:     Alexey Dobriyan <adobriyan@gmail.com>,
        Borislav Petkov <bp@alien8.de>, linux-crypto@vger.kernel.org,
        Mike Galbraith <efault@gmx.de>, torvalds@linux-foundation.org,
        tglx@linutronix.de, luto@kernel.org, peterz@infradead.org,
        brgerst@gmail.com, hpa@zytor.com, linux-kernel@vger.kernel.org,
        dvlasenk@redhat.com, h.peter.anvin@intel.com,
        linux-tip-commits <linux-tip-commits@vger.kernel.org>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [PATCH] x86/crypto: Add missing RETs
Message-ID: <20180705075815.GA20903@gmail.com>
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* Josh Poimboeuf <jpoimboe@redhat.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