Re: [PATCH] x86/crypto: Add missing RETs
From: Ingo Molnar
Date: Thu Jul 05 2018 - 03:58:28 EST
* Josh Poimboeuf <jpoimboe@xxxxxxxxxx> 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