linux-kernel - Re: [PATCH v3 04/37] x86/cpufeatures: Enable CET CR4 bit for shadow stack

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Date:   Mon, 7 Nov 2022 19:19:21 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
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Subject: Re: [PATCH v3 04/37] x86/cpufeatures: Enable CET CR4 bit for shadow
 stack

On Mon, 2022-11-07 at 19:37 +0100, Borislav Petkov wrote:
> On Mon, Nov 07, 2022 at 06:19:48PM +0000, Edgecombe, Rick P wrote:
> > It was to catch if the software user shadow stack feature gets
> > disabled
> > at boot with the "clearcpuid" command.
> 
> I don't understand. clearcpuid does setup_clear_cpu_cap() too. It
> would
> eventually clear the bit in boot_cpu_data.x86_capability's AFAICT.
> 
> cpu_feature_enabled() looks at boot_cpu_data too.
> 
> So what's the problem?

You are right, there actually is no problem. I thought the
apply_forced_caps() was happening too late, but it is not. So this
check you highlighted would not be needed if we kept the clearcpuid
method.

Thanks.

> 
> Oh, and also, you've added that clearcpuid thing to the help docs.
> Please remove it. clearcpuid= taints the kernel and we've left it in
> because some of your colleagues really wanted it for testing or
> whatnot.
> But it is crap and it was on its way out at some point so we better
> not
> proliferate its use any more.
> 
> > Is there a better way to do this?
> 
> Yeah, cpu_feature_enabled() should be enough and if it isn't, then we
> need to fix it to be.
> 
> Which reminds me, I'd need to take Maxim's patch too:
> 
> https://lore.kernel.org/r/20220718141123.136106-3-mlevitsk@redhat.com
> 
> as it is a simplification.
> 
> > > Here you need to do
> > > 
> > >      setup_clear_cpu_cap(X86_FEATURE_IBT);
> > >      setup_clear_cpu_cap(X86_FEATURE_SHSTK);
> > 
> > This only gets called by kexec way after boot, as kexec is prepping
> > to
> > transition to the new kernel. Do we want to be clearing feature
> > bits at
> > that time?
> 
> Hmm, I was under the impression you'll have the usual chicken bit
> "noshstk" which gets added with every big feature. So it'll call that
> thing here.

There was at one point, but there was a suggestion to remove in favor
of clearcpuid. I can add it back.