linux-kernel - Re: [PATCH 03/19] x86/cpufeatures: Enable CET CR4 bit for shadow stack

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Message-ID: <YqxnwRn+/c+i1vL6@worktop.programming.kicks-ass.net>
Date:   Fri, 17 Jun 2022 13:38:41 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc:     "Yang, Weijiang" <weijiang.yang@...el.com>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Yu, Yu-cheng" <yu-cheng.yu@...el.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 03/19] x86/cpufeatures: Enable CET CR4 bit for shadow
 stack

On Thu, Jun 16, 2022 at 05:12:47PM +0000, Edgecombe, Rick P wrote:
> On Thu, 2022-06-16 at 12:24 +0200, Peter Zijlstra wrote:
> > On Thu, Jun 16, 2022 at 04:46:27AM -0400, Yang Weijiang wrote:
> > > --- a/arch/x86/include/asm/cpu.h
> > > +++ b/arch/x86/include/asm/cpu.h
> > > @@ -74,7 +74,7 @@ void init_ia32_feat_ctl(struct cpuinfo_x86 *c);
> > >   static inline void init_ia32_feat_ctl(struct cpuinfo_x86 *c) {}
> > >   #endif
> > >   
> > > -extern __noendbr void cet_disable(void);
> > > +extern __noendbr void ibt_disable(void);
> > >   
> > >   struct ucode_cpu_info;
> > >   
> > > diff --git a/arch/x86/kernel/cpu/common.c
> > > b/arch/x86/kernel/cpu/common.c
> > > index c296cb1c0113..86102a8d451e 100644
> > > --- a/arch/x86/kernel/cpu/common.c
> > > +++ b/arch/x86/kernel/cpu/common.c
> > > @@ -598,23 +598,23 @@ __noendbr void ibt_restore(u64 save)
> > >   
> > > -__noendbr void cet_disable(void)
> > > +__noendbr void ibt_disable(void)
> > >   {
> > >        if (cpu_feature_enabled(X86_FEATURE_IBT))
> > >                wrmsrl(MSR_IA32_S_CET, 0);
> > 
> > Not sure about this rename; it really disables all of (S) CET.
> > 
> > Specifically, once we do S-SHSTK (after FRED) we might also very much
> > need to kill that for kexec.
> 
> Sure, what about something like sup_cet_disable()?

Why bother? Arguably kexec should clear U_CET too.