Re: [tip: x86/urgent] x86/cpu: Disable CR pinning during CPU bringup

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Mar 18, 2026 at 09:47:22PM +0100, Peter Zijlstra wrote:
> On Wed, Mar 18, 2026 at 06:51:10PM -0000, tip-bot2 for Dave Hansen wrote:
> > --- a/arch/x86/kernel/cpu/common.c
> > +++ b/arch/x86/kernel/cpu/common.c
> > @@ -437,6 +437,21 @@ static const unsigned long cr4_pinned_mask = X86_CR4_SMEP | X86_CR4_SMAP | X86_C
> >  static DEFINE_STATIC_KEY_FALSE_RO(cr_pinning);
> >  static unsigned long cr4_pinned_bits __ro_after_init;
> >  
> > +static bool cr_pinning_enabled(void)
> > +{
> > +	if (!static_branch_likely(&cr_pinning))
> > +		return false;
> > +
> > +	/*
> > +	 * Do not enforce pinning during CPU bringup. It might
> > +	 * turn on features that are not set up yet, like FRED.
> > +	 */
> > +	if (!cpu_online(smp_processor_id()))
> > +		return false;
> > +
> > +	return true;
> > +}
> 
> Urgh, so this means all an attack needs to do is disable the online bit
> and it gets to poke CR4 bits.
> 
> This seems unfortunate.
> 
> And sure, randomly clearing the online bit will eventually cause havoc,
> but I suspect you still get plenty time until the system goes wobbly.

So what is the problem with removing FRED from cr4_pinned_mask?
Specifically, set it up such that if you 'accidentally' clear that, the
machines insta dies a horrible death.

So currently we setup an IDT and everything, then setup the FRED MSRs,
flip CR4_FRED and call it a day. But we could just explicitly poison all
the IDT stuff to cause tripple faults.

Fixing that up is a much bigger ask of an attacker, no?