Re: [PATCHv8 04/17] x86/cpu: Defer CR pinning setup until after EFI initialization

[Kirill A. Shutemov]

On Tue, Jul 01, 2025 at 12:03:01PM -0700, Sohil Mehta wrote:
> On 7/1/2025 2:58 AM, Kirill A. Shutemov wrote:
> > From: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
> > 
> > In order to map the EFI runtime services, set_virtual_address_map()
> > needs to be called, which resides in the lower half of the address
> > space. This means that LASS needs to be temporarily disabled around
> > this call. This can only be done before the CR pinning is set up.
> > 
> > Move CR pinning setup behind the EFI initialization.
> > 
> > Wrapping efi_enter_virtual_mode() into lass_disable/enable_enforcement()
> 
> I believe this should be lass_stac()/clac() since we reverted to the
> original naming.

Doh. Will fix.

> > is not enough because AC flag gates data accesses, but not instruction
> > fetch. Clearing the CR4 bit is required.
> > 
> > Signed-off-by: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
> > Suggested-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > ---
> >  arch/x86/kernel/cpu/common.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> > index 4f430be285de..9918121e0adc 100644
> > --- a/arch/x86/kernel/cpu/common.c
> > +++ b/arch/x86/kernel/cpu/common.c
> > @@ -2081,7 +2081,6 @@ static __init void identify_boot_cpu(void)
> >  	enable_sep_cpu();
> >  #endif
> >  	cpu_detect_tlb(&boot_cpu_data);
> > -	setup_cr_pinning();
> >  
> >  	tsx_init();
> >  	tdx_init();
> > @@ -2532,10 +2531,14 @@ void __init arch_cpu_finalize_init(void)
> >  
> >  	/*
> >  	 * This needs to follow the FPU initializtion, since EFI depends on it.
> > +	 *
> > +	 * EFI twiddles CR4.LASS. Do it before CR pinning.
> >  	 */
> >  	if (efi_enabled(EFI_RUNTIME_SERVICES))
> >  		efi_enter_virtual_mode();
> >  
> > +	setup_cr_pinning();
> > +
> 
> Instead of EFI toggling CR4.LASS, why not defer the first LASS
> activation itself?
> 
> i.e.
> 
> 	if (efi_enabled(EFI_RUNTIME_SERVICES))
> 		efi_enter_virtual_mode();
> 
> 	setup_lass();
> 
> 	setup_cr_pinning();
> 
> 
> This way, we can avoid the following patch (#5) altogether.

That's definitely an option.

The benefit of current approach is that the enforcement is enabled
earlier and cover more boot code, providing marginal protection
improvement.

I also like that related security features (SMEP/SMAP/UMIP/LASS) are
enabled in the same place.

In the end it is a judgement call.

Maintainers, any preference?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov