Re: [PATCH v3] x86/dom0: delay setting SMAP after dom0 build is done

["Roger Pau Monné" <roger.pau@citrix.com>]

On Thu, Aug 01, 2024 at 05:58:38PM +0200, Jan Beulich wrote:
> On 01.08.2024 17:36, Roger Pau Monné wrote:
> > On Thu, Aug 01, 2024 at 12:28:06PM +0200, Jan Beulich wrote:
> >> On 01.08.2024 11:52, Roger Pau Monne wrote:
> >>> @@ -1907,16 +1890,25 @@ void asmlinkage __init noreturn __start_xen(unsigned long mbi_p)
> >>>      if ( cpu_has_smep && opt_smep != SMEP_HVM_ONLY )
> >>>          setup_force_cpu_cap(X86_FEATURE_XEN_SMEP);
> >>>      if ( boot_cpu_has(X86_FEATURE_XEN_SMEP) )
> >>> +    {
> >>>          set_in_cr4(X86_CR4_SMEP);
> >>> +        cr4_pv32_mask = mmu_cr4_features & XEN_CR4_PV32_BITS;
> >>
> >> Could be just "cr4_pv32_mask = X86_CR4_SMEP" now?
> > 
> > Yes, indeed, same below then.
> > 
> >>> +    }
> >>>  
> >>>      if ( !opt_smap )
> >>>          setup_clear_cpu_cap(X86_FEATURE_SMAP);
> >>>      if ( cpu_has_smap && opt_smap != SMAP_HVM_ONLY )
> >>>          setup_force_cpu_cap(X86_FEATURE_XEN_SMAP);
> >>>      if ( boot_cpu_has(X86_FEATURE_XEN_SMAP) )
> >>> -        set_in_cr4(X86_CR4_SMAP);
> >>> -
> >>> -    cr4_pv32_mask = mmu_cr4_features & XEN_CR4_PV32_BITS;
> >>> +        /*
> >>> +         * Set SMAP on the %cr4 mask so that it's set for APs on bringup, but
> >>> +         * don't set for the BSP until domain building is done.
> >>> +         *
> >>> +         * Don't set it in cr4_pv32_mask either, until it's also set on the
> >>> +         * BSP.  Otherwise the BUG in cr4_pv32_restore would trigger for events
> >>> +         * received on the BSP.
> >>> +         */
> >>> +        mmu_cr4_features |= X86_CR4_SMAP;
> >>
> >> Don't you put APs at risk this way of triggering the BUG in cr4_pv32_restore()?
> >> They'll have the bit set in %cr4, but the bit remains clear in cr4_pv32_mask
> >> until much later.
> > 
> > As long as the bit is set in %cr4, but not in cr4_pv32_mask the BUG in
> > cr4_pv32_restore won't hit.
> 
> Hmm, you're right. Despite staring at that for quite some time, I got
> it wrong. Feel free to add
> Reviewed-by: Jan Beulich <jbeulich@suse.com>
> then, preferably with said minor adjustment (left in context above).

Sure, thanks!

Will adjust and send v4.