Re: [PATCH v1 5/9] KVM: x86: Add new hypercall to lock control registers

[Kees Cook <keescook@xxxxxxxxxxxx>]

On Mon, May 29, 2023 at 06:48:03PM +0200, Mickaël Salaün wrote:
> 
> On 08/05/2023 23:11, Wei Liu wrote:
> > On Fri, May 05, 2023 at 05:20:42PM +0200, Mickaël Salaün wrote:
> > > This enables guests to lock their CR0 and CR4 registers with a subset of
> > > X86_CR0_WP, X86_CR4_SMEP, X86_CR4_SMAP, X86_CR4_UMIP, X86_CR4_FSGSBASE
> > > and X86_CR4_CET flags.
> > > 
> > > The new KVM_HC_LOCK_CR_UPDATE hypercall takes two arguments.  The first
> > > is to identify the control register, and the second is a bit mask to
> > > pin (i.e. mark as read-only).
> > > 
> > > These register flags should already be pinned by Linux guests, but once
> > > compromised, this self-protection mechanism could be disabled, which is
> > > not the case with this dedicated hypercall.
> > > 
> > > Cc: Borislav Petkov <bp@xxxxxxxxx>
> > > Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > > Cc: H. Peter Anvin <hpa@xxxxxxxxx>
> > > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > > Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> > > Cc: Madhavan T. Venkataraman <madvenka@xxxxxxxxxxxxxxxxxxx>
> > > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > > Cc: Sean Christopherson <seanjc@xxxxxxxxxx>
> > > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > > Cc: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> > > Cc: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> > > Signed-off-by: Mickaël Salaün <mic@xxxxxxxxxxx>
> > > Link: https://lore.kernel.org/r/20230505152046.6575-6-mic@xxxxxxxxxxx
> > [...]
> > >           hw_cr4 = (cr4_read_shadow() & X86_CR4_MCE) | (cr4 & 
> > > ~X86_CR4_MCE);
> > >           if (is_unrestricted_guest(vcpu))
> > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > index ffab64d08de3..a529455359ac 100644
> > > --- a/arch/x86/kvm/x86.c
> > > +++ b/arch/x86/kvm/x86.c
> > > @@ -7927,11 +7927,77 @@ static unsigned long emulator_get_cr(struct 
> > > x86_emulate_ctxt *ctxt, int cr)
> > >           return value;
> > >   }
> > > +#ifdef CONFIG_HEKI
> > > +
> > > +extern unsigned long cr4_pinned_mask;
> > > +
> > 
> > Can this be moved to a header file?
> 
> Yep, but I'm not sure which one. Any preference Kees?

Uh, er, I was never expecting that mask to be non-static. ;) To that
end, how about putting it in arch/x86/kvm/x86.h ?

-- 
Kees Cook