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

[Mickaël Salaün]

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@alien8.de>
> > Cc: Dave Hansen <dave.hansen@linux.intel.com>
> > Cc: H. Peter Anvin <hpa@zytor.com>
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Cc: Kees Cook <keescook@chromium.org>
> > Cc: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
> > Cc: Paolo Bonzini <pbonzini@redhat.com>
> > Cc: Sean Christopherson <seanjc@google.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
> > Cc: Wanpeng Li <wanpengli@tencent.com>
> > Signed-off-by: Mickaël Salaün <mic@digikod.net>
> > Link: 20230505152046.6575-6-mic@digikod.net">https://lore.kernel.org/r/20230505152046.6575-6-mic@digikod.net
> [...]
> >         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?


> > +static int heki_lock_cr(struct kvm *const kvm, const unsigned long cr,
> > +                       unsigned long pin)
> > +{
> > +       if (!pin)
> > +               return -KVM_EINVAL;
> > +
> > +       switch (cr) {
> > +       case 0:
> > +               /* Cf. arch/x86/kernel/cpu/common.c */
> > +               if (!(pin & X86_CR0_WP))
> > +                       return -KVM_EINVAL;
> > +
> > +               if ((read_cr0() & pin) != pin)
> > +                       return -KVM_EINVAL;
> > +
> > +               atomic_long_or(pin, &kvm->heki_pinned_cr0);
> > +               return 0;
> > +       case 4:
> > +               /* Checks for irrelevant bits. */
> > +               if ((pin & cr4_pinned_mask) != pin)
> > +                       return -KVM_EINVAL;
> > +
>
> It is enforcing the host mask on the guest, right? If the guest's set is a
> super set of the host's then it will get rejected.
>
>
> > +               /* Ignores bits not present in host. */
> > +               pin &= __read_cr4();
> > +               atomic_long_or(pin, &kvm->heki_pinned_cr4);

We assume that the host's mask is a superset of the guest's mask. I 
guess we should check the absolute supported bits instead, even if it 
would be weird for the host to not support these bits.


> > +               return 0;
> > +       }
> > +       return -KVM_EINVAL;
> > +}
> > +
> > +int heki_check_cr(const struct kvm *const kvm, const unsigned long cr,
> > +                 const unsigned long val)
> > +{
> > +       unsigned long pinned;
> > +
> > +       switch (cr) {
> > +       case 0:
> > +               pinned = atomic_long_read(&kvm->heki_pinned_cr0);
> > +               if ((val & pinned) != pinned) {
> > +                       pr_warn_ratelimited(
> > +                               "heki-kvm: Blocked CR0 update: 0x%lx\n", val);
>
> I think if the message contains the VM and VCPU identifier it will
> become more useful.

Indeed, and this should be the case for all log messages, but I'd left 
that for future work. ;) I'll update the logs for the next series with a 
new kvm_warn_ratelimited() helper using VCPU's PID.