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

[Wei Liu]

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?

> +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);
> +		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.

Thanks,
Wei.