Re: [PATCH 2/2] KVM: x86: Allow userspace to update tracked sregs for protected guests

[Tom Lendacky]

On 5/7/21 11:59 AM, Sean Christopherson wrote:
> Allow userspace to set CR0, CR4, CR8, and EFER via KVM_SET_SREGS for
> protected guests, e.g. for SEV-ES guests with an encrypted VMSA.  KVM
> tracks the aforementioned registers by trapping guest writes, and also
> exposes the values to userspace via KVM_GET_SREGS.  Skipping the regs
> in KVM_SET_SREGS prevents userspace from updating KVM's CPU model to
> match the known hardware state.

This is very similar to the original patch I had proposed that you were
against :)

I'm assuming it's meant to make live migration a bit easier?

> 
> Fixes: 5265713a0737 ("KVM: x86: Update __get_sregs() / __set_sregs() to support SEV-ES")
> Reported-by: Peter Gonda <pgonda@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

Acked-by: Tom Lendacky <thomas.lendacky@xxxxxxx>

> ---
>  arch/x86/kvm/x86.c | 73 ++++++++++++++++++++++++++--------------------
>  1 file changed, 42 insertions(+), 31 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 3bf52ba5f2bb..1b7d0e97c82b 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9963,21 +9963,25 @@ static int __set_sregs(struct kvm_vcpu *vcpu, struct kvm_sregs *sregs)
>  	if (kvm_set_apic_base(vcpu, &apic_base_msr))
>  		goto out;
>  
> -	if (vcpu->arch.guest_state_protected)
> -		goto skip_protected_regs;
> +	if (!vcpu->arch.guest_state_protected) {
> +		dt.size = sregs->idt.limit;
> +		dt.address = sregs->idt.base;
> +		static_call(kvm_x86_set_idt)(vcpu, &dt);
> +		dt.size = sregs->gdt.limit;
> +		dt.address = sregs->gdt.base;
> +		static_call(kvm_x86_set_gdt)(vcpu, &dt);
>  
> -	dt.size = sregs->idt.limit;
> -	dt.address = sregs->idt.base;
> -	static_call(kvm_x86_set_idt)(vcpu, &dt);
> -	dt.size = sregs->gdt.limit;
> -	dt.address = sregs->gdt.base;
> -	static_call(kvm_x86_set_gdt)(vcpu, &dt);
> -
> -	vcpu->arch.cr2 = sregs->cr2;
> -	mmu_reset_needed |= kvm_read_cr3(vcpu) != sregs->cr3;
> -	vcpu->arch.cr3 = sregs->cr3;
> -	kvm_register_mark_available(vcpu, VCPU_EXREG_CR3);
> +		vcpu->arch.cr2 = sregs->cr2;
> +		mmu_reset_needed |= kvm_read_cr3(vcpu) != sregs->cr3;
> +		vcpu->arch.cr3 = sregs->cr3;
> +		kvm_register_mark_available(vcpu, VCPU_EXREG_CR3);
> +	}
>  
> +	/*
> +	 * Writes to CR0, CR4, CR8, and EFER are trapped (after the instruction
> +	 * completes) for SEV-EV guests, thus userspace is allowed to set them
> +	 * so that KVM's model can be updated to mirror hardware state.
> +	 */
>  	kvm_set_cr8(vcpu, sregs->cr8);
>  
>  	mmu_reset_needed |= vcpu->arch.efer != sregs->efer;
> @@ -9990,35 +9994,42 @@ static int __set_sregs(struct kvm_vcpu *vcpu, struct kvm_sregs *sregs)
>  	mmu_reset_needed |= kvm_read_cr4(vcpu) != sregs->cr4;
>  	static_call(kvm_x86_set_cr4)(vcpu, sregs->cr4);
>  
> -	idx = srcu_read_lock(&vcpu->kvm->srcu);
> -	if (is_pae_paging(vcpu)) {
> +	/*
> +	 * PDPTEs, like regular PTEs, are always encrypted, thus reading them
> +	 * will return garbage.  Shadow paging, including nested NPT, isn't
> +	 * compatible with protected guests, so ignoring the PDPTEs is a-ok.
> +	 */
> +	if (!vcpu->arch.guest_state_protected && is_pae_paging(vcpu)) {
> +		idx = srcu_read_lock(&vcpu->kvm->srcu);
>  		load_pdptrs(vcpu, vcpu->arch.walk_mmu, kvm_read_cr3(vcpu));
> +		srcu_read_unlock(&vcpu->kvm->srcu, idx);
> +
>  		mmu_reset_needed = 1;
>  	}
> -	srcu_read_unlock(&vcpu->kvm->srcu, idx);
>  
>  	if (mmu_reset_needed)
>  		kvm_mmu_reset_context(vcpu);
>  
> -	kvm_set_segment(vcpu, &sregs->cs, VCPU_SREG_CS);
> -	kvm_set_segment(vcpu, &sregs->ds, VCPU_SREG_DS);
> -	kvm_set_segment(vcpu, &sregs->es, VCPU_SREG_ES);
> -	kvm_set_segment(vcpu, &sregs->fs, VCPU_SREG_FS);
> -	kvm_set_segment(vcpu, &sregs->gs, VCPU_SREG_GS);
> -	kvm_set_segment(vcpu, &sregs->ss, VCPU_SREG_SS);
> +	if (!vcpu->arch.guest_state_protected) {
> +		kvm_set_segment(vcpu, &sregs->cs, VCPU_SREG_CS);
> +		kvm_set_segment(vcpu, &sregs->ds, VCPU_SREG_DS);
> +		kvm_set_segment(vcpu, &sregs->es, VCPU_SREG_ES);
> +		kvm_set_segment(vcpu, &sregs->fs, VCPU_SREG_FS);
> +		kvm_set_segment(vcpu, &sregs->gs, VCPU_SREG_GS);
> +		kvm_set_segment(vcpu, &sregs->ss, VCPU_SREG_SS);
>  
> -	kvm_set_segment(vcpu, &sregs->tr, VCPU_SREG_TR);
> -	kvm_set_segment(vcpu, &sregs->ldt, VCPU_SREG_LDTR);
> +		kvm_set_segment(vcpu, &sregs->tr, VCPU_SREG_TR);
> +		kvm_set_segment(vcpu, &sregs->ldt, VCPU_SREG_LDTR);
>  
> -	update_cr8_intercept(vcpu);
> +		update_cr8_intercept(vcpu);
>  
> -	/* Older userspace won't unhalt the vcpu on reset. */
> -	if (kvm_vcpu_is_bsp(vcpu) && kvm_rip_read(vcpu) == 0xfff0 &&
> -	    sregs->cs.selector == 0xf000 && sregs->cs.base == 0xffff0000 &&
> -	    !is_protmode(vcpu))
> -		vcpu->arch.mp_state = KVM_MP_STATE_RUNNABLE;
> +		/* Older userspace won't unhalt the vcpu on reset. */
> +		if (kvm_vcpu_is_bsp(vcpu) && kvm_rip_read(vcpu) == 0xfff0 &&
> +		    sregs->cs.selector == 0xf000 &&
> +		    sregs->cs.base == 0xffff0000 && !is_protmode(vcpu))
> +			vcpu->arch.mp_state = KVM_MP_STATE_RUNNABLE;
> +	}
>  
> -skip_protected_regs:
>  	max_bits = KVM_NR_INTERRUPTS;
>  	pending_vec = find_first_bit(
>  		(const unsigned long *)sregs->interrupt_bitmap, max_bits);
>