Re: [PATCH v2] KVM: x86: Assume a 64-bit hypercall for guests with protected state

[Paolo Bonzini]

On 5/24/21 19:48, Tom Lendacky wrote:
> When processing a hypercall for a guest with protected state, currently
> SEV-ES guests, the guest CS segment register can't be checked to
> determine if the guest is in 64-bit mode. For an SEV-ES guest, it is
> expected that communication between the guest and the hypervisor is
> performed to shared memory using the GHCB. In order to use the GHCB, the
> guest must have been in long mode, otherwise writes by the guest to the
> GHCB would be encrypted and not be able to be comprehended by the
> hypervisor.
>
> Create a new helper function, is_64_bit_hypercall(), that assumes the
> guest is in 64-bit mode when the guest has protected state, and returns
> true, otherwise invoking is_64_bit_mode() to determine the mode. Update
> the hypercall related routines to use is_64_bit_hypercall() instead of
> is_64_bit_mode().
>
> Add a WARN_ON_ONCE() to is_64_bit_mode() to catch occurences of calls to
> this helper function for a guest running with protected state.
>
> Fixes: f1c6366e3043 ("KVM: SVM: Add required changes to support intercepts under SEV-ES")
> Reported-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
> ---
>
> Changes since v1:
> - Create a new helper routine, is_64_bit_hypercall(), and use it in place
>    of is_64_bit_mode() in hypercall related areas.
> - Add a WARN_ON_ONCE() to is_64_bit_mode() to issue a warning if invoked
>    for a guest with protected state.
> ---
>   arch/x86/kvm/hyperv.c |  4 ++--
>   arch/x86/kvm/x86.c    |  2 +-
>   arch/x86/kvm/x86.h    | 12 ++++++++++++
>   arch/x86/kvm/xen.c    |  2 +-
>   4 files changed, 16 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> index f98370a39936..1cdf2b213f41 100644
> --- a/arch/x86/kvm/hyperv.c
> +++ b/arch/x86/kvm/hyperv.c
> @@ -1818,7 +1818,7 @@ static void kvm_hv_hypercall_set_result(struct kvm_vcpu *vcpu, u64 result)
>   {
>   	bool longmode;
>   
> -	longmode = is_64_bit_mode(vcpu);
> +	longmode = is_64_bit_hypercall(vcpu);
>   	if (longmode)
>   		kvm_rax_write(vcpu, result);
>   	else {
> @@ -1895,7 +1895,7 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
>   	}
>   
>   #ifdef CONFIG_X86_64
> -	if (is_64_bit_mode(vcpu)) {
> +	if (is_64_bit_hypercall(vcpu)) {
>   		param = kvm_rcx_read(vcpu);
>   		ingpa = kvm_rdx_read(vcpu);
>   		outgpa = kvm_r8_read(vcpu);
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 9b6bca616929..dc72f0a1609a 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8403,7 +8403,7 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
>   
>   	trace_kvm_hypercall(nr, a0, a1, a2, a3);
>   
> -	op_64_bit = is_64_bit_mode(vcpu);
> +	op_64_bit = is_64_bit_hypercall(vcpu);
>   	if (!op_64_bit) {
>   		nr &= 0xFFFFFFFF;
>   		a0 &= 0xFFFFFFFF;
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index 521f74e5bbf2..3102caf689d2 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -151,12 +151,24 @@ static inline bool is_64_bit_mode(struct kvm_vcpu *vcpu)
>   {
>   	int cs_db, cs_l;
>   
> +	WARN_ON_ONCE(vcpu->arch.guest_state_protected);
> +
>   	if (!is_long_mode(vcpu))
>   		return false;
>   	static_call(kvm_x86_get_cs_db_l_bits)(vcpu, &cs_db, &cs_l);
>   	return cs_l;
>   }
>   
> +static inline bool is_64_bit_hypercall(struct kvm_vcpu *vcpu)
> +{
> +	/*
> +	 * If running with protected guest state, the CS register is not
> +	 * accessible. The hypercall register values will have had to been
> +	 * provided in 64-bit mode, so assume the guest is in 64-bit.
> +	 */
> +	return vcpu->arch.guest_state_protected || is_64_bit_mode(vcpu);
> +}
> +
>   static inline bool is_la57_mode(struct kvm_vcpu *vcpu)
>   {
>   #ifdef CONFIG_X86_64
> diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
> index ae17250e1efe..c58f6369e668 100644
> --- a/arch/x86/kvm/xen.c
> +++ b/arch/x86/kvm/xen.c
> @@ -680,7 +680,7 @@ int kvm_xen_hypercall(struct kvm_vcpu *vcpu)
>   	    kvm_hv_hypercall_enabled(vcpu))
>   		return kvm_hv_hypercall(vcpu);
>   
> -	longmode = is_64_bit_mode(vcpu);
> +	longmode = is_64_bit_hypercall(vcpu);
>   	if (!longmode) {
>   		params[0] = (u32)kvm_rbx_read(vcpu);
>   		params[1] = (u32)kvm_rcx_read(vcpu);

Queued, thanks.

Paolo