Re: [PATCH 1/2] KVM: VMX: fix instruction skipping when handling UD exception

[Sean Christopherson <seanjc@google.com>]

On Fri, Oct 22, 2021, Hou Wenlong wrote:
> When kvm.force_emulation_prefix is enabled, instruction with
> kvm prefix would trigger an UD exception and do instruction
> emulation. The emulation may need to exit to userspace due
> to userspace io, and the complete_userspace_io callback may
> skip instruction, i.e. MSR accesses emulation would exit to
> userspace if userspace wanted to know about the MSR fault.
> However, VM_EXIT_INSTRUCTION_LEN in vmcs is invalid now, it
> should use kvm_emulate_instruction() to skip instruction.
> 
> Signed-off-by: Hou Wenlong <houwenlong93@linux.alibaba.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 4 ++--
>  arch/x86/kvm/vmx/vmx.h | 9 +++++++++
>  2 files changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 1c8b2b6e7ed9..01049d65da26 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1501,8 +1501,8 @@ static int skip_emulated_instruction(struct kvm_vcpu *vcpu)
>  	 * (namely Hyper-V) don't set it due to it being undefined behavior,
>  	 * i.e. we end up advancing IP with some random value.
>  	 */
> -	if (!static_cpu_has(X86_FEATURE_HYPERVISOR) ||
> -	    exit_reason.basic != EXIT_REASON_EPT_MISCONFIG) {
> +	if (!is_ud_exit(vcpu) && (!static_cpu_has(X86_FEATURE_HYPERVISOR) ||

This is incomplete and is just a workaround for the underlying bug.  The same
mess can occur if the emulator triggers an exit to userspace during "normal"
emulation, e.g. if unrestricted guest is disabled and the guest accesses an MSR
while in Big RM.  In that case, there's no #UD to key off of.

The correct way to fix this is to attach a different callback when the MSR access
comes from the emulator.  I'd say rename the existing complete_emulated_{rd,wr}msr()
callbacks to complete_fast_{rd,wr}msr() to match the port I/O nomenclature.

Something like this (which also has some opportunistic simplification of the
error handling in kvm_emulate_{rd,wr}msr()).

---
 arch/x86/kvm/x86.c | 82 +++++++++++++++++++++++++---------------------
 1 file changed, 45 insertions(+), 37 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index ac83d873d65b..7ff5b8d58ca3 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1814,18 +1814,44 @@ int kvm_set_msr(struct kvm_vcpu *vcpu, u32 index, u64 data)
 }
 EXPORT_SYMBOL_GPL(kvm_set_msr);

-static int complete_emulated_rdmsr(struct kvm_vcpu *vcpu)
+static void __complete_emulated_rdmsr(struct kvm_vcpu *vcpu)
 {
-	int err = vcpu->run->msr.error;
-	if (!err) {
+	if (!vcpu->run->msr.error) {
 		kvm_rax_write(vcpu, (u32)vcpu->run->msr.data);
 		kvm_rdx_write(vcpu, vcpu->run->msr.data >> 32);
 	}
+}

-	return static_call(kvm_x86_complete_emulated_msr)(vcpu, err);
+static int complete_emulated_msr_access(struct kvm_vcpu *vcpu)
+{
+	if (vcpu->run->msr.error) {
+		kvm_inject_gp(vcpu, 0);
+		return 1;
+	}
+
+	return kvm_emulate_instruction(vcpu, EMULTYPE_SKIP);
+}
+
+static int complete_emulated_rdmsr(struct kvm_vcpu *vcpu)
+{
+	__complete_emulated_rdmsr(vcpu);
+
+	return complete_emulated_msr_access(vcpu);
 }

 static int complete_emulated_wrmsr(struct kvm_vcpu *vcpu)
+{
+	return complete_emulated_msr_access(vcpu);
+}
+
+static int complete_fast_rdmsr(struct kvm_vcpu *vcpu)
+{
+	__complete_emulated_rdmsr(vcpu);
+
+	return static_call(kvm_x86_complete_emulated_msr)(vcpu, vcpu->run->msr.error);
+}
+
+static int complete_fast_wrmsr(struct kvm_vcpu *vcpu)
 {
 	return static_call(kvm_x86_complete_emulated_msr)(vcpu, vcpu->run->msr.error);
 }
@@ -1864,18 +1890,6 @@ static int kvm_msr_user_space(struct kvm_vcpu *vcpu, u32 index,
 	return 1;
 }

-static int kvm_get_msr_user_space(struct kvm_vcpu *vcpu, u32 index, int r)
-{
-	return kvm_msr_user_space(vcpu, index, KVM_EXIT_X86_RDMSR, 0,
-				   complete_emulated_rdmsr, r);
-}
-
-static int kvm_set_msr_user_space(struct kvm_vcpu *vcpu, u32 index, u64 data, int r)
-{
-	return kvm_msr_user_space(vcpu, index, KVM_EXIT_X86_WRMSR, data,
-				   complete_emulated_wrmsr, r);
-}
-
 int kvm_emulate_rdmsr(struct kvm_vcpu *vcpu)
 {
 	u32 ecx = kvm_rcx_read(vcpu);
@@ -1883,19 +1897,15 @@ int kvm_emulate_rdmsr(struct kvm_vcpu *vcpu)
 	int r;

 	r = kvm_get_msr(vcpu, ecx, &data);
-
-	/* MSR read failed? See if we should ask user space */
-	if (r && kvm_get_msr_user_space(vcpu, ecx, r)) {
-		/* Bounce to user space */
-		return 0;
-	}
-
 	if (!r) {
 		trace_kvm_msr_read(ecx, data);

 		kvm_rax_write(vcpu, data & -1u);
 		kvm_rdx_write(vcpu, (data >> 32) & -1u);
 	} else {
+		if (kvm_msr_user_space(vcpu, ecx, KVM_EXIT_X86_RDMSR, 0,
+				       complete_fast_rdmsr, r))
+			return 0;
 		trace_kvm_msr_read_ex(ecx);
 	}

@@ -1910,20 +1920,16 @@ int kvm_emulate_wrmsr(struct kvm_vcpu *vcpu)
 	int r;

 	r = kvm_set_msr(vcpu, ecx, data);
-
-	/* MSR write failed? See if we should ask user space */
-	if (r && kvm_set_msr_user_space(vcpu, ecx, data, r))
-		/* Bounce to user space */
-		return 0;
-
-	/* Signal all other negative errors to userspace */
-	if (r < 0)
-		return r;
-
-	if (!r)
+	if (!r) {
 		trace_kvm_msr_write(ecx, data);
-	else
+	} else {
+		if (kvm_msr_user_space(vcpu, ecx, KVM_EXIT_X86_WRMSR, data,
+				       complete_fast_wrmsr, r))
+			return 0;
+		if (r < 0)
+			return r;
 		trace_kvm_msr_write_ex(ecx, data);
+	}

 	return static_call(kvm_x86_complete_emulated_msr)(vcpu, r);
 }
@@ -7387,7 +7393,8 @@ static int emulator_get_msr(struct x86_emulate_ctxt *ctxt,

 	r = kvm_get_msr(vcpu, msr_index, pdata);

-	if (r && kvm_get_msr_user_space(vcpu, msr_index, r)) {
+	if (r && kvm_msr_user_space(vcpu, msr_index, KVM_EXIT_X86_RDMSR, 0,
+				    complete_emulated_rdmsr, r)) {
 		/* Bounce to user space */
 		return X86EMUL_IO_NEEDED;
 	}
@@ -7403,7 +7410,8 @@ static int emulator_set_msr(struct x86_emulate_ctxt *ctxt,

 	r = kvm_set_msr(vcpu, msr_index, data);

-	if (r && kvm_set_msr_user_space(vcpu, msr_index, data, r)) {
+	if (r && kvm_msr_user_space(vcpu, msr_index, KVM_EXIT_X86_WRMSR, data,
+				    complete_emulated_wrmsr, r)) {
 		/* Bounce to user space */
 		return X86EMUL_IO_NEEDED;
 	}
--