KVM: x86: use positive error values for msr emulation that causes #GP

Recent introduction of the userspace msr filtering added code that uses
negative error codes for cases that result in either #GP delivery to
the guest, or handled by the userspace msr filtering.

This breaks an assumption that a negative error code returned from the
msr emulation code is a semi-fatal error which should be returned
to userspace via KVM_RUN ioctl and usually kill the guest.

Fix this by reusing the already existing KVM_MSR_RET_INVALID error code,
and by adding a new KVM_MSR_RET_FILTERED error code for the
userspace filtered msrs.

Fixes: 291f35fb2c1d1 ("KVM: x86: report negative values from wrmsr emulation to userspace")
Reported-by: Qian Cai <cai@redhat.com>
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20201101115523.115780-1-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -255,10 +255,9 @@ static struct kmem_cache *x86_emulator_cache;
 
 /*
  * When called, it means the previous get/set msr reached an invalid msr.
- * Return 0 if we want to ignore/silent this failed msr access, or 1 if we want
- * to fail the caller.
+ * Return true if we want to ignore/silent this failed msr access.
  */
-static int kvm_msr_ignored_check(struct kvm_vcpu *vcpu, u32 msr,
+static bool kvm_msr_ignored_check(struct kvm_vcpu *vcpu, u32 msr,
 				  u64 data, bool write)
 {
 	const char *op = write ? "wrmsr" : "rdmsr";
@@ -268,11 +267,11 @@ static int kvm_msr_ignored_check(struct kvm_vcpu *vcpu, u32 msr,
 			kvm_pr_unimpl("ignored %s: 0x%x data 0x%llx\n",
 				      op, msr, data);
 		/* Mask the error */
-		return 0;
+		return true;
 	} else {
 		kvm_debug_ratelimited("unhandled %s: 0x%x data 0x%llx\n",
 				      op, msr, data);
-		return -ENOENT;
+		return false;
 	}
 }
 
@@ -1416,7 +1415,8 @@ static int do_get_msr_feature(struct kvm_vcpu *vcpu, unsigned index, u64 *data)
 	if (r == KVM_MSR_RET_INVALID) {
 		/* Unconditionally clear the output for simplicity */
 		*data = 0;
-		r = kvm_msr_ignored_check(vcpu, index, 0, false);
+		if (kvm_msr_ignored_check(vcpu, index, 0, false))
+			r = 0;
 	}
 
 	if (r)
@@ -1540,7 +1540,7 @@ static int __kvm_set_msr(struct kvm_vcpu *vcpu, u32 index, u64 data,
 	struct msr_data msr;
 
 	if (!host_initiated && !kvm_msr_allowed(vcpu, index, KVM_MSR_FILTER_WRITE))
-		return -EPERM;
+		return KVM_MSR_RET_FILTERED;
 
 	switch (index) {
 	case MSR_FS_BASE:
@@ -1581,7 +1581,8 @@ static int kvm_set_msr_ignored_check(struct kvm_vcpu *vcpu,
 	int ret = __kvm_set_msr(vcpu, index, data, host_initiated);
 
 	if (ret == KVM_MSR_RET_INVALID)
-		ret = kvm_msr_ignored_check(vcpu, index, data, true);
+		if (kvm_msr_ignored_check(vcpu, index, data, true))
+			ret = 0;
 
 	return ret;
 }
@@ -1599,7 +1600,7 @@ int __kvm_get_msr(struct kvm_vcpu *vcpu, u32 index, u64 *data,
 	int ret;
 
 	if (!host_initiated && !kvm_msr_allowed(vcpu, index, KVM_MSR_FILTER_READ))
-		return -EPERM;
+		return KVM_MSR_RET_FILTERED;
 
 	msr.index = index;
 	msr.host_initiated = host_initiated;
@@ -1618,7 +1619,8 @@ static int kvm_get_msr_ignored_check(struct kvm_vcpu *vcpu,
 	if (ret == KVM_MSR_RET_INVALID) {
 		/* Unconditionally clear *data for simplicity */
 		*data = 0;
-		ret = kvm_msr_ignored_check(vcpu, index, 0, false);
+		if (kvm_msr_ignored_check(vcpu, index, 0, false))
+			ret = 0;
 	}
 
 	return ret;
@@ -1662,9 +1664,9 @@ static int complete_emulated_wrmsr(struct kvm_vcpu *vcpu)
 static u64 kvm_msr_reason(int r)
 {
 	switch (r) {
-	case -ENOENT:
+	case KVM_MSR_RET_INVALID:
 		return KVM_MSR_EXIT_REASON_UNKNOWN;
-	case -EPERM:
+	case KVM_MSR_RET_FILTERED:
 		return KVM_MSR_EXIT_REASON_FILTER;
 	default:
 		return KVM_MSR_EXIT_REASON_INVAL;

arch/x86/kvm/x86.h

@@ -376,7 +376,13 @@ int kvm_handle_memory_failure(struct kvm_vcpu *vcpu, int r,
 int kvm_handle_invpcid(struct kvm_vcpu *vcpu, unsigned long type, gva_t gva);
 bool kvm_msr_allowed(struct kvm_vcpu *vcpu, u32 index, u32 type);
 
-#define  KVM_MSR_RET_INVALID  2
+/*
+ * Internal error codes that are used to indicate that MSR emulation encountered
+ * an error that should result in #GP in the guest, unless userspace
+ * handles it.
+ */
+#define  KVM_MSR_RET_INVALID	2	/* in-kernel MSR emulation #GP condition */
+#define  KVM_MSR_RET_FILTERED	3	/* #GP due to userspace MSR filter */
 
 #define __cr4_reserved_bits(__cpu_has, __c)             \
 ({                                                      \