KVM: x86: Introduce MSR filtering

It's not desireable to have all MSRs always handled by KVM kernel space. Some
MSRs would be useful to handle in user space to either emulate behavior (like
uCode updates) or differentiate whether they are valid based on the CPU model.

To allow user space to specify which MSRs it wants to see handled by KVM,
this patch introduces a new ioctl to push filter rules with bitmaps into
KVM. Based on these bitmaps, KVM can then decide whether to reject MSR access.
With the addition of KVM_CAP_X86_USER_SPACE_MSR it can also deflect the
denied MSR events to user space to operate on.

If no filter is populated, MSR handling stays identical to before.

Signed-off-by: Alexander Graf <graf@amazon.com>

Message-Id: <20200925143422.21718-8-graf@amazon.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 144 insertions, 1 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 60219882fee2..72f91f3640f3 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1490,7 +1490,35 @@ EXPORT_SYMBOL_GPL(kvm_enable_efer_bits);
 
 bool kvm_msr_allowed(struct kvm_vcpu *vcpu, u32 index, u32 type)
 {
-	return true;
+	struct kvm *kvm = vcpu->kvm;
+	struct msr_bitmap_range *ranges = kvm->arch.msr_filter.ranges;
+	u32 count = kvm->arch.msr_filter.count;
+	u32 i;
+	bool r = kvm->arch.msr_filter.default_allow;
+	int idx;
+
+	/* MSR filtering not set up, allow everything */
+	if (!count)
+		return true;
+
+	/* Prevent collision with set_msr_filter */
+	idx = srcu_read_lock(&kvm->srcu);
+
+	for (i = 0; i < count; i++) {
+		u32 start = ranges[i].base;
+		u32 end = start + ranges[i].nmsrs;
+		u32 flags = ranges[i].flags;
+		unsigned long *bitmap = ranges[i].bitmap;
+
+		if ((index >= start) && (index < end) && (flags & type)) {
+			r = !!test_bit(index - start, bitmap);
+			break;
+		}
+	}
+
+	srcu_read_unlock(&kvm->srcu, idx);
+
+	return r;
 }
 EXPORT_SYMBOL_GPL(kvm_msr_allowed);
 
@@ -1505,6 +1533,9 @@ 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;
+
 	switch (index) {
 	case MSR_FS_BASE:
 	case MSR_GS_BASE:
@@ -1561,6 +1592,9 @@ int __kvm_get_msr(struct kvm_vcpu *vcpu, u32 index, u64 *data,
 	struct msr_data msr;
 	int ret;
 
+	if (!host_initiated && !kvm_msr_allowed(vcpu, index, KVM_MSR_FILTER_READ))
+		return -EPERM;
+
 	msr.index = index;
 	msr.host_initiated = host_initiated;
 
@@ -1624,6 +1658,8 @@ static u64 kvm_msr_reason(int r)
 	switch (r) {
 	case -ENOENT:
 		return KVM_MSR_EXIT_REASON_UNKNOWN;
+	case -EPERM:
+		return KVM_MSR_EXIT_REASON_FILTER;
 	default:
 		return KVM_MSR_EXIT_REASON_INVAL;
 	}
@@ -3620,6 +3656,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 	case KVM_CAP_SET_GUEST_DEBUG:
 	case KVM_CAP_LAST_CPU:
 	case KVM_CAP_X86_USER_SPACE_MSR:
+	case KVM_CAP_X86_MSR_FILTER:
 		r = 1;
 		break;
 	case KVM_CAP_SYNC_REGS:
@@ -5151,6 +5188,103 @@ split_irqchip_unlock:
 	return r;
 }
 
+static void kvm_clear_msr_filter(struct kvm *kvm)
+{
+	u32 i;
+	u32 count = kvm->arch.msr_filter.count;
+	struct msr_bitmap_range ranges[16];
+
+	mutex_lock(&kvm->lock);
+	kvm->arch.msr_filter.count = 0;
+	memcpy(ranges, kvm->arch.msr_filter.ranges, count * sizeof(ranges[0]));
+	mutex_unlock(&kvm->lock);
+	synchronize_srcu(&kvm->srcu);
+
+	for (i = 0; i < count; i++)
+		kfree(ranges[i].bitmap);
+}
+
+static int kvm_add_msr_filter(struct kvm *kvm, struct kvm_msr_filter_range *user_range)
+{
+	struct msr_bitmap_range *ranges = kvm->arch.msr_filter.ranges;
+	struct msr_bitmap_range range;
+	unsigned long *bitmap = NULL;
+	size_t bitmap_size;
+	int r;
+
+	if (!user_range->nmsrs)
+		return 0;
+
+	bitmap_size = BITS_TO_LONGS(user_range->nmsrs) * sizeof(long);
+	if (!bitmap_size || bitmap_size > KVM_MSR_FILTER_MAX_BITMAP_SIZE)
+		return -EINVAL;
+
+	bitmap = memdup_user((__user u8*)user_range->bitmap, bitmap_size);
+	if (IS_ERR(bitmap))
+		return PTR_ERR(bitmap);
+
+	range = (struct msr_bitmap_range) {
+		.flags = user_range->flags,
+		.base = user_range->base,
+		.nmsrs = user_range->nmsrs,
+		.bitmap = bitmap,
+	};
+
+	if (range.flags & ~(KVM_MSR_FILTER_READ | KVM_MSR_FILTER_WRITE)) {
+		r = -EINVAL;
+		goto err;
+	}
+
+	if (!range.flags) {
+		r = -EINVAL;
+		goto err;
+	}
+
+	/* Everything ok, add this range identifier to our global pool */
+	ranges[kvm->arch.msr_filter.count] = range;
+	/* Make sure we filled the array before we tell anyone to walk it */
+	smp_wmb();
+	kvm->arch.msr_filter.count++;
+
+	return 0;
+err:
+	kfree(bitmap);
+	return r;
+}
+
+static int kvm_vm_ioctl_set_msr_filter(struct kvm *kvm, void __user *argp)
+{
+	struct kvm_msr_filter __user *user_msr_filter = argp;
+	struct kvm_msr_filter filter;
+	bool default_allow;
+	int r = 0;
+	u32 i;
+
+	if (copy_from_user(&filter, user_msr_filter, sizeof(filter)))
+		return -EFAULT;
+
+	kvm_clear_msr_filter(kvm);
+
+	default_allow = !(filter.flags & KVM_MSR_FILTER_DEFAULT_DENY);
+	kvm->arch.msr_filter.default_allow = default_allow;
+
+	/*
+	 * Protect from concurrent calls to this function that could trigger
+	 * a TOCTOU violation on kvm->arch.msr_filter.count.
+	 */
+	mutex_lock(&kvm->lock);
+	for (i = 0; i < ARRAY_SIZE(filter.ranges); i++) {
+		r = kvm_add_msr_filter(kvm, &filter.ranges[i]);
+		if (r)
+			break;
+	}
+
+	kvm_make_all_cpus_request(kvm, KVM_REQ_MSR_FILTER_CHANGED);
+	mutex_unlock(&kvm->lock);
+
+	return r;
+}
+
 long kvm_arch_vm_ioctl(struct file *filp,
 		       unsigned int ioctl, unsigned long arg)
 {
@@ -5457,6 +5591,9 @@ set_pit2_out:
 	case KVM_SET_PMU_EVENT_FILTER:
 		r = kvm_vm_ioctl_set_pmu_event_filter(kvm, argp);
 		break;
+	case KVM_X86_SET_MSR_FILTER:
+		r = kvm_vm_ioctl_set_msr_filter(kvm, argp);
+		break;
 	default:
 		r = -ENOTTY;
 	}
@@ -8611,6 +8748,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 			kvm_vcpu_update_apicv(vcpu);
 		if (kvm_check_request(KVM_REQ_APF_READY, vcpu))
 			kvm_check_async_pf_completion(vcpu);
+		if (kvm_check_request(KVM_REQ_MSR_FILTER_CHANGED, vcpu))
+			kvm_x86_ops.msr_filter_changed(vcpu);
 	}
 
 	if (kvm_check_request(KVM_REQ_EVENT, vcpu) || req_int_win) {
@@ -10163,6 +10302,8 @@ void kvm_arch_pre_destroy_vm(struct kvm *kvm)
 
 void kvm_arch_destroy_vm(struct kvm *kvm)
 {
+	u32 i;
+
 	if (current->mm == kvm->mm) {
 		/*
 		 * Free memory regions allocated on behalf of userspace,
@@ -10179,6 +10320,8 @@ void kvm_arch_destroy_vm(struct kvm *kvm)
 	}
 	if (kvm_x86_ops.vm_destroy)
 		kvm_x86_ops.vm_destroy(kvm);
+	for (i = 0; i < kvm->arch.msr_filter.count; i++)
+		kfree(kvm->arch.msr_filter.ranges[i].bitmap);
 	kvm_pic_destroy(kvm);
 	kvm_ioapic_destroy(kvm);
 	kvm_free_vcpus(kvm);