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>

Documentation/virt/kvm/api.rst

@@ -4707,6 +4707,99 @@ KVM_PV_VM_VERIFY
   Verify the integrity of the unpacked image. Only if this succeeds,
   KVM is allowed to start protected VCPUs.
 
+4.126 KVM_X86_SET_MSR_FILTER
+----------------------------
+
+:Capability: KVM_X86_SET_MSR_FILTER
+:Architectures: x86
+:Type: vm ioctl
+:Parameters: struct kvm_msr_filter
+:Returns: 0 on success, < 0 on error
+
+::
+
+  struct kvm_msr_filter_range {
+  #define KVM_MSR_FILTER_READ  (1 << 0)
+  #define KVM_MSR_FILTER_WRITE (1 << 1)
+	__u32 flags;
+	__u32 nmsrs; /* number of msrs in bitmap */
+	__u32 base;  /* MSR index the bitmap starts at */
+	__u8 *bitmap; /* a 1 bit allows the operations in flags, 0 denies */
+  };
+
+  #define KVM_MSR_FILTER_MAX_RANGES 16
+  struct kvm_msr_filter {
+  #define KVM_MSR_FILTER_DEFAULT_ALLOW (0 << 0)
+  #define KVM_MSR_FILTER_DEFAULT_DENY  (1 << 0)
+	__u32 flags;
+	struct kvm_msr_filter_range ranges[KVM_MSR_FILTER_MAX_RANGES];
+  };
+
+flags values for struct kvm_msr_filter_range:
+
+KVM_MSR_FILTER_READ
+
+  Filter read accesses to MSRs using the given bitmap. A 0 in the bitmap
+  indicates that a read should immediately fail, while a 1 indicates that
+  a read for a particular MSR should be handled regardless of the default
+  filter action.
+
+KVM_MSR_FILTER_WRITE
+
+  Filter write accesses to MSRs using the given bitmap. A 0 in the bitmap
+  indicates that a write should immediately fail, while a 1 indicates that
+  a write for a particular MSR should be handled regardless of the default
+  filter action.
+
+KVM_MSR_FILTER_READ | KVM_MSR_FILTER_WRITE
+
+  Filter both read and write accesses to MSRs using the given bitmap. A 0
+  in the bitmap indicates that both reads and writes should immediately fail,
+  while a 1 indicates that reads and writes for a particular MSR are not
+  filtered by this range.
+
+flags values for struct kvm_msr_filter:
+
+KVM_MSR_FILTER_DEFAULT_ALLOW
+
+  If no filter range matches an MSR index that is getting accessed, KVM will
+  fall back to allowing access to the MSR.
+
+KVM_MSR_FILTER_DEFAULT_DENY
+
+  If no filter range matches an MSR index that is getting accessed, KVM will
+  fall back to rejecting access to the MSR. In this mode, all MSRs that should
+  be processed by KVM need to explicitly be marked as allowed in the bitmaps.
+
+This ioctl allows user space to define up to 16 bitmaps of MSR ranges to
+specify whether a certain MSR access should be explicitly filtered for or not.
+
+If this ioctl has never been invoked, MSR accesses are not guarded and the
+old KVM in-kernel emulation behavior is fully preserved.
+
+As soon as the filtering is in place, every MSR access is processed through
+the filtering. If a bit is within one of the defined ranges, read and write
+accesses are guarded by the bitmap's value for the MSR index. If it is not
+defined in any range, whether MSR access is rejected is determined by the flags
+field in the kvm_msr_filter struct: KVM_MSR_FILTER_DEFAULT_ALLOW and
+KVM_MSR_FILTER_DEFAULT_DENY.
+
+Calling this ioctl with an empty set of ranges (all nmsrs == 0) disables MSR
+filtering. In that mode, KVM_MSR_FILTER_DEFAULT_DENY no longer has any effect.
+
+Each bitmap range specifies a range of MSRs to potentially allow access on.
+The range goes from MSR index [base .. base+nmsrs]. The flags field
+indicates whether reads, writes or both reads and writes are filtered
+by setting a 1 bit in the bitmap for the corresponding MSR index.
+
+If an MSR access is not permitted through the filtering, it generates a
+#GP inside the guest. When combined with KVM_CAP_X86_USER_SPACE_MSR, that
+allows user space to deflect and potentially handle various MSR accesses
+into user space.
+
+If a vCPU is in running state while this ioctl is invoked, the vCPU may
+experience inconsistent filtering behavior on MSR accesses.
+
 
 5. The kvm_run structure
 ========================
@@ -5187,6 +5280,7 @@ ENABLE_CAP. Currently valid exit reasons are:
 
 	KVM_MSR_EXIT_REASON_UNKNOWN - access to MSR that is unknown to KVM
 	KVM_MSR_EXIT_REASON_INVAL - access to invalid MSRs or reserved bits
+	KVM_MSR_EXIT_REASON_FILTER - access blocked by KVM_X86_SET_MSR_FILTER
 
 For KVM_EXIT_X86_RDMSR, the "index" field tells user space which MSR the guest
 wants to read. To respond to this request with a successful read, user space
@@ -6265,3 +6359,17 @@ writes to user space. It can be enabled on a VM level. If enabled, MSR
 accesses that would usually trigger a #GP by KVM into the guest will
 instead get bounced to user space through the KVM_EXIT_X86_RDMSR and
 KVM_EXIT_X86_WRMSR exit notifications.
+
+8.25 KVM_X86_SET_MSR_FILTER
+---------------------------
+
+:Architectures: x86
+
+This capability indicates that KVM supports that accesses to user defined MSRs
+may be rejected. With this capability exposed, KVM exports new VM ioctl
+KVM_X86_SET_MSR_FILTER which user space can call to specify bitmaps of MSR
+ranges that KVM should reject access to.
+
+In combination with KVM_CAP_X86_USER_SPACE_MSR, this allows user space to
+trap and emulate MSRs that are outside of the scope of KVM as well as
+limit the attack surface on KVM's MSR emulation code.

arch/x86/include/asm/kvm_host.h

@@ -87,6 +87,7 @@
 #define KVM_REQ_HV_TLB_FLUSH \
 	KVM_ARCH_REQ_FLAGS(27, KVM_REQUEST_NO_WAKEUP)
 #define KVM_REQ_APF_READY		KVM_ARCH_REQ(28)
+#define KVM_REQ_MSR_FILTER_CHANGED	KVM_ARCH_REQ(29)
 
 #define CR0_RESERVED_BITS                                               \
 	(~(unsigned long)(X86_CR0_PE | X86_CR0_MP | X86_CR0_EM | X86_CR0_TS \
@@ -860,6 +861,13 @@ struct kvm_hv {
 	struct kvm_hv_syndbg hv_syndbg;
 };
 
+struct msr_bitmap_range {
+	u32 flags;
+	u32 nmsrs;
+	u32 base;
+	unsigned long *bitmap;
+};
+
 enum kvm_irqchip_mode {
 	KVM_IRQCHIP_NONE,
 	KVM_IRQCHIP_KERNEL,       /* created with KVM_CREATE_IRQCHIP */
@@ -964,6 +972,12 @@ struct kvm_arch {
 	/* Deflect RDMSR and WRMSR to user space when they trigger a #GP */
 	u32 user_space_msr_mask;
 
+	struct {
+		u8 count;
+		bool default_allow:1;
+		struct msr_bitmap_range ranges[16];
+	} msr_filter;
+
 	struct kvm_pmu_event_filter *pmu_event_filter;
 	struct task_struct *nx_lpage_recovery_thread;
 };

arch/x86/include/uapi/asm/kvm.h

@@ -192,8 +192,26 @@ struct kvm_msr_list {
 	__u32 indices[0];
 };
 
+/* Maximum size of any access bitmap in bytes */
+#define KVM_MSR_FILTER_MAX_BITMAP_SIZE 0x600
+
+/* for KVM_X86_SET_MSR_FILTER */
+struct kvm_msr_filter_range {
 #define KVM_MSR_FILTER_READ  (1 << 0)
 #define KVM_MSR_FILTER_WRITE (1 << 1)
+	__u32 flags;
+	__u32 nmsrs; /* number of msrs in bitmap */
+	__u32 base;  /* MSR index the bitmap starts at */
+	__u8 *bitmap; /* a 1 bit allows the operations in flags, 0 denies */
+};
+
+#define KVM_MSR_FILTER_MAX_RANGES 16
+struct kvm_msr_filter {
+#define KVM_MSR_FILTER_DEFAULT_ALLOW (0 << 0)
+#define KVM_MSR_FILTER_DEFAULT_DENY  (1 << 0)
+	__u32 flags;
+	struct kvm_msr_filter_range ranges[KVM_MSR_FILTER_MAX_RANGES];
+};
 
 struct kvm_cpuid_entry {
 	__u32 function;