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SubjectRe: [PATCH v2 2/3] KVM: x86: Introduce allow list for MSR emulation
Date
Alexander Graf <graf@amazon.com> writes:

> 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 allow lists of 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 allowlist is populated, MSR handling stays identical to before.
>
> Signed-off-by: KarimAllah Ahmed <karahmed@amazon.de>
> Signed-off-by: Alexander Graf <graf@amazon.com>
> ---
> Documentation/virt/kvm/api.rst | 53 ++++++++++++++
> arch/x86/include/asm/kvm_host.h | 10 +++
> arch/x86/include/uapi/asm/kvm.h | 15 ++++
> arch/x86/kvm/x86.c | 123 ++++++++++++++++++++++++++++++++
> include/uapi/linux/kvm.h | 4 ++
> 5 files changed, 205 insertions(+)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index c1f991c1ffa6..ca92b9e2cded 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -4697,6 +4697,45 @@ 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_ADD_MSR_ALLOWLIST
> +-------------------------
> +
> +:Capability: KVM_CAP_ADD_MSR_ALLOWLIST
> +:Architectures: x86
> +:Type: vm ioctl
> +:Parameters: struct kvm_msr_allowlist
> +:Returns: 0 on success, < 0 on error
> +
> +::
> +
> + struct kvm_msr_allowlist {
> + __u32 flags;
> + __u32 nmsrs; /* number of msrs in bitmap */
> + __u32 base; /* base address for the MSRs bitmap */
> + __u32 pad;
> +
> + __u8 bitmap[0]; /* a set bit allows that the operation set in flags */
> + };
> +
> +This ioctl allows user space to define a set of bitmaps of MSR ranges to
> +specify whether a certain MSR access is allowed 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 first allow list was specified, only allowed MSR accesses
> +are permitted inside of KVM's MSR code.
> +
> +Each allowlist 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 permitted
> +by setting a 1 bit in the bitmap for the corresponding MSR index.

I think it would make sense to add KVM_MSR_ALLOW_READ/WRITE definitions
here as well to make the doc complete.

> +
> +If an MSR access is not permitted through the allow list, 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.
> +
>
> 5. The kvm_run structure
> ========================
> @@ -6213,3 +6252,17 @@ 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_RDMSR and
> KVM_EXIT_WRMSR exit notifications.
> +
> +8.25 KVM_CAP_ADD_MSR_ALLOWLIST
> +------------------------------
> +
> +:Architectures: x86
> +
> +This capability indicates that KVM supports emulation of only select MSR
> +registers. With this capability exposed, KVM exports a new VM ioctl
> +KVM_ADD_MSR_ALLOWLIST which allows user space to specify bitmaps of MSR
> +ranges that KVM should emulate in kernel space.
> +
> +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.
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 2f2307e71342..4b1ff7cb848f 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -901,6 +901,13 @@ 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 */
> @@ -1005,6 +1012,9 @@ struct kvm_arch {
> /* Deflect RDMSR and WRMSR to user space when they trigger a #GP */
> bool user_space_msr_enabled;
>
> + struct msr_bitmap_range msr_allowlist_ranges[10];
> + int msr_allowlist_ranges_count;
> +
> struct kvm_pmu_event_filter *pmu_event_filter;
> struct task_struct *nx_lpage_recovery_thread;
> };
> diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
> index 0780f97c1850..bd640a43cad6 100644
> --- a/arch/x86/include/uapi/asm/kvm.h
> +++ b/arch/x86/include/uapi/asm/kvm.h
> @@ -192,6 +192,21 @@ struct kvm_msr_list {
> __u32 indices[0];
> };
>
> +#define KVM_MSR_ALLOW_READ (1 << 0)
> +#define KVM_MSR_ALLOW_WRITE (1 << 1)

Nit: BIT(0)/BIT(1) maybe?

> +
> +/* Maximum size of the of the bitmap in bytes */
> +#define KVM_MSR_ALLOWLIST_MAX_LEN 0x600
> +
> +/* for KVM_ADD_MSR_ALLOWLIST */
> +struct kvm_msr_allowlist {
> + __u32 flags;
> + __u32 nmsrs; /* number of msrs in bitmap */
> + __u32 base; /* base address for the MSRs bitmap */
> + __u32 pad;
> +
> + __u8 bitmap[0]; /* a set bit allows that the operation set in flags */
> +};
>
> struct kvm_cpuid_entry {
> __u32 function;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 11e94a780656..924baec58d87 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1472,6 +1472,29 @@ void kvm_enable_efer_bits(u64 mask)
> }
> EXPORT_SYMBOL_GPL(kvm_enable_efer_bits);
>
> +static bool kvm_msr_allowed(struct kvm_vcpu *vcpu, u32 index, u32 type)
> +{
> + struct msr_bitmap_range *ranges = vcpu->kvm->arch.msr_allowlist_ranges;
> + u32 count = vcpu->kvm->arch.msr_allowlist_ranges_count;
> + u32 i;
> +
> + /* MSR allowlist not set up, allow everything */
> + if (!count)
> + return true;
> +
> + for (i = 0; i < count; i++) {
> + u32 start = ranges[i].base;
> + u32 end = start + ranges[i].nmsrs;
> + int flags = ranges[i].flags;
> + unsigned long *bitmap = ranges[i].bitmap;
> +
> + if ((index >= start) && (index < end) && (flags & type))
> + return !!test_bit(index - start, bitmap);
> + }
> +
> + return false;
> +}
> +
> /*
> * Write @data into the MSR specified by @index. Select MSR specific fault
> * checks are bypassed if @host_initiated is %true.
> @@ -1483,6 +1506,9 @@ 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_ALLOW_WRITE))
> + return -ENOENT;
> +
> switch (index) {
> case MSR_FS_BASE:
> case MSR_GS_BASE:
> @@ -1528,6 +1554,9 @@ 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_ALLOW_READ))
> + return -ENOENT;
> +
> msr.index = index;
> msr.host_initiated = host_initiated;
>
> @@ -3549,6 +3578,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
> case KVM_CAP_EXCEPTION_PAYLOAD:
> case KVM_CAP_SET_GUEST_DEBUG:
> case KVM_CAP_X86_USER_SPACE_MSR:
> + case KVM_CAP_ADD_MSR_ALLOWLIST:
> r = 1;
> break;
> case KVM_CAP_SYNC_REGS:
> @@ -5074,6 +5104,92 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
> return r;
> }
>
> +static bool msr_range_overlaps(struct kvm *kvm, struct msr_bitmap_range *range)
> +{
> + struct msr_bitmap_range *ranges = kvm->arch.msr_allowlist_ranges;
> + u32 i, count = kvm->arch.msr_allowlist_ranges_count;
> +
> + for (i = 0; i < count; i++) {
> + u32 start = max(range->base, ranges[i].base);
> + u32 end = min(range->base + range->nmsrs,
> + ranges[i].base + ranges[i].nmsrs);
> +
> + if ((start < end) && (range->flags & ranges[i].flags))
> + return true;
> + }
> +
> + return false;
> +}

It is a bit weird that we can only add something to allowlist, there is
no way to remove anything/everything from it.

E.g. if I add a range of msrs allowing read access only but later some
feature gets enabled and I'd like to convert some of these MSRs to
read/write, I, apparently can add overlapping ranges with "write-only"
access (as range->flags & ranges[i].flags allows me to do that) but I
can't add an overlapping 'read/write' region. This is not obvious.

> +
> +static int kvm_vm_ioctl_add_msr_allowlist(struct kvm *kvm, void __user *argp)
> +{
> + struct msr_bitmap_range *ranges = kvm->arch.msr_allowlist_ranges;
> + struct kvm_msr_allowlist __user *user_msr_allowlist = argp;
> + struct msr_bitmap_range range;
> + struct kvm_msr_allowlist kernel_msr_allowlist;
> + unsigned long *bitmap = NULL;
> + size_t bitmap_size;
> + int r;
> +
> + if (copy_from_user(&kernel_msr_allowlist, user_msr_allowlist,
> + sizeof(kernel_msr_allowlist))) {
> + r = -EFAULT;
> + goto out_err;
> + }
> +
> + bitmap_size = BITS_TO_LONGS(kernel_msr_allowlist.nmsrs) * sizeof(long);
> + if (bitmap_size > KVM_MSR_ALLOWLIST_MAX_LEN) {
> + r = -EINVAL;
> + goto out_err;
> + }
> +
> + bitmap = memdup_user(user_msr_allowlist->bitmap, bitmap_size);
> + if (IS_ERR(bitmap)) {
> + r = PTR_ERR(bitmap);
> + goto out_err;
> + }
> +
> + range = (struct msr_bitmap_range) {
> + .flags = kernel_msr_allowlist.flags,
> + .base = kernel_msr_allowlist.base,
> + .nmsrs = kernel_msr_allowlist.nmsrs,
> + .bitmap = bitmap,
> + };
> +
> + if (range.flags & ~(KVM_MSR_ALLOW_READ | KVM_MSR_ALLOW_WRITE)) {
> + r = -EINVAL;
> + goto out_err;
> + }
> +
> + /*
> + * Protect from concurrent calls to this function that could trigger
> + * a TOCTOU violation on kvm->arch.msr_allowlist_ranges_count.
> + */
> + mutex_lock(&kvm->lock);
> +
> + if (kvm->arch.msr_allowlist_ranges_count >=
> + ARRAY_SIZE(kvm->arch.msr_allowlist_ranges)) {
> + r = -E2BIG;
> + goto out_err;
> + }
> +
> + if (msr_range_overlaps(kvm, &range)) {
> + r = -EINVAL;
> + goto out_err;
> + }
> +
> + /* Everything ok, add this range identifier to our global pool */
> + ranges[kvm->arch.msr_allowlist_ranges_count++] = range;
> +
> + mutex_unlock(&kvm->lock);
> +
> + return 0;
> +
> +out_err:

You seem to forget to unlock &kvm->lock here.

> + kfree(bitmap);
> + return r;
> +}
> +
> long kvm_arch_vm_ioctl(struct file *filp,
> unsigned int ioctl, unsigned long arg)
> {
> @@ -5380,6 +5496,9 @@ long kvm_arch_vm_ioctl(struct file *filp,
> case KVM_SET_PMU_EVENT_FILTER:
> r = kvm_vm_ioctl_set_pmu_event_filter(kvm, argp);
> break;
> + case KVM_ADD_MSR_ALLOWLIST:
> + r = kvm_vm_ioctl_add_msr_allowlist(kvm, argp);
> + break;
> default:
> r = -ENOTTY;
> }
> @@ -10091,6 +10210,8 @@ void kvm_arch_pre_destroy_vm(struct kvm *kvm)
>
> void kvm_arch_destroy_vm(struct kvm *kvm)
> {
> + int i;
> +
> if (current->mm == kvm->mm) {
> /*
> * Free memory regions allocated on behalf of userspace,
> @@ -10107,6 +10228,8 @@ 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_allowlist_ranges_count; i++)
> + kfree(kvm->arch.msr_allowlist_ranges[i].bitmap);
> kvm_pic_destroy(kvm);
> kvm_ioapic_destroy(kvm);
> kvm_free_vcpus(kvm);
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index df237bf2bdc2..44ee9df8007f 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -1042,6 +1042,7 @@ struct kvm_ppc_resize_hpt {
> #define KVM_CAP_HALT_POLL 182
> #define KVM_CAP_ASYNC_PF_INT 183
> #define KVM_CAP_X86_USER_SPACE_MSR 184
> +#define KVM_CAP_ADD_MSR_ALLOWLIST 185

X86?

>
> #ifdef KVM_CAP_IRQ_ROUTING
>
> @@ -1543,6 +1544,9 @@ struct kvm_pv_cmd {
> /* Available with KVM_CAP_S390_PROTECTED */
> #define KVM_S390_PV_COMMAND _IOWR(KVMIO, 0xc5, struct kvm_pv_cmd)
>
> +/* Available with KVM_CAP_ADD_MSR_ALLOWLIST */
> +#define KVM_ADD_MSR_ALLOWLIST _IOW(KVMIO, 0xc6, struct kvm_msr_allowlist)
> +
> /* Secure Encrypted Virtualization command */
> enum sev_cmd_id {
> /* Guest initialization commands */

--
Vitaly

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