Re: [PATCH v4 2/3] KVM: x86: Introduce allow list for MSR emulation

[Alexander Graf <graf@amazon.com>]

On 20.08.20 00:49, Jim Mattson wrote:
> 
> On Mon, Aug 3, 2020 at 2:14 PM Alexander Graf <graf@amazon.com> wrote:
> 
>> --- 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];
> 
> Why 10? I think this is the only use of this constant, but a macro
> would still be nice, especially since the number appears to be
> arbitrary.
> 
>> diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
>> index 0780f97c1850..c33fb1d72d52 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)
>> +
>> +/* Maximum size of the of the bitmap in bytes */
>> +#define KVM_MSR_ALLOWLIST_MAX_LEN 0x600
> 
> Wouldn't 0x400 be a more natural size, since both Intel and AMD MSR
> permission bitmaps cover ranges of 8192 MSRs?

You can always make your bitmaps 0x400 :). I had to choose something 
that limits our memory footprint, so that user space can't allocate 
infinite amounts of memory.

> 
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index e1139124350f..25e58ceb19de 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -1472,6 +1472,38 @@ 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)
> 
> In another thread, when I suggested that a function should return
> bool, you said, "'I'm not a big fan of bool returning APIs unless they
> have an "is" in their name.' This function doesn't have "is" in its
> name. :-)

I've left this unanswered for way too long :). IMHO, passive is fine 
too, as it implies an "is" in my brain. Or to put it differently:

   bad: bool kvm_get_msr()
   bad: bool kvm_get_msr_user_space()
   good: bool kvm_msr_blocked()
   good: bool kvm_msr_allowed()
   good: bool is_kvm_msr_allowed()

> 
>> +{
>> +       struct kvm *kvm = vcpu->kvm;
>> +       struct msr_bitmap_range *ranges = kvm->arch.msr_allowlist_ranges;
>> +       u32 count = kvm->arch.msr_allowlist_ranges_count;
> 
> Shouldn't the read of kvm->arch.msr_allowlist_ranges_count be guarded
> by the mutex, below?
> 
>> +       u32 i;
>> +       bool r = false;
>> +
>> +       /* MSR allowlist not set up, allow everything */
>> +       if (!count)
>> +               return true;
>> +
>> +       /* Prevent collision with clear_msr_allowlist */
>> +       mutex_lock(&kvm->lock);
>> +
>> +       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);
> 
> The !! seems gratuitous, since r is of type bool.
> 
>> @@ -1483,6 +1515,9 @@ 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;
> 
> Perhaps -EPERM is more appropriate here?
> 
>>          switch (index) {
>>          case MSR_FS_BASE:
>>          case MSR_GS_BASE:
>> @@ -1528,6 +1563,9 @@ 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;
> 
> ...and here?
> 
>> +static bool msr_range_overlaps(struct kvm *kvm, struct msr_bitmap_range *range)
> 
> Another bool function with no "is"? :-)
> 
>> +{
>> +       struct msr_bitmap_range *ranges = kvm->arch.msr_allowlist_ranges;
>> +       u32 i, count = kvm->arch.msr_allowlist_ranges_count;
>> +       bool r = false;
>> +
>> +       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)) {
>> +                       r = true;
>> +                       break;
>> +               }
>> +       }
>> +
>> +       return r;
>> +}
> 
> This seems like an awkward constraint. Would it be possible to allow
> overlapping ranges as long as the access types don't clash? So, for
> example, could I specify an allow list for READ of MSRs 0-0x1ffff and
> an allow list for WRITE of MSRs 0-0x1ffff? Actually, I don't see why
> you have to prohibit overlapping ranges at all.

I tend to agree. Now that the order is obvious through the new API, we 
no longer need to check for overlaps.

> 
> 
>> +static int kvm_vm_ioctl_clear_msr_allowlist(struct kvm *kvm)
>> +{
>> +       int i;
> 
> Nit: In earlier code, you use u32 for this index. (I'm actually a fan
> of int, myself.)

I usually use int as well because it's easier to type, but doing signed 
indexes is just so wrong on so many levels :). I'll fix them up too be 
all u32.


Alex



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