Re: [PATCH v2 1/4] kvm: cpuid: adjust the returned nent field of kvm_cpuid2 for KVM_GET_SUPPORTED_CPUID and KVM_GET_EMULATED_CPUID

[Sean Christopherson <seanjc@google.com>]

On Wed, Mar 31, 2021, Emanuele Giuseppe Esposito wrote:
> Calling the kvm KVM_GET_[SUPPORTED/EMULATED]_CPUID ioctl requires
> a nent field inside the kvm_cpuid2 struct to be big enough to contain
> all entries that will be set by kvm.
> Therefore if the nent field is too high, kvm will adjust it to the
> right value. If too low, -E2BIG is returned.
> 
> However, when filling the entries do_cpuid_func() requires an
> additional entry, so if the right nent is known in advance,
> giving the exact number of entries won't work because it has to be 
> increased by one.

I'd strong prefer to reword the shortlog and changelog.  It's not immediately
obvious what this is changing without the context from the v1 thread.  E.g.

  KVM: x86: Fix a spurious -E2BIG in KVM_GET_EMULATED_CPUID

  When retrieving emulated CPUID entries, check for an insufficient array size
  if and only if KVM is actually inserting an entry.  If userspace has a priori
  knowledge of the exact array size, KVM_GET_EMULATED_CPUID will incorrectly
  fail due to effectively requiring an extra, unused entry.

> Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
> ---
>  arch/x86/kvm/cpuid.c | 35 ++++++++++++++++++-----------------
>  1 file changed, 18 insertions(+), 17 deletions(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 6bd2f8b830e4..02a51f921548 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -567,34 +567,34 @@ static struct kvm_cpuid_entry2 *do_host_cpuid(struct kvm_cpuid_array *array,
>  
>  static int __do_cpuid_func_emulated(struct kvm_cpuid_array *array, u32 func)
>  {
> -	struct kvm_cpuid_entry2 *entry;
> -
> -	if (array->nent >= array->maxnent)
> -		return -E2BIG;
> +	struct kvm_cpuid_entry2 entry;
>  
> -	entry = &array->entries[array->nent];
> -	entry->function = func;
> -	entry->index = 0;
> -	entry->flags = 0;
> +	entry.function = func;
> +	entry.index = 0;
> +	entry.flags = 0;

Depending on the leaf, eax/ebx/ecx/edx will be left uninitialized.  This wasn't
a bug before since @array is zeroed on allocation.

What about pre-checking @func?  I don't particular like the duplicate checks,
but none of the solutions are particularly elegant.  E.g.

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 6bd2f8b830e4..9824947bd5ad 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -565,14 +565,18 @@ static struct kvm_cpuid_entry2 *do_host_cpuid(struct kvm_cpuid_array *array,
        return entry;
 }

-static int __do_cpuid_func_emulated(struct kvm_cpuid_array *array, u32 func)
+static noinline int __do_cpuid_func_emulated(struct kvm_cpuid_array *array, u32 func)
 {
        struct kvm_cpuid_entry2 *entry;

+       if (func != 0 && func != 1 && func != 7)
+               return 0;
+
        if (array->nent >= array->maxnent)
                return -E2BIG;

-       entry = &array->entries[array->nent];
+       entry = &array->entries[array->nent++];
+
        entry->function = func;
        entry->index = 0;
        entry->flags = 0;
@@ -580,19 +584,17 @@ static int __do_cpuid_func_emulated(struct kvm_cpuid_array *array, u32 func)
        switch (func) {
        case 0:
                entry->eax = 7;
-               ++array->nent;
                break;
        case 1:
                entry->ecx = F(MOVBE);
-               ++array->nent;
                break;
        case 7:
                entry->flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
                entry->eax = 0;
                entry->ecx = F(RDPID);
-               ++array->nent;
-       default:
                break;
+       default:
+               BUG();
        }

        return 0


>  
>  	switch (func) {
>  	case 0:
> -		entry->eax = 7;
> -		++array->nent;
> +		entry.eax = 7;
>  		break;
>  	case 1:
> -		entry->ecx = F(MOVBE);
> -		++array->nent;
> +		entry.ecx = F(MOVBE);
>  		break;
>  	case 7:
> -		entry->flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
> -		entry->eax = 0;
> -		entry->ecx = F(RDPID);
> -		++array->nent;
> -	default:
> +		entry.flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
> +		entry.eax = 0;
> +		entry.ecx = F(RDPID);
>  		break;
> +	default:
> +		goto out;
>  	}
>  
> +	if (array->nent >= array->maxnent)
> +		return -E2BIG;
> +
> +	memcpy(&array->entries[array->nent++], &entry, sizeof(entry));
> +
> +out:
>  	return 0;
>  }
>  
> @@ -975,6 +975,7 @@ int kvm_dev_ioctl_get_cpuid(struct kvm_cpuid2 *cpuid,
>  
>  	if (cpuid->nent < 1)
>  		return -E2BIG;
> +
>  	if (cpuid->nent > KVM_MAX_CPUID_ENTRIES)
>  		cpuid->nent = KVM_MAX_CPUID_ENTRIES;
>  
> -- 
> 2.30.2
>