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

[Sean Christopherson]

On Tue, Mar 30, 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.
> 
> Signed-off-by: Emanuele Giuseppe Esposito <eesposit@xxxxxxxxxx>
> ---
>  arch/x86/kvm/cpuid.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 6bd2f8b830e4..5412b48b9103 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -975,6 +975,12 @@ int kvm_dev_ioctl_get_cpuid(struct kvm_cpuid2 *cpuid,
>  
>  	if (cpuid->nent < 1)
>  		return -E2BIG;
> +
> +	/* if there are X entries, we need to allocate at least X+1
> +	 * entries but return the actual number of entries
> +	 */
> +	cpuid->nent++;

I don't see how this can be correct.

If this bonus entry really is needed, then won't that be reflected in array.nent?
I.e won't KVM overrun the userspace buffer?

If it's not reflected in array.nent, that would imply there's an off-by-one check
somewhere, or KVM is creating an entry that it doesn't copy to userspace.  The
former seems unlikely as there are literally only two checks against maxnent,
and they both look correct (famous last words...).

KVM does decrement array->nent in one specific case (CPUID.0xD.2..64), i.e. a
false positive is theoretically possible, but that carries a WARN and requires a
kernel or CPU bug as well.  And fudging nent for that case would still break
normal use cases due to the overrun problem.

What am I missing?

> +
>  	if (cpuid->nent > KVM_MAX_CPUID_ENTRIES)
>  		cpuid->nent = KVM_MAX_CPUID_ENTRIES;
>  
> -- 
> 2.30.2
>