Re: [PATCH] kvm/x86: actually verify that reading MSR_IA32_UCODE_REV succeeds

[Sean Christopherson <seanjc@google.com>]

On Wed, Mar 15, 2023, Daniil Tatianin wrote:
> ...and return KVM_MSR_RET_INVALID otherwise.
> 
> Found by Linux Verification Center (linuxtesting.org) with the SVACE
> static analysis tool.
> 
> Fixes: cd28325249a1 ("KVM: VMX: support MSR_IA32_ARCH_CAPABILITIES as a feature MSR")
> Signed-off-by: Daniil Tatianin <d-tatianin@yandex-team.ru>
> ---
>  arch/x86/kvm/x86.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 7713420abab0..7de6939fc371 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1661,7 +1661,8 @@ static int kvm_get_msr_feature(struct kvm_msr_entry *msr)
>  		msr->data = kvm_caps.supported_perf_cap;
>  		break;
>  	case MSR_IA32_UCODE_REV:
> -		rdmsrl_safe(msr->index, &msr->data);
> +		if (rdmsrl_safe(msr->index, &msr->data))
> +			return KVM_MSR_RET_INVALID;

This is unnecessary and would arguably break KVM's ABI.  KVM unconditionally emulates
MSR_IA32_UCODE_REV in software and rdmsrl_safe() zeros the result on a fault (see
ex_handler_msr()).  '0' is a legitimate ucode revid and a reasonable fallback for
a theoretical (virtual) CPU that doesn't support the MSR.