Re: [PATCH] KVM: x86/pmu: Fix type length error when reading pmu->fixed_ctr_ctrl

[Sean Christopherson <seanjc@google.com>]

On Tue, Jan 23, 2024, Mingwei Zhang wrote:
> Fix type length error since pmu->fixed_ctr_ctrl is u64 but the local
> variable old_fixed_ctr_ctrl is u8. Truncating the value leads to
> information loss at runtime. This leads to incorrect value in old_ctrl
> retrieved from each field of old_fixed_ctr_ctrl and causes incorrect code
> execution within the for loop of reprogram_fixed_counters(). So fix this
> type to u64.

But what is the actual fallout from this?  Stating that the bug causes incorrect
code execution isn't helpful, that's akin to saying water is wet.

If I'm following the code correctly, the only fallout is that KVM may unnecessarily
mark a fixed PMC as in use and reprogram it.  I.e. the bug can result in (minor?)
performance issues, but it won't cause functional problems.

Understanding what actually goes wrong matters, because I'm trying to determine
whether or not this needs to be fixed in 6.8 and backported to stable trees.  If
the bug is relatively benign, then this is fodder for 6.9.

> Fixes: 76d287b2342e ("KVM: x86/pmu: Drop "u8 ctrl, int idx" for reprogram_fixed_counter()")
> Signed-off-by: Mingwei Zhang <mizhang@google.com>
> ---
>  arch/x86/kvm/vmx/pmu_intel.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> index a6216c874729..315c7c2ba89b 100644
> --- a/arch/x86/kvm/vmx/pmu_intel.c
> +++ b/arch/x86/kvm/vmx/pmu_intel.c
> @@ -71,7 +71,7 @@ static int fixed_pmc_events[] = {
>  static void reprogram_fixed_counters(struct kvm_pmu *pmu, u64 data)
>  {
>  	struct kvm_pmc *pmc;
> -	u8 old_fixed_ctr_ctrl = pmu->fixed_ctr_ctrl;
> +	u64 old_fixed_ctr_ctrl = pmu->fixed_ctr_ctrl;
>  	int i;
>  
>  	pmu->fixed_ctr_ctrl = data;
> 
> base-commit: 6613476e225e090cc9aad49be7fa504e290dd33d
> -- 
> 2.43.0.429.g432eaa2c6b-goog
>