Re: [PATCH 1/6] KVM: x86: Fix tracing of CPUID.function when function is out-of-range

[Jan Kiszka]

On 02.03.20 21:49, Sean Christopherson wrote:
> On Mon, Mar 02, 2020 at 09:26:54PM +0100, Jan Kiszka wrote:
> > On 02.03.20 20:57, Sean Christopherson wrote:
> > > Rework kvm_cpuid() to query entry->function when adjusting the output
> > > values so that the original function (in the aptly named "function") is
> > > preserved for tracing.  This fixes a bug where trace_kvm_cpuid() will
> > > trace the max function for a range instead of the requested function if
> > > the requested function is out-of-range and an entry for the max function
> > > exists.
> > >
> > > Fixes: 43561123ab37 ("kvm: x86: Improve emulation of CPUID leaves 0BH and 1FH")
> > > Reported-by: Jan Kiszka <jan.kiszka@xxxxxxxxxxx>
> > > Cc: Jim Mattson <jmattson@xxxxxxxxxx>
> > > Cc: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
> > > Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> > > ---
> > >   arch/x86/kvm/cpuid.c | 15 +++++++--------
> > >   1 file changed, 7 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > > index b1c469446b07..6be012937eba 100644
> > > --- a/arch/x86/kvm/cpuid.c
> > > +++ b/arch/x86/kvm/cpuid.c
> > > @@ -997,12 +997,12 @@ static bool cpuid_function_in_range(struct kvm_vcpu *vcpu, u32 function)
> > >   	return max && function <= max->eax;
> > >   }
> > > +/* Returns true if the requested leaf/function exists in guest CPUID. */
> > >   bool kvm_cpuid(struct kvm_vcpu *vcpu, u32 *eax, u32 *ebx,
> > >   	       u32 *ecx, u32 *edx, bool check_limit)
> > >   {
> > > -	u32 function = *eax, index = *ecx;
> > > +	const u32 function = *eax, index = *ecx;
> > >   	struct kvm_cpuid_entry2 *entry;
> > > -	struct kvm_cpuid_entry2 *max;
> > >   	bool found;
> > >   	entry = kvm_find_cpuid_entry(vcpu, function, index);
> > > @@ -1015,18 +1015,17 @@ bool kvm_cpuid(struct kvm_vcpu *vcpu, u32 *eax, u32 *ebx,
> > >   	 */
> > >   	if (!entry && check_limit && !guest_cpuid_is_amd(vcpu) &&
> > >   	    !cpuid_function_in_range(vcpu, function)) {
> > > -		max = kvm_find_cpuid_entry(vcpu, 0, 0);
> > > -		if (max) {
> > > -			function = max->eax;
> > > -			entry = kvm_find_cpuid_entry(vcpu, function, index);
> > > -		}
> > > +		entry = kvm_find_cpuid_entry(vcpu, 0, 0);
> > > +		if (entry)
> > > +			entry = kvm_find_cpuid_entry(vcpu, entry->eax, index);
> > >   	}
> > >   	if (entry) {
> > >   		*eax = entry->eax;
> > >   		*ebx = entry->ebx;
> > >   		*ecx = entry->ecx;
> > >   		*edx = entry->edx;
> > > -		if (function == 7 && index == 0) {
> > > +
> > > +		if (entry->function == 7 && index == 0) {
> > >   			u64 data;
> > >   		        if (!__kvm_get_msr(vcpu, MSR_IA32_TSX_CTRL, &data, true) &&
> > >   			    (data & TSX_CTRL_CPUID_CLEAR))
> >
> > What about the !entry case below this? It was impacted by the function
> > capping so far, not it's no longer.
>
> Hmm, the only way the output would be different is in a really contrived
> scenario where userspace doesn't provide an entry for the max basic leaf.

I think I've seen that, a cap to 0x10, with QEMU and '-cpu host# when 
providing intentionally bogus values to cpuid.

Jan

> The !entry path can only be reached with "orig_function != function" if
> orig_function is out of range and there is no entry for the max basic leaf.
> The adjustments for 0xb/0x1f require the max basic leaf to be 0xb or 0x1f,
> and to take effect with !entry would require there to be a CPUID.max.1 but
> not a CPUID.max.0.  That'd be a violation of Intel's SDM, i.e. it's bogus
> userspace input and IMO can be ignored.

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