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

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Date:   Mon, 2 Mar 2020 12:49:40 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Jan Kiszka <jan.kiszka@...mens.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Xiaoyao Li <xiaoyao.li@...el.com>
Subject: Re: [PATCH 1/6] KVM: x86: Fix tracing of CPUID.function when
 function is out-of-range

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@...mens.com>
> >Cc: Jim Mattson <jmattson@...gle.com>
> >Cc: Xiaoyao Li <xiaoyao.li@...el.com>
> >Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> >---
> >  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.

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.