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SubjectRe: [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@siemens.com>
> >Cc: Jim Mattson <jmattson@google.com>
> >Cc: Xiaoyao Li <xiaoyao.li@intel.com>
> >Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.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.

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