Commit 25b97845 authored Sep 29, 2021 by Sean Christopherson Committed by Paolo Bonzini Sep 30, 2021

KVM: x86: Manually retrieve CPUID.0x1 when getting FMS for RESET/INIT



Manually look for a CPUID.0x1 entry instead of bouncing through
kvm_cpuid() when retrieving the Family-Model-Stepping information for
vCPU RESET/INIT.  This fixes a potential undefined behavior bug due to
kvm_cpuid() using the uninitialized "dummy" param as the ECX _input_,
a.k.a. the index.

A more minimal fix would be to simply zero "dummy", but the extra work in
kvm_cpuid() is wasteful, and KVM should be treating the FMS retrieval as
an out-of-band access, e.g. same as how KVM computes guest.MAXPHYADDR.
Both Intel's SDM and AMD's APM describe the RDX value at RESET/INIT as
holding the CPU's FMS information, not as holding CPUID.0x1.EAX.  KVM's
usage of CPUID entries to get FMS is simply a pragmatic approach to avoid
having yet another way for userspace to provide inconsistent data.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Message-Id: <20210929222426.1855730-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

parent 62dd57dd

arch/x86/kvm/x86.c

+5 −7

Original line number	Diff line number	Diff line
		@@ -10779,9 +10779,9 @@ void kvm_arch_vcpu_destroy(struct kvm_vcpu *vcpu)

		void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
		{
		struct kvm_cpuid_entry2 *cpuid_0x1;
		unsigned long old_cr0 = kvm_read_cr0(vcpu);
		unsigned long new_cr0;
		u32 eax, dummy;

		/*
		* Several of the "set" flows, e.g. ->set_cr0(), read other registers
		@@ -10862,13 +10862,11 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
		* Fall back to KVM's default Family/Model/Stepping of 0x600 (P6/Athlon)
		* if no CPUID match is found. Note, it's impossible to get a match at
		* RESET since KVM emulates RESET before exposing the vCPU to userspace,
		* i.e. it'simpossible for kvm_cpuid() to find a valid entry on RESET.
		* But, go through the motions in case that's ever remedied.
		* i.e. it's impossible for kvm_find_cpuid_entry() to find a valid entry
		* on RESET. But, go through the motions in case that's ever remedied.
		*/
		eax = 1;
		if (!kvm_cpuid(vcpu, &eax, &dummy, &dummy, &dummy, true))
		eax = 0x600;
		kvm_rdx_write(vcpu, eax);
		cpuid_0x1 = kvm_find_cpuid_entry(vcpu, 1, 0);
		kvm_rdx_write(vcpu, cpuid_0x1 ? cpuid_0x1->eax : 0x600);

		vcpu->arch.ia32_xss = 0;