linux-kernel - [PATCH 2/2] KVM: x86: Manually retrieve CPUID.0x1 when getting FMS for RESET/INIT

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Message-Id: <20210929222426.1855730-3-seanjc@google.com>
Date:   Wed, 29 Sep 2021 15:24:26 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.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,
        syzbot+f3985126b746b3d59c9d@...kaller.appspotmail.com,
        Alexander Potapenko <glider@...gle.com>
Subject: [PATCH 2/2] 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@...gle.com>
---
 arch/x86/kvm/x86.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 46ee9bf61df4..14562ea5d78d 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -10941,9 +10941,9 @@ 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
@@ -11025,12 +11025,11 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
 	 * 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.
+	 * But, go through the motions in case that's ever remedied.  Note, the
+	 * index for CPUID.0x1 is not significant, arbitrarily specify '0'.
 	 */
-	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;
 
-- 
2.33.0.685.g46640cef36-goog