[PATCH v2] KVM: VMX: Fix a TSX_CTRL_CPUID_CLEAR field mask issue

From: Zhenzhong Duan
Date: Sat Sep 25 2021 - 21:55:04 EST

When updating the host's mask for its MSR_IA32_TSX_CTRL user return entry,
clear the mask in the found uret MSR instead of vmx->guest_uret_msrs[i].
Modifying guest_uret_msrs directly is completely broken as 'i' does not
point at the MSR_IA32_TSX_CTRL entry. In fact, it's guaranteed to be an
out-of-bounds accesses as is always set to kvm_nr_uret_msrs in a prior
loop. By sheer dumb luck, the fallout is limited to "only" failing to
preserve the host's TSX_CTRL_CPUID_CLEAR. The out-of-bounds access is
benign as it's guaranteed to clear a bit in a guest MSR value, which are
always zero at vCPU creation on both x86-64 and i386.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 8ea8b8d6f869 ("KVM: VMX: Use common x86's uret MSR list as the one true list")
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@xxxxxxxxx>
Reviewed-by: Sean Christopherson <seanjc@xxxxxxxxxx>
arch/x86/kvm/vmx/vmx.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 0c2c0d5ae873..cbf3d33432b9 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6833,7 +6833,7 @@ static int vmx_create_vcpu(struct kvm_vcpu *vcpu)
tsx_ctrl = vmx_find_uret_msr(vmx, MSR_IA32_TSX_CTRL);
if (tsx_ctrl)
- vmx->guest_uret_msrs[i].mask = ~(u64)TSX_CTRL_CPUID_CLEAR;
+ tsx_ctrl->mask = ~(u64)TSX_CTRL_CPUID_CLEAR;

err = alloc_loaded_vmcs(&vmx->vmcs01);