Host value of TSX_CTRL_CPUID_CLEAR field should be unchangable by guest,
but the mask for this purpose is set to a wrong value. So it doesn't
take effect.
Fixes: 8ea8b8d6f869 ("KVM: VMX: Use common x86's uret MSR list as the one true list")
Signed-off-by: Zhenzhong Duan <[email protected]>
---
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 927a552393b9..36588b5feee6 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6812,7 +6812,7 @@ 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);
--
2.25.1
On Mon, Sep 06, 2021, Zhenzhong Duan wrote:
> Host value of TSX_CTRL_CPUID_CLEAR field should be unchangable by guest,
> but the mask for this purpose is set to a wrong value. So it doesn't
> take effect.
It would be helpful to provide a bit more info as to just how bad/boneheaded this
bug is. E.g.
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.
> Fixes: 8ea8b8d6f869 ("KVM: VMX: Use common x86's uret MSR list as the one true list")
Cc: [email protected]
> Signed-off-by: Zhenzhong Duan <[email protected]>
> ---
> 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 927a552393b9..36588b5feee6 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6812,7 +6812,7 @@ 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;
Egad, that's a horrific oversight on my part.
Reviewed-by: Sean Christopherson <[email protected]>
>-----Original Message-----
>From: Sean Christopherson <[email protected]>
>Sent: Wednesday, September 8, 2021 8:08 AM
>To: Duan, Zhenzhong <[email protected]>
>Cc: [email protected]; [email protected];
>[email protected]; [email protected]; [email protected];
>[email protected]; [email protected]
>Subject: Re: [PATCH] KVM: VMX: Fix a TSX_CTRL_CPUID_CLEAR field mask
>issue
>
>On Mon, Sep 06, 2021, Zhenzhong Duan wrote:
>> Host value of TSX_CTRL_CPUID_CLEAR field should be unchangable by
>> guest, but the mask for this purpose is set to a wrong value. So it
>> doesn't take effect.
>
>It would be helpful to provide a bit more info as to just how bad/boneheaded
>this bug is. E.g.
>
> 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.
Sorry for late response, I missed this mail by a wrong mail rule.
Your comment is more clear, I'll use it in v2.
Thanks
Zhenzhong