Consider the case that userspace enables KVM_CAP_HYPERV_ENLIGHTENED_VMCS twice:
1) kvm_vcpu_ioctl_enable_cap() is called to enable
KVM_CAP_HYPERV_ENLIGHTENED_VMCS which calls nested_enable_evmcs().
2) nested_enable_evmcs() sets enlightened_vmcs_enabled to true and fills
vmcs_version which is then copied to userspace.
3) kvm_vcpu_ioctl_enable_cap() is called again to enable
KVM_CAP_HYPERV_ENLIGHTENED_VMCS which calls nested_enable_evmcs().
4) This time nested_enable_evmcs() just returns 0 as
enlightened_vmcs_enabled is already true. *Without filling
vmcs_version*.
5) kvm_vcpu_ioctl_enable_cap() continues as usual and copies
*uninitialized* vmcs_version to userspace which leads to kernel info-leak.
Fix this issue by simply changing nested_enable_evmcs() to always fill
vmcs_version output argument. Even when enlightened_vmcs_enabled is
already set to true.
Note that SVM's nested_enable_evmcs() should not be modified because it
always returns a non-zero value (-ENODEV) which results in
kvm_vcpu_ioctl_enable_cap() skipping the copy of vmcs_version to
userspace (as it should).
Fixes: 57b119da3594 ("KVM: nVMX: add KVM_CAP_HYPERV_ENLIGHTENED_VMCS capability")
Reported-by: syzbot+cfbc368e283d381f8cef@xxxxxxxxxxxxxxxxxxxxxxxxx
Reviewed-by: Krish Sadhukhan <krish.sadhukhan@xxxxxxxxxx>
Signed-off-by: Liran Alon <liran.alon@xxxxxxxxxx>
---
arch/x86/kvm/vmx.c | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 6e92624fe8d6..be1c6e37b57c 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -1610,12 +1610,6 @@ static int nested_enable_evmcs(struct kvm_vcpu *vcpu,
{
struct vcpu_vmx *vmx = to_vmx(vcpu);
- /* We don't support disabling the feature for simplicity. */
- if (vmx->nested.enlightened_vmcs_enabled)
- return 0;
-
- vmx->nested.enlightened_vmcs_enabled = true;
-
/*
* vmcs_version represents the range of supported Enlightened VMCS
* versions: lower 8 bits is the minimal version, higher 8 bits is the
@@ -1625,6 +1619,12 @@ static int nested_enable_evmcs(struct kvm_vcpu *vcpu,
if (vmcs_version)
*vmcs_version = (KVM_EVMCS_VERSION << 8) | 1;
+ /* We don't support disabling the feature for simplicity. */
+ if (vmx->nested.enlightened_vmcs_enabled)
+ return 0;
+
+ vmx->nested.enlightened_vmcs_enabled = true;
+
vmx->nested.msrs.pinbased_ctls_high &= ~EVMCS1_UNSUPPORTED_PINCTRL;
vmx->nested.msrs.entry_ctls_high &= ~EVMCS1_UNSUPPORTED_VMENTRY_CTRL;
vmx->nested.msrs.exit_ctls_high &= ~EVMCS1_UNSUPPORTED_VMEXIT_CTRL;
--
2.16.1
