Re: KMSAN: kernel-infoleak in kvm_vcpu_write_guest_page

[Liran Alon]

> On 7 Nov 2018, at 14:47, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
> 
> On 07/11/2018 13:10, Alexander Potapenko wrote:
>> This appears to be a real bug in KVM.
>> Please see a simplified reproducer attached.
> 
> Thanks, I agree it's a reael bug.  The basic issue is that the
> kvm_state->size member is too small (1040) in the KVM_SET_NESTED_STATE
> ioctl, aka 0x4080aebf.
> 
> One way to fix it would be to just change kmalloc to kzalloc when
> allocating cached_vmcs12 and cached_shadow_vmcs12, but really the ioctl
> is wrong and should be rejected.  And the case where a shadow VMCS has
> to be loaded is even more wrong, and we have to fix it anyway, so I
> don't really like the idea of papering over the bug in the allocation.
> 
> I'll test this patch and submit it formally:
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index c645f777b425..c546f0b1f3e0 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -14888,10 +14888,13 @@ static int vmx_set_nested_state(struct
> kvm_vcpu *vcpu,
> 	if (ret)
> 		return ret;
> 
> -	/* Empty 'VMXON' state is permitted */
> -	if (kvm_state->size < sizeof(kvm_state) + sizeof(*vmcs12))
> +	/* Empty 'VMXON' state is permitted.  A partial VMCS12 is not.  */
> +	if (kvm_state->size == sizeof(kvm_state))
> 		return 0;
> 
> +	if (kvm_state->size < sizeof(kvm_state) + VMCS12_SIZE)
> +		return -EINVAL;
> +

I donât think that this test is sufficient to fully resolve issue.
What if malicious userspace supplies valid size but pages containing nested_state->vmcs12 is unmapped?
This will result in vmx_set_nested_state() still calling set_current_vmptr() but failing on copy_from_user()
which still leaks cached_vmcs12 on next VMPTRLD of guest.

Therefore, I think that the correct patch should be to change vmx_set_nested_state() to
first gather all relevant information from userspace and validate it,
and only then start applying it to KVMâs internal vCPU state.

> 	if (kvm_state->vmx.vmcs_pa != -1ull) {
> 		if (kvm_state->vmx.vmcs_pa == kvm_state->vmx.vmxon_pa ||
> 		    !page_address_valid(vcpu, kvm_state->vmx.vmcs_pa))
> @@ -14917,6 +14920,7 @@ static int vmx_set_nested_state(struct kvm_vcpu
> *vcpu,
> 	}
> 
> 	vmcs12 = get_vmcs12(vcpu);
> +	BUILD_BUG_ON(sizeof(*vmcs12) > VMCS12_SIZE);

Why put this BUILD_BUG_ON() specifically here?
There are many places which assumes cached_vmcs12 is of size VMCS12_SIZE.
(Such as nested_release_vmcs12() and handle_vmptrld()).

> 	if (copy_from_user(vmcs12, user_kvm_nested_state->data, sizeof(*vmcs12)))
> 		return -EFAULT;
> 
> @@ -14932,7 +14936,7 @@ static int vmx_set_nested_state(struct kvm_vcpu
> *vcpu,
> 	if (nested_cpu_has_shadow_vmcs(vmcs12) &&
> 	    vmcs12->vmcs_link_pointer != -1ull) {
> 		struct vmcs12 *shadow_vmcs12 = get_shadow_vmcs12(vcpu);
> -		if (kvm_state->size < sizeof(kvm_state) + 2 * sizeof(*vmcs12))
> +		if (kvm_state->size < sizeof(kvm_state) + 2 * VMCS12_SIZE)
> 			return -EINVAL;
> 
> 		if (copy_from_user(shadow_vmcs12,
> 
> Paolo

-Liran