On 07/11/2018 13:58, Liran Alon wrote:
>
>
>> 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 don’t 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.
Makes sense; since SET_NESTED_STATE is not a fast path, we can just
memdup_user and pass a kernel pointer to vmx_set_nested_state.
> 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 KVM’s 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()).
Unlike those places, here the copy has sizeof(*vmcs12) bytes and an
overflow would cause a userspace write to kernel memory. Though, that
means there is still a possibility of leaking kernel data when
nested_release_vmcs12 is called. So it also makes sense to use
VMCS12_SIZE for the memory copies, and kzalloc.
Thanks,
Paolo
>> 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
>
>
