On Fri, Apr 15, 2022 at 03:25:06PM +0000, Sean Christopherson wrote:
>On Mon, Apr 11, 2022, Zeng Guang wrote:
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index d1a39285deab..23fbf52f7bea 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -11180,11 +11180,15 @@ static int sync_regs(struct kvm_vcpu *vcpu)
>>
>> int kvm_arch_vcpu_precreate(struct kvm *kvm, unsigned int id)
>> {
>> + int ret = 0;
>> +
>> if (kvm_check_tsc_unstable() && atomic_read(&kvm->online_vcpus) != 0)
>> pr_warn_once("kvm: SMP vm created on host with unstable TSC; "
>> "guest TSC will not be reliable\n");
>>
>> - return 0;
>> + if (kvm_x86_ops.alloc_ipiv_pid_table)
>> + ret = static_call(kvm_x86_alloc_ipiv_pid_table)(kvm);
>
>Add a generic kvm_x86_ops.vcpu_precreate, no reason to make this so specific.
>And use KVM_X86_OP_RET0 instead of KVM_X86_OP_OPTIONAL, then this can simply be
>
> return static_call(kvm_x86_vcpu_precreate);
>
>That said, there's a flaw in my genius plan.
>
> 1. KVM_CREATE_VM
> 2. KVM_CAP_MAX_VCPU_ID, set max_vcpu_ids=1
> 3. KVM_CREATE_VCPU, create IPIv table but ultimately fails
> 4. KVM decrements created_vcpus back to '0'
> 5. KVM_CAP_MAX_VCPU_ID, set max_vcpu_ids=4096
> 6. KVM_CREATE_VCPU w/ ID out of range
>
>In other words, malicious userspace could trigger buffer overflow.
can we simply return an error (e.g., -EEXIST) on step 5 (i.e.,
max_vcpu_ids cannot be changed after being set once)?
or
can we detect the change of max_vcpu_ids in step 6 and re-allocate PID
table?
>
>That could be solved by adding an arch hook to undo precreate, but that's gross
>and a good indication that we're trying to solve this the wrong way.
>
>I think it's high time we add KVM_FINALIZE_VM, though that's probably a bad name
>since e.g. TDX wants to use that name for VM really, really, being finalized[*],
>i.e. after all vCPUs have been created.
>
>KVM_POST_CREATE_VM? That's not very good either.
>
>Paolo or anyone else, thoughts?
>
>[*] https://lore.kernel.org/all/83768bf0f786d24f49d9b698a45ba65441ef5ef0.1646422845.git.isaku.yamahata@xxxxxxxxx
