Re: [PATCH 1/2] KVM: x86: Make x2APIC ID 100% readonly

[Paolo Bonzini <pbonzini@redhat.com>]

On 8/5/24 18:26, Sean Christopherson wrote:
> On Sun, Aug 04, 2024, Michal Luczaj wrote:
>> On 8/2/24 22:29, Sean Christopherson wrote:
>>> [...]
>>> Making the x2APIC ID fully readonly fixes a WARN in KVM's optimized map
>>> calculation, which expects the LDR to align with the x2APIC ID.
>>>
>>>    WARNING: CPU: 2 PID: 958 at arch/x86/kvm/lapic.c:331 kvm_recalculate_apic_map+0x609/0xa00 [kvm]
>>>    CPU: 2 PID: 958 Comm: recalc_apic_map Not tainted 6.4.0-rc3-vanilla+ #35
>>>    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Arch Linux 1.16.2-1-1 04/01/2014
>>>    RIP: 0010:kvm_recalculate_apic_map+0x609/0xa00 [kvm]
>>>    Call Trace:
>>>     <TASK>
>>>     kvm_apic_set_state+0x1cf/0x5b0 [kvm]
>>>     kvm_arch_vcpu_ioctl+0x1806/0x2100 [kvm]
>>>     kvm_vcpu_ioctl+0x663/0x8a0 [kvm]
>>>     __x64_sys_ioctl+0xb8/0xf0
>>>     do_syscall_64+0x56/0x80
>>>     entry_SYSCALL_64_after_hwframe+0x46/0xb0
>>>    RIP: 0033:0x7fade8b9dd6f
>>
>> Isn't this WARN_ON_ONCE() inherently racy, though? With your patch applied,
>> it can still be hit by juggling the APIC modes.
> 
> Doh, right, the logic is unfortunately cross-vCPU.  The sanity check could be
> conditioned on the APIC belonging to the running/loaded vCPU, but I'm leaning
> towards deleting it entirely.  Though it did detect the KVM_SET_LAPIC backdoor...

Yeah, let's drop it since we do have a test (in userspace).

Paolo