Ignore the userspace provided x2APIC ID when fixing up APIC state for
KVM_SET_LAPIC, i.e. make the x2APIC fully readonly in KVM.  Commit
a92e2543 ("KVM: x86: use hardware-compatible format for APIC ID
register"), which added the fixup, didn't intend to allow userspace to
modify the x2APIC ID.  In fact, that commit is when KVM first started
treating the x2APIC ID as readonly, apparently to fix some race:

 static inline u32 kvm_apic_id(struct kvm_lapic *apic)
 {
-       return (kvm_lapic_get_reg(apic, APIC_ID) >> 24) & 0xff;
+       /* To avoid a race between apic_base and following APIC_ID update when
+        * switching to x2apic_mode, the x2apic mode returns initial x2apic id.
+        */
+       if (apic_x2apic_mode(apic))
+               return apic->vcpu->vcpu_id;
+
+       return kvm_lapic_get_reg(apic, APIC_ID) >> 24;
 }

Furthermore, KVM doesn't support delivering interrupts to vCPUs with a
modified x2APIC ID, but KVM *does* return the modified value on a guest
RDMSR and for KVM_GET_LAPIC.  I.e. no remotely sane setup can actually
work with a modified x2APIC ID.

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

Unfortunately, the WARN can still trigger for other CPUs than the current
one by racing against KVM_SET_LAPIC, so remove it completely.
Reported-by: Michal Luczaj <mhal@rbox.co>
Closes: https://lore.kernel.org/all/814baa0c-1eaa-4503-129f-059917365e80@rbox.coReported-by: Haoyu Wu <haoyuwu254@gmail.com>
Closes: https://lore.kernel.org/all/20240126161633.62529-1-haoyuwu254@gmail.com
Reported-by: syzbot+545f1326f405db4e1c3e@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/000000000000c2a6b9061cbca3c3@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20240802202941.344889-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>