KVM: Fix a data race on last_boosted_vcpu in kvm_vcpu_on_spin()

Use {READ,WRITE}_ONCE() to access kvm->last_boosted_vcpu to ensure the
loads and stores are atomic.  In the extremely unlikely scenario the
compiler tears the stores, it's theoretically possible for KVM to attempt
to get a vCPU using an out-of-bounds index, e.g. if the write is split
into multiple 8-bit stores, and is paired with a 32-bit load on a VM with
257 vCPUs:

  CPU0                              CPU1
  last_boosted_vcpu = 0xff;

                                    (last_boosted_vcpu = 0x100)
                                    last_boosted_vcpu[15:8] = 0x01;
  i = (last_boosted_vcpu = 0x1ff)
                                    last_boosted_vcpu[7:0] = 0x00;

  vcpu = kvm->vcpu_array[0x1ff];

As detected by KCSAN:

  BUG: KCSAN: data-race in kvm_vcpu_on_spin [kvm] / kvm_vcpu_on_spin [kvm]

  write to 0xffffc90025a92344 of 4 bytes by task 4340 on cpu 16:
  kvm_vcpu_on_spin (arch/x86/kvm/../../../virt/kvm/kvm_main.c:4112) kvm
  handle_pause (arch/x86/kvm/vmx/vmx.c:5929) kvm_intel
  vmx_handle_exit (arch/x86/kvm/vmx/vmx.c:?
		 arch/x86/kvm/vmx/vmx.c:6606) kvm_intel
  vcpu_run (arch/x86/kvm/x86.c:11107 arch/x86/kvm/x86.c:11211) kvm
  kvm_arch_vcpu_ioctl_run (arch/x86/kvm/x86.c:?) kvm
  kvm_vcpu_ioctl (arch/x86/kvm/../../../virt/kvm/kvm_main.c:?) kvm
  __se_sys_ioctl (fs/ioctl.c:52 fs/ioctl.c:904 fs/ioctl.c:890)
  __x64_sys_ioctl (fs/ioctl.c:890)
  x64_sys_call (arch/x86/entry/syscall_64.c:33)
  do_syscall_64 (arch/x86/entry/common.c:?)
  entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)

  read to 0xffffc90025a92344 of 4 bytes by task 4342 on cpu 4:
  kvm_vcpu_on_spin (arch/x86/kvm/../../../virt/kvm/kvm_main.c:4069) kvm
  handle_pause (arch/x86/kvm/vmx/vmx.c:5929) kvm_intel
  vmx_handle_exit (arch/x86/kvm/vmx/vmx.c:?
			arch/x86/kvm/vmx/vmx.c:6606) kvm_intel
  vcpu_run (arch/x86/kvm/x86.c:11107 arch/x86/kvm/x86.c:11211) kvm
  kvm_arch_vcpu_ioctl_run (arch/x86/kvm/x86.c:?) kvm
  kvm_vcpu_ioctl (arch/x86/kvm/../../../virt/kvm/kvm_main.c:?) kvm
  __se_sys_ioctl (fs/ioctl.c:52 fs/ioctl.c:904 fs/ioctl.c:890)
  __x64_sys_ioctl (fs/ioctl.c:890)
  x64_sys_call (arch/x86/entry/syscall_64.c:33)
  do_syscall_64 (arch/x86/entry/common.c:?)
  entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)

  value changed: 0x00000012 -> 0x00000000

Fixes: 217ece6 ("KVM: use yield_to instead of sleep in kvm_vcpu_on_spin")
Cc: stable@vger.kernel.org
Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://lore.kernel.org/r/20240510092353.2261824-1-leitao@debian.org
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: leitao

virt/kvm/kvm_main.c

@@ -4025,12 +4025,13 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool yield_to_kernel_mode)
 {
 	struct kvm *kvm = me->kvm;
 	struct kvm_vcpu *vcpu;
-	int last_boosted_vcpu = me->kvm->last_boosted_vcpu;
+	int last_boosted_vcpu;
 	unsigned long i;
 	int yielded = 0;
 	int try = 3;
 	int pass;
 
+	last_boosted_vcpu = READ_ONCE(kvm->last_boosted_vcpu);
 	kvm_vcpu_set_in_spin_loop(me, true);
 	/*
 	 * We boost the priority of a VCPU that is runnable but not
@@ -4068,7 +4069,7 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool yield_to_kernel_mode)
 
 			yielded = kvm_vcpu_yield_to(vcpu);
 			if (yielded > 0) {
-				kvm->last_boosted_vcpu = i;
+				WRITE_ONCE(kvm->last_boosted_vcpu, i);
 				break;
 			} else if (yielded < 0) {
 				try--;