KVM: x86: Drop WARN on INIT/SIPI being blocked when vCPU is in Wait-For-SIPI

Drop the sanity check in kvm_apic_accept_events() that attempts to detect
KVM bugs by asserting that a vCPU isn't in Wait-For-SIPI if INIT/SIPI are
blocked, because if INIT is blocked, then it should be impossible for a
vCPU to get into WFS in the first place.  Unfortunately, syzbot is smarter
than KVM (and its maintainers), and circumvented the guards put in place
by commit 0fe3e8d804fd ("KVM: x86: Move INIT_RECEIVED vs. INIT/SIPI blocked
check to KVM_RUN") by swapping the order and stuffing VMXON after INIT, and
then triggering kvm_apic_accept_events() by way of KVM_GET_MP_STATE.

Simply drop the WARN as it hasn't detected any meaningful KVM bugs in
years (if ever?), and preventing userspace from clobbering guest state is
generally a non-goal.  More importantly, fully closing the hole would
likely require enforcing some amount of ordering in KVM's ioctls, which is
a much bigger risk than simply deleting the WARN.

Reported-by: syzbot+59f2c3a3fc4f6c09b8cd@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/6925da1b.a70a0220.d98e3.00b0.GAE@google.com
Link: https://patch.msgid.link/20260123022816.2283567-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 1597dd0b0cc664f87dfb81edb763a32e70b8b753..01c2557bfa055c5fccd1fb4b5074232ee2eb1dcb 100644 (file)
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -3498,7 +3498,6 @@ int kvm_apic_accept_events(struct kvm_vcpu *vcpu)
         * wait-for-SIPI (WFS).
         */
        if (!kvm_apic_init_sipi_allowed(vcpu)) {
-               WARN_ON_ONCE(vcpu->arch.mp_state == KVM_MP_STATE_INIT_RECEIVED);
                clear_bit(KVM_APIC_SIPI, &apic->pending_events);
                return 0;
        }