linux-kernel - [PATCH] KVM: x86: Drop WARN on INIT/SIPI being blocked when vCPU is in Wait-For-SIPI

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Message-ID: <20260123022816.2283567-1-seanjc@google.com>
Date: Thu, 22 Jan 2026 18:28:16 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	syzbot+59f2c3a3fc4f6c09b8cd@...kaller.appspotmail.com
Subject: [PATCH] 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@...kaller.appspotmail.com
Closes: https://lore.kernel.org/all/6925da1b.a70a0220.d98e3.00b0.GAE@google.com
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/lapic.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 2e513f1c8988..46ba9df3b81d 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -3487,7 +3487,6 @@ 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;
 	}

base-commit: acdc5446135932ca974b82d9d9a17762c7a82493
-- 
2.52.0.457.g6b5491de43-goog