[PATCH] KVM: x86: Drop WARN_ON_ONCE() for concurrently disappearing interrupts

["syzbot" <syzbot@kernel.org>]

From: Alexander Potapenko <glider@google.com>

A warning can be triggered in kvm_check_and_inject_events() when an
interrupt disappears between the time it is checked via
kvm_cpu_has_injectable_intr() and the time it is fetched via
kvm_cpu_get_interrupt(). This occurs because the warning incorrectly
assumes that if an interrupt is injectable, fetching it must always return
a valid interrupt vector (i.e., not -1).

However, this assumption is broken by level-triggered interrupts that are
deasserted concurrently by another thread. For example, if a misconfigured
PIT or a PCI device asserts and then immediately deasserts a
level-triggered interrupt, the VCPU thread might see the pending interrupt
during the check but find it gone during the fetch, resulting in
kvm_cpu_get_interrupt() returning -1.

The warning manifests as follows:

------------[ cut here ]------------
irq == -1
WARNING: arch/x86/kvm/x86.c:10860 at kvm_check_and_inject_events
arch/x86/kvm/x86.c:10860 [inline]
WARNING: arch/x86/kvm/x86.c:10860 at vcpu_enter_guest
arch/x86/kvm/x86.c:11356 [inline]
WARNING: arch/x86/kvm/x86.c:10860 at vcpu_run+0x57ec/0x7950
arch/x86/kvm/x86.c:11770
RIP: 0010:kvm_check_and_inject_events arch/x86/kvm/x86.c:10860 [inline]
RIP: 0010:vcpu_enter_guest arch/x86/kvm/x86.c:11356 [inline]
RIP: 0010:vcpu_run+0x57ec/0x7950 arch/x86/kvm/x86.c:11770
Call Trace:
 <TASK>
 kvm_arch_vcpu_ioctl_run+0x1193/0x2070 arch/x86/kvm/x86.c:12125
 kvm_vcpu_ioctl+0xa61/0xfd0 virt/kvm/kvm_main.c:4470
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:597 [inline]
 __se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x174/0x580 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
 </TASK>

Since this is a legitimate Time-Of-Check to Time-Of-Use (TOCTOU) race
condition that can occur during normal operation, WARN_ON_ONCE() must not
be used for conditions that can legitimately happen. The patch removes the
WARN_ON_ONCE() in kvm_check_and_inject_events() and replaces it with a
pr_err_ratelimited() to log the event instead.

Fixes: bf672720e83c ("KVM: x86: check the kvm_cpu_get_interrupt result before using it")
Assisted-by: Gemini:gemini-3.1-pro-preview Gemini:gemini-3-flash-preview syzbot
Reported-by: syzbot+dd769db18693736eee89@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=dd769db18693736eee89
Link: https://syzkaller.appspot.com/ai_job?id=35cad3cd-95fd-4c0d-8ca8-812f58d56e59
Signed-off-by: Alexander Potapenko <glider@google.com>

---
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 0550359ed..c5b4cddd9 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -10857,10 +10857,13 @@ static int kvm_check_and_inject_events(struct kvm_vcpu *vcpu,
 		if (r) {
 			int irq = kvm_cpu_get_interrupt(vcpu);
 
-			if (!WARN_ON_ONCE(irq == -1)) {
+			if (irq != -1) {
 				kvm_queue_interrupt(vcpu, irq, false);
 				kvm_x86_call(inject_irq)(vcpu, false);
 				WARN_ON(kvm_x86_call(interrupt_allowed)(vcpu, true) < 0);
+			} else {
+				pr_err_ratelimited(
+					"KVM: interrupt disappeared between checking and fetching\n");
 			}
 		}
 		if (kvm_cpu_has_injectable_intr(vcpu))


base-commit: 8cd9520d35a6c38db6567e97dd93b1f11f185dc6
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