linux-kernel - [PATCH v2 2/2] KVM: x86/xen: Stop Xen timer before changing the IRQ vector

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Message-Id: <20220729184640.244969-3-dietschc@csp.edu>
Date:   Fri, 29 Jul 2022 13:46:40 -0500
From:   Coleman Dietsch <dietschc@....edu>
To:     kvm@...r.kernel.org
Cc:     Coleman Dietsch <dietschc@....edu>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        skhan@...uxfoundation.org, Pavel Skripkin <paskripkin@...il.com>,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        syzbot+e54f930ed78eb0f85281@...kaller.appspotmail.com
Subject: [PATCH v2 2/2] KVM: x86/xen: Stop Xen timer before changing the IRQ vector

This moves the stop xen timer call outside of the previously unreachable
if else statement as well as making sure that the timer is stopped first
before changing IRQ vector. Code was streamlined a bit also.

This was contributing to the ODEBUG bug in kvm_xen_vcpu_set_attr crash that
was discovered by syzbot.

ODEBUG: init active (active state 0)
object type: hrtimer hint: xen_timer_callbac0
RIP: 0010:debug_print_object+0x16e/0x250 lib/debugobjects.c:502
Call Trace:
__debug_object_init
debug_hrtimer_init
debug_init
hrtimer_init
kvm_xen_init_timer
kvm_xen_vcpu_set_attr
kvm_arch_vcpu_ioctl
kvm_vcpu_ioctl
vfs_ioctl

Link: https://syzkaller.appspot.com/bug?id=8234a9dfd3aafbf092cc5a7cd9842e3ebc45fc42
Reported-by: syzbot+e54f930ed78eb0f85281@...kaller.appspotmail.com
Signed-off-by: Coleman Dietsch <dietschc@....edu>
---
 arch/x86/kvm/xen.c | 37 ++++++++++++++++++-------------------
 1 file changed, 18 insertions(+), 19 deletions(-)

diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index 2dd0f72a62f2..f612fac0e379 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -707,27 +707,26 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
 		break;
 
 	case KVM_XEN_VCPU_ATTR_TYPE_TIMER:
-		if (data->u.timer.port) {
-			if (data->u.timer.priority != KVM_IRQ_ROUTING_XEN_EVTCHN_PRIO_2LEVEL) {
-				r = -EINVAL;
-				break;
-			}
-			vcpu->arch.xen.timer_virq = data->u.timer.port;
-
-			/* Check for existing timer */
-			if (!vcpu->arch.xen.timer.function)
-				kvm_xen_init_timer(vcpu);
-
-			/* Restart the timer if it's set */
-			if (data->u.timer.expires_ns)
-				kvm_xen_start_timer(vcpu, data->u.timer.expires_ns,
-						    data->u.timer.expires_ns -
-						    get_kvmclock_ns(vcpu->kvm));
-		} else if (kvm_xen_timer_enabled(vcpu)) {
-			kvm_xen_stop_timer(vcpu);
-			vcpu->arch.xen.timer_virq = 0;
+		if (data->u.timer.port &&
+		    data->u.timer.priority != KVM_IRQ_ROUTING_XEN_EVTCHN_PRIO_2LEVEL) {
+			r = -EINVAL;
+			break;
 		}
 
+		/* Check for existing timer */
+		if (!vcpu->arch.xen.timer.function)
+			kvm_xen_init_timer(vcpu);
+
+		/* Stop the timer (if it's running) before changing the vector */
+		kvm_xen_stop_timer(vcpu);
+		vcpu->arch.xen.timer_virq = data->u.timer.port;
+
+		/* Restart the timer if it's set */
+		if (data->u.timer.port && data->u.timer.expires_ns)
+			kvm_xen_start_timer(vcpu, data->u.timer.expires_ns,
+					    data->u.timer.expires_ns -
+					    get_kvmclock_ns(vcpu->kvm));
+
 		r = 0;
 		break;
 
-- 
2.34.1