linux-kernel - [PATCH 3.16 251/306] KVM: Disable irq while unregistering user notifier

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Message-ID: <lsq.1487198500.346788549@decadent.org.uk>
Date:   Wed, 15 Feb 2017 22:41:40 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org,
        "Radim Krčmář" <rkrcmar@...hat.com>,
        "Paolo Bonzini" <pbonzini@...hat.com>,
        "Ignacio Alvarado" <ikalvarado@...gle.com>
Subject: [PATCH 3.16 251/306] KVM: Disable irq while unregistering user
 notifier

3.16.40-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Ignacio Alvarado <ikalvarado@...gle.com>

commit 1650b4ebc99da4c137bfbfc531be4a2405f951dd upstream.

Function user_notifier_unregister should be called only once for each
registered user notifier.

Function kvm_arch_hardware_disable can be executed from an IPI context
which could cause a race condition with a VCPU returning to user mode
and attempting to unregister the notifier.

Signed-off-by: Ignacio Alvarado <ikalvarado@...gle.com>
Fixes: 18863bdd60f8 ("KVM: x86 shared msr infrastructure")
Reviewed-by: Paolo Bonzini <pbonzini@...hat.com>
Signed-off-by: Radim Krčmář <rkrcmar@...hat.com>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 arch/x86/kvm/x86.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -180,7 +180,18 @@ static void kvm_on_user_return(struct us
 	struct kvm_shared_msrs *locals
 		= container_of(urn, struct kvm_shared_msrs, urn);
 	struct kvm_shared_msr_values *values;
+	unsigned long flags;
 
+	/*
+	 * Disabling irqs at this point since the following code could be
+	 * interrupted and executed through kvm_arch_hardware_disable()
+	 */
+	local_irq_save(flags);
+	if (locals->registered) {
+		locals->registered = false;
+		user_return_notifier_unregister(urn);
+	}
+	local_irq_restore(flags);
 	for (slot = 0; slot < shared_msrs_global.nr; ++slot) {
 		values = &locals->values[slot];
 		if (values->host != values->curr) {
@@ -188,8 +199,6 @@ static void kvm_on_user_return(struct us
 			values->curr = values->host;
 		}
 	}
-	locals->registered = false;
-	user_return_notifier_unregister(urn);
 }
 
 static void shared_msr_update(unsigned slot, u32 msr)