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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, syzbot <syzkaller@googlegroups.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        David Hildenbrand <david@redhat.com>,
        Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Subject: [PATCH 4.4 67/79] KVM: irqfd: fix race between EPOLLHUP and irq_bypass_register_consumer
Date:   Thu, 23 Aug 2018 09:53:43 +0200
Message-Id: <20180823074923.544978272@linuxfoundation.org>
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4.4-stable review patch.  If anyone has any objections, please let me know.

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

From: Paolo Bonzini <pbonzini@redhat.com>

commit 9432a3175770e06cb83eada2d91fac90c977cb99 upstream.

A comment warning against this bug is there, but the code is not doing what
the comment says.  Therefore it is possible that an EPOLLHUP races against
irq_bypass_register_consumer.  The EPOLLHUP handler schedules irqfd_shutdown,
and if that runs soon enough, you get a use-after-free.

Reported-by: syzbot <syzkaller@googlegroups.com>
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 virt/kvm/eventfd.c |   11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

--- a/virt/kvm/eventfd.c
+++ b/virt/kvm/eventfd.c
@@ -405,11 +405,6 @@ kvm_irqfd_assign(struct kvm *kvm, struct
 	if (events & POLLIN)
 		schedule_work(&irqfd->inject);
 
-	/*
-	 * do not drop the file until the irqfd is fully initialized, otherwise
-	 * we might race against the POLLHUP
-	 */
-	fdput(f);
 #ifdef CONFIG_HAVE_KVM_IRQ_BYPASS
 	irqfd->consumer.token = (void *)irqfd->eventfd;
 	irqfd->consumer.add_producer = kvm_arch_irq_bypass_add_producer;
@@ -423,6 +418,12 @@ kvm_irqfd_assign(struct kvm *kvm, struct
 #endif
 
 	srcu_read_unlock(&kvm->irq_srcu, idx);
+
+	/*
+	 * do not drop the file until the irqfd is fully initialized, otherwise
+	 * we might race against the POLLHUP
+	 */
+	fdput(f);
 	return 0;
 
 fail: