Patch "KVM: eventfd: fix NULL deref irqbypass consumer" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

KVM: eventfd: fix NULL deref irqbypass consumer

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
kvm-eventfd-fix-null-deref-irqbypass-consumer.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Wanpeng Li <[email protected]>
Date: Thu, 5 Jan 2017 17:39:42 -0800
Subject: KVM: eventfd: fix NULL deref irqbypass consumer
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From: Wanpeng Li <[email protected]>

commit 4f3dbdf47e150016aacd734e663347fcaa768303 upstream.

Reported syzkaller:

    BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
    IP: irq_bypass_unregister_consumer+0x9d/0xb70 [irqbypass]
    PGD 0

    Oops: 0002 [#1] SMP
    CPU: 1 PID: 125 Comm: kworker/1:1 Not tainted 4.9.0+ #1
    Workqueue: kvm-irqfd-cleanup irqfd_shutdown [kvm]
    task: ffff9bbe0dfbb900 task.stack: ffffb61802014000
    RIP: 0010:irq_bypass_unregister_consumer+0x9d/0xb70 [irqbypass]
    Call Trace:
     irqfd_shutdown+0x66/0xa0 [kvm]
     process_one_work+0x16b/0x480
     worker_thread+0x4b/0x500
     kthread+0x101/0x140
     ? process_one_work+0x480/0x480
     ? kthread_create_on_node+0x60/0x60
     ret_from_fork+0x25/0x30
    RIP: irq_bypass_unregister_consumer+0x9d/0xb70 [irqbypass] RSP: ffffb61802017e20
    CR2: 0000000000000008

The syzkaller folks reported a NULL pointer dereference that due to unregister an consumer which fails registration before. The syzkaller creates two VMs w/ an equal eventfd occasionally. So the second VM fails to register an irqbypass consumer. It will make irqfd as inactive and queue an workqueue work to shutdown irqfd and unregister the irqbypass consumer when eventfd is closed. However, the second consumer has been initialized though it fails registration. So the token(same as the first VM's) is taken to unregister the consumer through the workqueue, the consumer of the first VM is found and unregistered, then NULL deref incurred in the path of deleting consumer from the consumers list.

This patch fixes it by making irq_bypass_register/unregister_consumer() looks for the consumer entry based on consumer pointer itself instead of token matching.

---
 virt/lib/irqbypass.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

@@ -195,7 +195,7 @@ int irq_bypass_register_consumer(struct
 	mutex_lock(&lock);
 
 	list_for_each_entry(tmp, &consumers, node) {
-		if (tmp->token == consumer->token) {
+		if (tmp->token == consumer->token || tmp == consumer) {
 			mutex_unlock(&lock);
 			module_put(THIS_MODULE);
 			return -EBUSY;

@@ -245,7 +245,7 @@ void irq_bypass_unregister_consumer(stru
 	mutex_lock(&lock);
 
 	list_for_each_entry(tmp, &consumers, node) {
-		if (tmp->token != consumer->token)
+		if (tmp != consumer)
 			continue;
 
 		list_for_each_entry(producer, &producers, node) {

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/kvm-eventfd-fix-null-deref-irqbypass-consumer.patch queue-4.9/kvm-x86-fix-null-deref-in-vcpu_scan_ioapic.patch