3.16.63-rc1 review patch. If anyone has any objections, please let me know.
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From: Halil Pasic pasic@linux.ibm.com
commit 78b1a52e05c9db11d293342e8d6d8a230a04b4e7 upstream.
While ccw_io_helper() seems like intended to be exclusive in a sense that it is supposed to facilitate I/O for at most one thread at any given time, there is actually nothing ensuring that threads won't pile up at vcdev->wait_q. If they do, all threads get woken up and see the status that belongs to some other request than their own. This can lead to bugs. For an example see: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1788432
This race normally does not cause any problems. The operations provided by struct virtio_config_ops are usually invoked in a well defined sequence, normally don't fail, and are normally used quite infrequent too.
Yet, if some of the these operations are directly triggered via sysfs attributes, like in the case described by the referenced bug, userspace is given an opportunity to force races by increasing the frequency of the given operations.
Let us fix the problem by ensuring, that for each device, we finish processing the previous request before starting with a new one.
Signed-off-by: Halil Pasic pasic@linux.ibm.com Reported-by: Colin Ian King colin.king@canonical.com Message-Id: 20180925121309.58524-3-pasic@linux.ibm.com Signed-off-by: Cornelia Huck cohuck@redhat.com Signed-off-by: Michael S. Tsirkin mst@redhat.com [bwh: Backported to 3.16: adjust filename] Signed-off-by: Ben Hutchings ben@decadent.org.uk --- drivers/s390/kvm/virtio_ccw.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-)
--- a/drivers/s390/kvm/virtio_ccw.c +++ b/drivers/s390/kvm/virtio_ccw.c @@ -57,6 +57,7 @@ struct virtio_ccw_device { int err; wait_queue_head_t wait_q; spinlock_t lock; + struct mutex io_lock; /* Serializes I/O requests */ struct list_head virtqueues; unsigned long indicators; unsigned long indicators2; @@ -282,6 +283,7 @@ static int ccw_io_helper(struct virtio_c unsigned long flags; int flag = intparm & VIRTIO_CCW_INTPARM_MASK;
+ mutex_lock(&vcdev->io_lock); do { spin_lock_irqsave(get_ccwdev_lock(vcdev->cdev), flags); ret = ccw_device_start(vcdev->cdev, ccw, intparm, 0, 0); @@ -294,7 +296,9 @@ static int ccw_io_helper(struct virtio_c cpu_relax(); } while (ret == -EBUSY); wait_event(vcdev->wait_q, doing_io(vcdev, flag) == 0); - return ret ? ret : vcdev->err; + ret = ret ? ret : vcdev->err; + mutex_unlock(&vcdev->io_lock); + return ret; }
static void virtio_ccw_drop_indicator(struct virtio_ccw_device *vcdev, @@ -1086,6 +1090,7 @@ static int virtio_ccw_online(struct ccw_ init_waitqueue_head(&vcdev->wait_q); INIT_LIST_HEAD(&vcdev->virtqueues); spin_lock_init(&vcdev->lock); + mutex_init(&vcdev->io_lock);
spin_lock_irqsave(get_ccwdev_lock(cdev), flags); dev_set_drvdata(&cdev->dev, vcdev);