Drivers: hv: utils: fix a race on userspace daemons registration

Background: userspace daemons registration protocol for Hyper-V utilities
drivers has two steps:
1) daemon writes its own version to kernel
2) kernel reads it and replies with module version
at this point we consider the handshake procedure being completed and we
do hv_poll_channel() transitioning the utility device to HVUTIL_READY
state. At this point we're ready to handle messages from kernel.

When hvutil_transport is in HVUTIL_TRANSPORT_CHARDEV mode we have a
single buffer for outgoing message. hvutil_transport_send() puts to this
buffer and till the buffer is cleared with hvt_op_read() returns -EFAULT
to all consequent calls. Host<->guest protocol guarantees there is no more
than one request at a time and we will not get new requests till we reply
to the previous one so this single message buffer is enough.

Now to the race. When we finish negotiation procedure and send kernel
module version to userspace with hvutil_transport_send() it goes into the
above mentioned buffer and if the daemon is slow enough to read it from
there we can get a collision when a request from the host comes, we won't
be able to put anything to the buffer so the request will be lost. To
solve the issue we need to know when the negotiation is really done (when
the version message is read by the daemon) and transition to HVUTIL_READY
state after this happens. Implement a callback on read to support this.
Old style netlink communication is not affected by the change, we don't
really know when these messages are delivered but we don't have a single
message buffer there.

Reported-by: Barry Davis <barry_davis@stormagic.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 14 insertions, 1 deletions

diff --git a/drivers/hv/hv_utils_transport.c b/drivers/hv/hv_utils_transport.c
index 9a9983fa4531..c235a9515267 100644
--- a/drivers/hv/hv_utils_transport.c
+++ b/drivers/hv/hv_utils_transport.c
@@ -72,6 +72,10 @@ static ssize_t hvt_op_read(struct file *file, char __user *buf,
 	hvt->outmsg = NULL;
 	hvt->outmsg_len = 0;
 
+	if (hvt->on_read)
+		hvt->on_read();
+	hvt->on_read = NULL;
+
 out_unlock:
 	mutex_unlock(&hvt->lock);
 	return ret;
@@ -219,7 +223,8 @@ static void hvt_cn_callback(struct cn_msg *msg, struct netlink_skb_parms *nsp)
 	mutex_unlock(&hvt->lock);
 }
 
-int hvutil_transport_send(struct hvutil_transport *hvt, void *msg, int len)
+int hvutil_transport_send(struct hvutil_transport *hvt, void *msg, int len,
+			  void (*on_read_cb)(void))
 {
 	struct cn_msg *cn_msg;
 	int ret = 0;
@@ -237,6 +242,13 @@ int hvutil_transport_send(struct hvutil_transport *hvt, void *msg, int len)
 		memcpy(cn_msg->data, msg, len);
 		ret = cn_netlink_send(cn_msg, 0, 0, GFP_ATOMIC);
 		kfree(cn_msg);
+		/*
+		 * We don't know when netlink messages are delivered but unlike
+		 * in CHARDEV mode we're not blocked and we can send next
+		 * messages right away.
+		 */
+		if (on_read_cb)
+			on_read_cb();
 		return ret;
 	}
 	/* HVUTIL_TRANSPORT_CHARDEV */
@@ -255,6 +267,7 @@ int hvutil_transport_send(struct hvutil_transport *hvt, void *msg, int len)
 	if (hvt->outmsg) {
 		memcpy(hvt->outmsg, msg, len);
 		hvt->outmsg_len = len;
+		hvt->on_read = on_read_cb;
 		wake_up_interruptible(&hvt->outmsg_q);
 	} else
 		ret = -ENOMEM;