[PATCH 6/6] Drivers: hv: vmbus: Do not allow overwriting vmbus_connection.channels[]

["Andrea Parri (Microsoft)" <parri.andrea@gmail.com>]

Currently, vmbus_onoffer() and vmbus_process_offer() are not validating
whether a given entry in the vmbus_connection.channels[] array is empty
before filling the entry with a call of vmbus_channel_map_relid().  An
erroneous or malicious host could rely on this to leak channel objects.
Do not allow overwriting an entry vmbus_connection.channels[].

Reported-by: Juan Vazquez <juvazq@microsoft.com>
Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
---
 drivers/hv/channel_mgmt.c | 30 ++++++++++++++++++------------
 drivers/hv/hyperv_vmbus.h |  2 +-
 2 files changed, 19 insertions(+), 13 deletions(-)

diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c
index 68950a1e4b638..841c0d4e101bd 100644
--- a/drivers/hv/channel_mgmt.c
+++ b/drivers/hv/channel_mgmt.c
@@ -354,10 +354,12 @@ static void free_channel(struct vmbus_channel *channel)
 	kobject_put(&channel->kobj);
 }
 
-void vmbus_channel_map_relid(struct vmbus_channel *channel)
+int vmbus_channel_map_relid(struct vmbus_channel *channel)
 {
-	if (WARN_ON(channel->offermsg.child_relid >= MAX_CHANNEL_RELIDS))
-		return;
+	u32 relid = channel->offermsg.child_relid;
+
+	if (WARN_ON(relid >= MAX_CHANNEL_RELIDS || vmbus_connection.channels[relid] != NULL))
+		return -EINVAL;
 	/*
 	 * The mapping of the channel's relid is visible from the CPUs that
 	 * execute vmbus_chan_sched() by the time that vmbus_chan_sched() will
@@ -383,18 +385,17 @@ void vmbus_channel_map_relid(struct vmbus_channel *channel)
 	 *      of the VMBus driver and vmbus_chan_sched() can not run before
 	 *      vmbus_bus_resume() has completed execution (cf. resume_noirq).
 	 */
-	smp_store_mb(
-		vmbus_connection.channels[channel->offermsg.child_relid],
-		channel);
+	smp_store_mb(vmbus_connection.channels[relid], channel);
+	return 0;
 }
 
 void vmbus_channel_unmap_relid(struct vmbus_channel *channel)
 {
-	if (WARN_ON(channel->offermsg.child_relid >= MAX_CHANNEL_RELIDS))
+	u32 relid = channel->offermsg.child_relid;
+
+	if (WARN_ON(relid >= MAX_CHANNEL_RELIDS))
 		return;
-	WRITE_ONCE(
-		vmbus_connection.channels[channel->offermsg.child_relid],
-		NULL);
+	WRITE_ONCE(vmbus_connection.channels[relid], NULL);
 }
 
 static void vmbus_release_relid(u32 relid)
@@ -601,6 +602,12 @@ static void vmbus_process_offer(struct vmbus_channel *newchannel)
 	 */
 	atomic_dec(&vmbus_connection.offer_in_progress);
 
+	if (vmbus_channel_map_relid(newchannel)) {
+		mutex_unlock(&vmbus_connection.channel_mutex);
+		kfree(newchannel);
+		return;
+	}
+
 	list_for_each_entry(channel, &vmbus_connection.chn_list, listentry) {
 		if (guid_equal(&channel->offermsg.offer.if_type,
 			       &newchannel->offermsg.offer.if_type) &&
@@ -619,6 +626,7 @@ static void vmbus_process_offer(struct vmbus_channel *newchannel)
 		 * Check to see if this is a valid sub-channel.
 		 */
 		if (newchannel->offermsg.offer.sub_channel_index == 0) {
+			vmbus_channel_unmap_relid(newchannel);
 			mutex_unlock(&vmbus_connection.channel_mutex);
 			/*
 			 * Don't call free_channel(), because newchannel->kobj
@@ -635,8 +643,6 @@ static void vmbus_process_offer(struct vmbus_channel *newchannel)
 		list_add_tail(&newchannel->sc_list, &channel->sc_list);
 	}
 
-	vmbus_channel_map_relid(newchannel);
-
 	mutex_unlock(&vmbus_connection.channel_mutex);
 	cpus_read_unlock();
 
diff --git a/drivers/hv/hyperv_vmbus.h b/drivers/hv/hyperv_vmbus.h
index e2064bf2b557d..89d7b95b3bdad 100644
--- a/drivers/hv/hyperv_vmbus.h
+++ b/drivers/hv/hyperv_vmbus.h
@@ -338,7 +338,7 @@ int vmbus_add_channel_kobj(struct hv_device *device_obj,
 
 void vmbus_remove_channel_attr_group(struct vmbus_channel *channel);
 
-void vmbus_channel_map_relid(struct vmbus_channel *channel);
+int vmbus_channel_map_relid(struct vmbus_channel *channel);
 void vmbus_channel_unmap_relid(struct vmbus_channel *channel);
 
 struct vmbus_channel *relid2channel(u32 relid);
-- 
2.25.1