[PATCH 5/6] Drivers: hv: vmbus: Resolve race condition in vmbus_onoffer_rescind()

[Andrea Parri (Microsoft)]

An erroneous or malicious host could send multiple rescind messages for
a same channel.  In vmbus_onoffer_rescind(), the guest maps the channel
ID to obtain a pointer to the channel object and it eventually releases
such object and associated data.  The host could time rescind messages
and lead to an use-after-free.  Add a new flag to the channel structure
to make sure that only one instance of vmbus_onoffer_rescind() can get
the reference to the channel object.

Reported-by: Juan Vazquez <juv...@microsoft.com>
Signed-off-by: Andrea Parri (Microsoft) <parri.and...@gmail.com>
---
 drivers/hv/channel_mgmt.c | 12 ++++++++++++
 include/linux/hyperv.h    |  1 +
 2 files changed, 13 insertions(+)

diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c
index 4072fd1f22146..68950a1e4b638 100644
--- a/drivers/hv/channel_mgmt.c
+++ b/drivers/hv/channel_mgmt.c
@@ -1063,6 +1063,18 @@ static void vmbus_onoffer_rescind(struct 
vmbus_channel_message_header *hdr)
 
        mutex_lock(&vmbus_connection.channel_mutex);
        channel = relid2channel(rescind->child_relid);
+       if (channel != NULL) {
+               /*
+                * Guarantee that no other instance of vmbus_onoffer_rescind()
+                * has got a reference to the channel object.  Synchronize on
+                * &vmbus_connection.channel_mutex.
+                */
+               if (channel->rescind_ref) {
+                       mutex_unlock(&vmbus_connection.channel_mutex);
+                       return;
+               }
+               channel->rescind_ref = true;
+       }
        mutex_unlock(&vmbus_connection.channel_mutex);
 
        if (channel == NULL) {
diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
index 2ea967bc17adf..f0d48a368f131 100644
--- a/include/linux/hyperv.h
+++ b/include/linux/hyperv.h
@@ -809,6 +809,7 @@ struct vmbus_channel {
        u8 monitor_bit;
 
        bool rescind; /* got rescind msg */
+       bool rescind_ref; /* got rescind msg, got channel reference */
        struct completion rescind_event;
 
        u32 ringbuffer_gpadlhandle;
-- 
2.25.1