cifs: fix race between call_async() and reconnect()

commit 820962dc upstream.

cifs_call_async() queues the MID to the pending list and calls
smb_send_rqst().  If smb_send_rqst() performs a partial send, it sets
the tcpStatus to CifsNeedReconnect and returns an error code to
cifs_call_async().  In this case, cifs_call_async() removes the MID
from the list and returns to the caller.

However, cifs_call_async() releases the server mutex _before_ removing
the MID.  This means that a cifs_reconnect() can race with this function
and manage to remove the MID from the list and delete the entry before
cifs_call_async() calls cifs_delete_mid().  This leads to various
crashes due to the use after free in cifs_delete_mid().

Task1				Task2

cifs_call_async():
 - rc = -EAGAIN
 - mutex_unlock(srv_mutex)

				cifs_reconnect():
				 - mutex_lock(srv_mutex)
				 - mutex_unlock(srv_mutex)
				 - list_delete(mid)
				 - mid->callback()
				 	cifs_writev_callback():
				 		- mutex_lock(srv_mutex)
						- delete(mid)
				 		- mutex_unlock(srv_mutex)

 - cifs_delete_mid(mid) <---- use after free

Fix this by removing the MID in cifs_call_async() before releasing the
srv_mutex.  Also hold the srv_mutex in cifs_reconnect() until the MIDs
are moved out of the pending list.
Signed-off-by: Rabin Vincent <rabin.vincent@axis.com>
Acked-by: Shirish Pargaonkar <shirishpargaonkar@gmail.com>
Signed-off-by: Steve French <sfrench@localhost.localdomain>
[bwh: Backported to 3.2:
 - In cifs_call_async() there are two error paths jumping to 'out_err';
   fix both of them
 - s/cifs_delete_mid/delete_mid/
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

fs/cifs/connect.c

@@ -134,7 +134,6 @@ cifs_reconnect(struct TCP_Server_Info *server)
 	server->session_key.response = NULL;
 	server->session_key.len = 0;
 	server->lstrp = jiffies;
-	mutex_unlock(&server->srv_mutex);
 
 	/* mark submitted MIDs for retry and issue callback */
 	INIT_LIST_HEAD(&retry_list);
@@ -147,6 +146,7 @@ cifs_reconnect(struct TCP_Server_Info *server)
 		list_move(&mid_entry->qhead, &retry_list);
 	}
 	spin_unlock(&GlobalMid_Lock);
+	mutex_unlock(&server->srv_mutex);
 
 	cFYI(1, "%s: issuing mid callbacks", __func__);
 	list_for_each_safe(tmp, tmp2, &retry_list) {

fs/cifs/transport.c

@@ -370,10 +370,8 @@ cifs_call_async(struct TCP_Server_Info *server, struct kvec *iov,
 	spin_unlock(&GlobalMid_Lock);
 
 	rc = cifs_sign_smb2(iov, nvec, server, &mid->sequence_number);
-	if (rc) {
-		mutex_unlock(&server->srv_mutex);
-		goto out_err;
-	}
+	if (rc)
+		goto out;
 
 	mid->receive = receive;
 	mid->callback = callback;
@@ -384,14 +382,15 @@ cifs_call_async(struct TCP_Server_Info *server, struct kvec *iov,
 	rc = smb_sendv(server, iov, nvec);
 	cifs_in_send_dec(server);
 	cifs_save_when_sent(mid);
+out:
+	if (rc < 0)
+		delete_mid(mid);
+
 	mutex_unlock(&server->srv_mutex);
 
-	if (rc)
-		goto out_err;
+	if (rc == 0)
+		return 0;
 
-	return rc;
-out_err:
-	delete_mid(mid);
 	atomic_dec(&server->inFlight);
 	wake_up(&server->request_q);
 	return rc;