eCryptfs: Fix lockdep warning in miscdev operations

commit 60d65f1 upstream.

Don't grab the daemon mutex while holding the message context mutex.
Addresses this lockdep warning:

 ecryptfsd/2141 is trying to acquire lock:
  (&ecryptfs_msg_ctx_arr[i].mux){+.+.+.}, at: [<ffffffffa029c213>] ecryptfs_miscdev_read+0x143/0x470 [ecryptfs]

 but task is already holding lock:
  (&(*daemon)->mux){+.+...}, at: [<ffffffffa029c2ec>] ecryptfs_miscdev_read+0x21c/0x470 [ecryptfs]

 which lock already depends on the new lock.

 the existing dependency chain (in reverse order) is:

 -> #1 (&(*daemon)->mux){+.+...}:
        [<ffffffff810a3b8d>] lock_acquire+0x9d/0x220
        [<ffffffff8151c6da>] __mutex_lock_common+0x5a/0x4b0
        [<ffffffff8151cc64>] mutex_lock_nested+0x44/0x50
        [<ffffffffa029c5d7>] ecryptfs_send_miscdev+0x97/0x120 [ecryptfs]
        [<ffffffffa029b744>] ecryptfs_send_message+0x134/0x1e0 [ecryptfs]
        [<ffffffffa029a24e>] ecryptfs_generate_key_packet_set+0x2fe/0xa80 [ecryptfs]
        [<ffffffffa02960f8>] ecryptfs_write_metadata+0x108/0x250 [ecryptfs]
        [<ffffffffa0290f80>] ecryptfs_create+0x130/0x250 [ecryptfs]
        [<ffffffff811963a4>] vfs_create+0xb4/0x120
        [<ffffffff81197865>] do_last+0x8c5/0xa10
        [<ffffffff811998f9>] path_openat+0xd9/0x460
        [<ffffffff81199da2>] do_filp_open+0x42/0xa0
        [<ffffffff81187998>] do_sys_open+0xf8/0x1d0
        [<ffffffff81187a91>] sys_open+0x21/0x30
        [<ffffffff81527d69>] system_call_fastpath+0x16/0x1b

 -> #0 (&ecryptfs_msg_ctx_arr[i].mux){+.+.+.}:
        [<ffffffff810a3418>] __lock_acquire+0x1bf8/0x1c50
        [<ffffffff810a3b8d>] lock_acquire+0x9d/0x220
        [<ffffffff8151c6da>] __mutex_lock_common+0x5a/0x4b0
        [<ffffffff8151cc64>] mutex_lock_nested+0x44/0x50
        [<ffffffffa029c213>] ecryptfs_miscdev_read+0x143/0x470 [ecryptfs]
        [<ffffffff811887d3>] vfs_read+0xb3/0x180
        [<ffffffff811888ed>] sys_read+0x4d/0x90
        [<ffffffff81527d69>] system_call_fastpath+0x16/0x1b

Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

fs/ecryptfs/miscdev.c

@@ -195,31 +195,32 @@ int ecryptfs_send_miscdev(char *data, size_t data_size,
 			  struct ecryptfs_msg_ctx *msg_ctx, u8 msg_type,
 			  u16 msg_flags, struct ecryptfs_daemon *daemon)
 {
-	int rc = 0;
+	struct ecryptfs_message *msg;
 
-	mutex_lock(&msg_ctx->mux);
-	msg_ctx->msg = kmalloc((sizeof(*msg_ctx->msg) + data_size),
-			       GFP_KERNEL);
-	if (!msg_ctx->msg) {
-		rc = -ENOMEM;
+	msg = kmalloc((sizeof(*msg) + data_size), GFP_KERNEL);
+	if (!msg) {
 		printk(KERN_ERR "%s: Out of memory whilst attempting "
 		       "to kmalloc(%zd, GFP_KERNEL)\n", __func__,
-		       (sizeof(*msg_ctx->msg) + data_size));
-		goto out_unlock;
+		       (sizeof(*msg) + data_size));
+		return -ENOMEM;
 	}
+
+	mutex_lock(&msg_ctx->mux);
+	msg_ctx->msg = msg;
 	msg_ctx->msg->index = msg_ctx->index;
 	msg_ctx->msg->data_len = data_size;
 	msg_ctx->type = msg_type;
 	memcpy(msg_ctx->msg->data, data, data_size);
 	msg_ctx->msg_size = (sizeof(*msg_ctx->msg) + data_size);
-	mutex_lock(&daemon->mux);
 	list_add_tail(&msg_ctx->daemon_out_list, &daemon->msg_ctx_out_queue);
+	mutex_unlock(&msg_ctx->mux);
+
+	mutex_lock(&daemon->mux);
 	daemon->num_queued_msg_ctx++;
 	wake_up_interruptible(&daemon->wait);
 	mutex_unlock(&daemon->mux);
-out_unlock:
-	mutex_unlock(&msg_ctx->mux);
-	return rc;
+
+	return 0;
 }
 
 /**