ext4: fix crashes in dioread_nolock mode

Competing overwrite DIO in dioread_nolock mode will just overwrite
pointer to io_end in the inode. This may result in data corruption or
extent conversion happening from IO completion interrupt because we
don't properly set buffer_defer_completion() when unlocked DIO races
with locked DIO to unwritten extent.

Since unlocked DIO doesn't need io_end for anything, just avoid
allocating it and corrupting pointer from inode for locked DIO.
A cleaner fix would be to avoid these games with io_end pointer from the
inode but that requires more intrusive changes so we leave that for
later.

Cc: stable@vger.kernel.org
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/inode.c

@@ -3281,6 +3281,9 @@ static ssize_t ext4_ext_direct_IO(struct kiocb *iocb, struct iov_iter *iter,
 	 * case, we allocate an io_end structure to hook to the iocb.
 	 */
 	iocb->private = NULL;
+	if (overwrite) {
+		get_block_func = ext4_get_block_overwrite;
+	} else {
 		ext4_inode_aio_set(inode, NULL);
 		if (!is_sync_kiocb(iocb)) {
 			io_end = ext4_init_io_end(inode, GFP_NOFS);
@@ -3289,7 +3292,8 @@ static ssize_t ext4_ext_direct_IO(struct kiocb *iocb, struct iov_iter *iter,
 				goto retake_lock;
 			}
 			/*
-		 * Grab reference for DIO. Will be dropped in ext4_end_io_dio()
+			 * Grab reference for DIO. Will be dropped in
+			 * ext4_end_io_dio()
 			 */
 			iocb->private = ext4_get_io_end(io_end);
 			/*
@@ -3300,10 +3304,6 @@ static ssize_t ext4_ext_direct_IO(struct kiocb *iocb, struct iov_iter *iter,
 			 */
 			ext4_inode_aio_set(inode, io_end);
 		}
-
-	if (overwrite) {
-		get_block_func = ext4_get_block_overwrite;
-	} else {
 		get_block_func = ext4_get_block_write;
 		dio_flags = DIO_LOCKING;
 	}