cifs_dbg() outputs an uninitialized buffer in cifs_readdir()

commit 01b9b0b2 upstream.

In some cases tmp_bug can be not filled in cifs_filldir and stay uninitialized,
therefore its printk with "%s" modifier can leak content of kernelspace memory.
If old content of this buffer does not contain '\0' access bejond end of
allocated object can crash the host.
Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: Steve French <sfrench@localhost.localdomain>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

fs/cifs/readdir.c

@@ -849,6 +849,7 @@ int cifs_readdir(struct file *file, struct dir_context *ctx)
 		 * if buggy server returns . and .. late do we want to
 		 * check for that here?
 		 */
+		*tmp_buf = 0;
 		rc = cifs_filldir(current_entry, file, ctx,
 				  tmp_buf, max_len);
 		if (rc) {