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>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

fs/cifs/readdir.c

@@ -823,6 +823,7 @@ int cifs_readdir(struct file *file, void *direntry, filldir_t filldir)
 			}
 			/* if buggy server returns . and .. late do
 			we want to check for that here? */
+			*tmp_buf = 0;
 			rc = cifs_filldir(current_entry, file,
 					filldir, direntry, tmp_buf, max_len);
 			if (rc == -EOVERFLOW) {