ext4: force revalidation of directory pointer after seekdir(2)

A malicious user could force the directory pointer to be in an invalid
spot by using seekdir(2). Use the mechanism we already have to notice
if the directory has changed since the last time we called
ext4_readdir() to force a revalidation of the pointer.

Reported-by: [e-mail]
Signed-off-by: Theodore Ts'o <[e-mail]>
Cc: [e-mail]

fs/ext4/dir.c

···
 {
 	struct inode *inode = file->f_mapping->host;
 	int dx_dir = is_dx_dir(inode);
-	loff_t htree_max = ext4_get_htree_eof(file);
+	loff_t ret, htree_max = ext4_get_htree_eof(file);
 
 	if (likely(dx_dir))
-		return generic_file_llseek_size(file, offset, whence,
+		ret = generic_file_llseek_size(file, offset, whence,
 						    htree_max, htree_max);
 	else
-		return ext4_llseek(file, offset, whence);
+		ret = ext4_llseek(file, offset, whence);
+	file->f_version = inode_peek_iversion(inode) - 1;
+	return ret;
 }
 
 /*