fs: prevent out-of-bounds array speculation when closing a file descriptor

Google-Bug-Id: 114199369
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Author: tytso

fs/file.c

@@ -642,6 +642,7 @@ static struct file *pick_file(struct files_struct *files, unsigned fd)
 	if (fd >= fdt->max_fds)
 		return NULL;
 
+	fd = array_index_nospec(fd, fdt->max_fds);
 	file = fdt->fd[fd];
 	if (file) {
 		rcu_assign_pointer(fdt->fd[fd], NULL);