From 9b854303a58e8e0f4d9c40010fc88d5c280706d9 Mon Sep 17 00:00:00 2001
From: Andrea Arcangeli <andrea@cpushare.com>
Date: Tue, 25 Jul 2017 22:22:45 +0200
Subject: [PATCH] fs/exec: fix use after free in execve

"file" can be already freed if bprm->file is NULL after
search_binary_handler() return. binfmt_script will do exactly that for
example. If the VM reuses the file after fput run(), this will result in
a use ater free.

So obtain d_is_su before search_binary_handler() runs.

This should explain this crash:

[25333.009554] Unable to handle kernel NULL pointer dereference at virtual address 00000185
[..]
[25333.009918] [2:             am:21861] PC is at do_execve+0x354/0x474

Change-Id: I2a8a814d1c0aa75625be83cb30432cf13f1a0681
Signed-off-by: Kevin F. Haggerty <haggertk@lineageos.org>
---

diff --git a/fs/exec.c b/fs/exec.c
index 0ca8cba..c98c680 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1490,6 +1490,7 @@
 	bool clear_in_exec;
 	int retval;
 	const struct cred *cred = current_cred();
+	bool is_su;
 
 	/*
 	 * We move the actual failure in case of RLIMIT_NPROC excess from
@@ -1566,11 +1567,14 @@
 	if (retval < 0)
 		goto out;
 
+	/* search_binary_handler can release file and it may be freed */
+	is_su = d_is_su(file->f_dentry);
+
 	retval = search_binary_handler(bprm);
 	if (retval < 0)
 		goto out;
 
-	if (d_is_su(file->f_dentry) && capable(CAP_SYS_ADMIN)) {
+	if (is_su && capable(CAP_SYS_ADMIN)) {
 		current->flags |= PF_SU;
 		su_exec();
 	}