autofs4: check dev ioctl size before allocating

commit e53d77eb upstream.

There wasn't any check of the size passed from userspace before trying
to allocate the memory required.

This meant that userspace might request more space than allowed,
triggering an OOM.
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Ian Kent <raven@themaw.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[lizf: Backported to 3.4: adjust context]
Signed-off-by: Zefan Li <lizefan@huawei.com>

fs/autofs4/dev-ioctl.c

@@ -103,6 +103,9 @@ static struct autofs_dev_ioctl *copy_dev_ioctl(struct autofs_dev_ioctl __user *i
 	if (tmp.size < sizeof(tmp))
 		return ERR_PTR(-EINVAL);
 
+	if (tmp.size > (PATH_MAX + sizeof(tmp)))
+		return ERR_PTR(-ENAMETOOLONG);
+
 	res = memdup_user(in, tmp.size);
 	if (!IS_ERR(res))
 		res->size = tmp.size;