autofs: fix autofs_sbi() does not check super block type

BugLink: https://bugs.launchpad.net/bugs/1798539

commit 0633da48 upstream.

autofs_sbi() does not check the superblock magic number to verify it has
been given an autofs super block.

Backport Note: autofs4 has been renamed to autofs upstream. As a result
the upstream patch does not apply cleanly onto 4.14.y.

Link: http://lkml.kernel.org/r/153475422934.17131.7563724552005298277.stgit@pluto.themaw.net
Reported-by: <syzbot+87c3c541582e56943277@syzkaller.appspotmail.com>
Signed-off-by: Ian Kent <raven@themaw.net>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Zubin Mithra <zsm@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

fs/autofs4/autofs_i.h

@@ -18,6 +18,7 @@
 #include <linux/mutex.h>
 #include <linux/spinlock.h>
 #include <linux/list.h>
+#include <linux/magic.h>
 
 /* This is the range of ioctl() numbers we claim as ours */
 #define AUTOFS_IOC_FIRST     AUTOFS_IOC_READY
@@ -135,7 +136,8 @@ struct autofs_sb_info {
 
 static inline struct autofs_sb_info *autofs4_sbi(struct super_block *sb)
 {
-	return (struct autofs_sb_info *)(sb->s_fs_info);
+	return sb->s_magic != AUTOFS_SUPER_MAGIC ?
+		NULL : (struct autofs_sb_info *)(sb->s_fs_info);
 }
 
 static inline struct autofs_info *autofs4_dentry_ino(struct dentry *dentry)

fs/autofs4/inode.c

@@ -18,7 +18,6 @@
 #include <linux/pagemap.h>
 #include <linux/parser.h>
 #include <linux/bitops.h>
-#include <linux/magic.h>
 #include "autofs_i.h"
 #include <linux/module.h>