[PATCH v2] ntfs: Ensure $Extend is a directory

[Soumya Negi <soumya.negi97@gmail.com>]

Fix Syzbot bug: kernel BUG in ntfs_lookup_inode_by_name
https://syzkaller.appspot.com/bug?id=32cf53b48c1846ffc25a185a2e92e170d1a95d71

Check whether $Extend is a directory or not( for NTFS3.0+) while
loading system files. If it isn't(as in the case of this bug where the
mft record for $Extend contains a regular file), load_system_files()
returns false.

Reported-by: syzbot+30b7f850c6d98ea461d2@syzkaller.appspotmail.com
CC: stable@vger.kernel.org # 4.9+
Signed-off-by: Soumya Negi <soumya.negi97@gmail.com>
---
Changes since v1:
* Added CC tag for stable
* Formatted changelog to fit within 72 cols

---
 fs/ntfs/super.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/fs/ntfs/super.c b/fs/ntfs/super.c
index 5ae8de09b271..18e2902531f9 100644
--- a/fs/ntfs/super.c
+++ b/fs/ntfs/super.c
@@ -2092,10 +2092,15 @@ static bool load_system_files(ntfs_volume *vol)
 	// TODO: Initialize security.
 	/* Get the extended system files' directory inode. */
 	vol->extend_ino = ntfs_iget(sb, FILE_Extend);
-	if (IS_ERR(vol->extend_ino) || is_bad_inode(vol->extend_ino)) {
+	if (IS_ERR(vol->extend_ino) || is_bad_inode(vol->extend_ino) ||
+	    !S_ISDIR(vol->extend_ino->i_mode)) {
+		static const char *es1 = "$Extend is not a directory";
+		static const char *es2 = "Failed to load $Extend";
+		const char *es = !S_ISDIR(vol->extend_ino->i_mode) ? es1 : es2;
+
 		if (!IS_ERR(vol->extend_ino))
 			iput(vol->extend_ino);
-		ntfs_error(sb, "Failed to load $Extend.");
+		ntfs_error(sb, "%s.", es);
 		goto iput_sec_err_out;
 	}
 #ifdef NTFS_RW
-- 
2.17.1