[PATCH] hfsplus: don't return 0 when fill_super() failed

[Tetsuo Handa]

syzbot is reporting NULL pointer dereference at mount_fs() [1].
This is because hfsplus_fill_super() is by error returning 0 when
hfsplus_fill_super() detected invalid filesystem image, and mount_bdev()
is returning NULL because dget(s->s_root) == NULL if s->s_root == NULL,
and mount_fs() is accessing root->d_sb because IS_ERR(root) == false
if root == NULL. Fix this by returning -EINVAL when hfsplus_fill_super()
detected invalid filesystem image.

[1] https://syzkaller.appspot.com/bug?id=21acb6850cecbc960c927229e597158cf35f33d0

Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Reported-by: syzbot <syzbot+01ffaf5d9568dd1609f7@xxxxxxxxxxxxxxxxxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
---
 fs/hfsplus/super.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/hfsplus/super.c b/fs/hfsplus/super.c
index 513c357..9e690ae 100644
--- a/fs/hfsplus/super.c
+++ b/fs/hfsplus/super.c
@@ -524,8 +524,10 @@ static int hfsplus_fill_super(struct super_block *sb, void *data, int silent)
 		goto out_put_root;
 	if (!hfs_brec_read(&fd, &entry, sizeof(entry))) {
 		hfs_find_exit(&fd);
-		if (entry.type != cpu_to_be16(HFSPLUS_FOLDER))
+		if (entry.type != cpu_to_be16(HFSPLUS_FOLDER)) {
+			err = -EINVAL;
 			goto out_put_root;
+		}
 		inode = hfsplus_iget(sb, be32_to_cpu(entry.folder.id));
 		if (IS_ERR(inode)) {
 			err = PTR_ERR(inode);
-- 
1.8.3.1