hfsplus_reconfigure() does not create the hidden directory when
remounting from read-only to read-write, leaving sbi->hidden_dir
as NULL. This causes a null-ptr-deref when any subsequent
link/unlink/rename operation dereferences it.

Extract hidden directory creation logic from hfsplus_fill_super()
into a new helper hfsplus_create_hidden_dir() and call it from
hfsplus_reconfigure() when switching to read-write mode and
hidden_dir is NULL, ensuring hidden_dir is always valid on any
read-write mount.

Reported-by: syzbot+c0ba772a362e70937dfb@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=c0ba772a362e70937dfb
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
---
Changes in v5:
  - Pass str as input argument to hfsplus_create_hidden_dir()
    to avoid duplication, as suggested by Vyacheslav Dubeyko.
  - Use !(fc->sb_flags & SB_RDONLY) as guard in reconfigure
    instead of !sb_rdonly(sb).
  - Restore cancel_delayed_work_sync() in cleanup path.
  - Restore HFSPLUS_CAT_TREE_I dirty mark.

Changes in v4:
  - Correct fix: extract hidden dir creation into helper and call
    from hfsplus_reconfigure() on remount rw, as suggested by
    Vyacheslav Dubeyko.

Changes in v3:
  - Correct fix location: guard sbi->hidden_dir in hfsplus_link()
    and hfsplus_unlink() in dir.c.

Changes in v2:
  - Fixed commit message: hfsplus_delete_cat() has multiple callers,
    not just hfsplus_unlink() as incorrectly stated in v1.
---
 fs/hfsplus/super.c | 97 +++++++++++++++++++++++++++++-----------------
 1 file changed, 61 insertions(+), 36 deletions(-)

diff --git a/fs/hfsplus/super.c b/fs/hfsplus/super.c
index 40a0feda716b..f37d5abc84cf 100644
--- a/fs/hfsplus/super.c
+++ b/fs/hfsplus/super.c
@@ -375,6 +375,54 @@ static int hfsplus_statfs(struct dentry *dentry, struct kstatfs *buf)
 	return 0;
 }
 
+static int hfsplus_create_hidden_dir(struct super_block *sb,
+				     const struct qstr *str)
+{
+	struct hfsplus_sb_info *sbi = HFSPLUS_SB(sb);
+	struct inode *root = d_inode(sb->s_root);
+	int err;
+
+	mutex_lock(&sbi->vh_mutex);
+	sbi->hidden_dir = hfsplus_new_inode(sb, root, S_IFDIR);
+	if (!sbi->hidden_dir) {
+		mutex_unlock(&sbi->vh_mutex);
+		return -ENOMEM;
+	}
+
+	err = hfsplus_create_cat(sbi->hidden_dir->i_ino, root,
+				 str, sbi->hidden_dir);
+	if (err) {
+		mutex_unlock(&sbi->vh_mutex);
+		goto out_put_hidden_dir;
+	}
+
+	err = hfsplus_init_security(sbi->hidden_dir, root, str);
+	if (err == -EOPNOTSUPP)
+		err = 0; /* Operation is not supported. */
+	else if (err) {
+		/*
+		 * Try to delete anyway without
+		 * error analysis.
+		 */
+		hfsplus_delete_cat(sbi->hidden_dir->i_ino, root, str);
+		mutex_unlock(&sbi->vh_mutex);
+		goto out_put_hidden_dir;
+	}
+
+	mutex_unlock(&sbi->vh_mutex);
+	hfsplus_mark_inode_dirty(HFSPLUS_CAT_TREE_I(sb),
+				 HFSPLUS_I_CAT_DIRTY);
+	hfsplus_mark_inode_dirty(sbi->hidden_dir,
+				 HFSPLUS_I_CAT_DIRTY);
+	return 0;
+
+out_put_hidden_dir:
+	cancel_delayed_work_sync(&sbi->sync_work);
+	iput(sbi->hidden_dir);
+	sbi->hidden_dir = NULL;
+	return err;
+}
+
 static int hfsplus_reconfigure(struct fs_context *fc)
 {
 	struct super_block *sb = fc->root->d_sb;
@@ -403,6 +451,18 @@ static int hfsplus_reconfigure(struct fs_context *fc)
 			sb->s_flags |= SB_RDONLY;
 			fc->sb_flags |= SB_RDONLY;
 		}
+
+		/*
+		 * Create hidden dir if remounting read-write and it
+		 * does not exist - required for link/unlink/rename.
+		 */
+		if (!(fc->sb_flags & SB_RDONLY) && !sbi->hidden_dir) {
+			struct qstr str;
+
+			str.len = sizeof(HFSP_HIDDENDIR_NAME) - 1;
+			str.name = HFSP_HIDDENDIR_NAME;
+			return hfsplus_create_hidden_dir(sb, &str);
+		}
 	}
 	return 0;
 }
@@ -620,40 +680,9 @@ static int hfsplus_fill_super(struct super_block *sb, struct fs_context *fc)
 		hfsplus_sync_fs(sb, 1);
 
 		if (!sbi->hidden_dir) {
-			mutex_lock(&sbi->vh_mutex);
-			sbi->hidden_dir = hfsplus_new_inode(sb, root, S_IFDIR);
-			if (!sbi->hidden_dir) {
-				mutex_unlock(&sbi->vh_mutex);
-				err = -ENOMEM;
+			err = hfsplus_create_hidden_dir(sb, &str);
+			if (err)
 				goto out_put_root;
-			}
-			err = hfsplus_create_cat(sbi->hidden_dir->i_ino, root,
-						 &str, sbi->hidden_dir);
-			if (err) {
-				mutex_unlock(&sbi->vh_mutex);
-				goto out_put_hidden_dir;
-			}
-
-			err = hfsplus_init_security(sbi->hidden_dir,
-							root, &str);
-			if (err == -EOPNOTSUPP)
-				err = 0; /* Operation is not supported. */
-			else if (err) {
-				/*
-				 * Try to delete anyway without
-				 * error analysis.
-				 */
-				hfsplus_delete_cat(sbi->hidden_dir->i_ino,
-							root, &str);
-				mutex_unlock(&sbi->vh_mutex);
-				goto out_put_hidden_dir;
-			}
-
-			mutex_unlock(&sbi->vh_mutex);
-			hfsplus_mark_inode_dirty(HFSPLUS_CAT_TREE_I(sb),
-						 HFSPLUS_I_CAT_DIRTY);
-			hfsplus_mark_inode_dirty(sbi->hidden_dir,
-						 HFSPLUS_I_CAT_DIRTY);
 		}
 	}
 
@@ -661,9 +690,6 @@ static int hfsplus_fill_super(struct super_block *sb, struct fs_context *fc)
 	sbi->nls = nls;
 	return 0;
 
-out_put_hidden_dir:
-	cancel_delayed_work_sync(&sbi->sync_work);
-	iput(sbi->hidden_dir);
 out_put_root:
 	dput(sb->s_root);
 	sb->s_root = NULL;
-- 
2.43.0