Re: [PATCH v4] hfsplus: fix null-ptr-deref by creating hidden dir on remount rw

[Viacheslav Dubeyko <vdubeyko@redhat.com>]

On Thu, 2026-04-30 at 07:33 +0530, Deepanshu Kartikey wrote:
> 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 into a helper and call it from
> hfsplus_reconfigure() when switching to read-write mode.
> 
> 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 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 | 88 ++++++++++++++++++++++++++++------------------
>  1 file changed, 53 insertions(+), 35 deletions(-)
> 
> diff --git a/fs/hfsplus/super.c b/fs/hfsplus/super.c
> index 7229a8ae89f9..8b5c39ce4d48 100644
> --- a/fs/hfsplus/super.c
> +++ b/fs/hfsplus/super.c
> @@ -372,15 +372,59 @@ static int hfsplus_statfs(struct dentry *dentry, struct kstatfs *buf)
>  	return 0;
>  }
>  
> +static int hfsplus_create_hidden_dir(struct super_block *sb)
> +{
> +	struct hfsplus_sb_info *sbi = HFSPLUS_SB(sb);
> +	struct inode *root = d_inode(sb->s_root);
> +	struct qstr str;
> +	int err;
> +
> +	str.len = sizeof(HFSP_HIDDENDIR_NAME) - 1;
> +	str.name = HFSP_HIDDENDIR_NAME;

The hfsplus_fill_super() has this initialization. It looks like duplication.
Maybe, we need to add this into hfsplus_reconfigure() and get str as input
argument?

> +
> +	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;
> +	else if (err) {
> +		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(sbi->hidden_dir,
> +				 HFSPLUS_I_CAT_DIRTY);
> +	return 0;
> +
> +out_put_hidden_dir:
> +	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;
> +	struct hfsplus_sb_info *sbi = HFSPLUS_SB(sb);
>  
>  	sync_filesystem(sb);
>  	if ((bool)(fc->sb_flags & SB_RDONLY) == sb_rdonly(sb))
>  		return 0;
>  	if (!(fc->sb_flags & SB_RDONLY)) {
> -		struct hfsplus_sb_info *sbi = HFSPLUS_SB(sb);
>  		struct hfsplus_vh *vhdr = sbi->s_vhdr;
>  
>  		if (!(vhdr->attributes & cpu_to_be32(HFSPLUS_VOL_UNMNT))) {
> @@ -401,6 +445,12 @@ static int hfsplus_reconfigure(struct fs_context *fc)
>  			fc->sb_flags |= SB_RDONLY;
>  		}
>  	}
> +	/*
> +	 * Create hidden dir if remounting read-write and it does
> +	 * not exist - required for link/unlink/rename operations.
> +	 */
> +	if (!sb_rdonly(sb) && !sbi->hidden_dir)
> +		return hfsplus_create_hidden_dir(sb);

It looks like hfsplus_reconfigure() is always false on a successful remount. The
sb->s_flags is updated by the VFS after reconfigure returns 0, not inside it.
The correct guard is !(fc->sb_flags & SB_RDONLY), which reflects the negotiated
new state.

>  	return 0;
>  }
>  
> @@ -595,38 +645,9 @@ 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);
> +			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.
> -				 */

What was wrong with the comment? Why have we lost it?

> -				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(sbi->hidden_dir,
> -						 HFSPLUS_I_CAT_DIRTY);

Are you sure that you work with the latest sources? Because, current state of
this code is:

  hfsplus_mark_inode_dirty(HFSPLUS_CAT_TREE_I(sb), HFSPLUS_I_CAT_DIRTY);       
  hfsplus_mark_inode_dirty(sbi->hidden_dir,        HFSPLUS_I_CAT_DIRTY);

>  		}
>  	}
>  
> @@ -634,9 +655,6 @@ 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);

Why cancel_delayed_work_sync() has been removed? The hfsplus_new_inode() calls
hfsplus_mark_mdb_dirty() which queues sync_work. So, nothing cancels the queued
work in the case of any issue.

Thanks,
Slava.

> -	iput(sbi->hidden_dir);
>  out_put_root:
>  	dput(sb->s_root);
>  	sb->s_root = NULL;