Re: [PATCH V3] fs/ntfs3: check if the inode is bad before creating symlink

[Al Viro]

On Wed, Nov 20, 2024 at 11:04:43AM +0800, Lizhi Xu wrote:
> syzbot reported a null-ptr-deref in pick_link. [1]
> 
> First, i_link and i_dir_seq are in the same union, they share the same memory
> address, and i_dir_seq will be updated during the execution of walk_component,
> which makes the value of i_link equal to i_dir_seq.
> 
> Secondly, the chmod execution failed, which resulted in setting the mode value
> of file0's inode to REG when executing ntfs_bad_inode.
> 
> Third, when creating a symbolic link using the file0 whose inode has been marked
> as bad, it is not determined whether its inode is bad, which ultimately leads to
> null-ptr-deref when performing a mount operation on the symbolic link bus because
> the i_link value is equal to i_dir_seq=2. 
> 
> Note: ("file0, bus" are defined in reproducer [2])
> 
> To avoid null-ptr-deref in pick_link, when creating a symbolic link, first check
> whether the inode of file is already bad.

I would really like to understand how the hell did that bad inode end up passed
to d_splice_alias()/d_instantiate()/whatever it had been.

That's the root cause - and it looks like ntfs is too free with make_bad_inode()
in general, which might cause other problems.