On Wed, 20 Nov 2024 16:10:45 +0000, Al Viro 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.
I will release the patch of the v4 version and add root cause:
During the execution of the link command, it sets the inode of the symlink file to the
already bad inode of file0 by calling d_instantiate, which ultimately leads to
null-ptr-deref when performing a mount operation on the symbolic link bus
because it use bad inode's i_link and its value is equal to i_dir_seq=2.
BR,
Lizhi
