Re: [PATCH] ovl: null dereference possibility fixup

[Amir Goldstein <amir73il@gmail.com>]

On Sun, Jun 21, 2020 at 3:33 PM youngjun <her0gyugyu@gmail.com> wrote:
>
> lowerdentry could be NULL, and dereferenced by calling d_inode.
> code flow which described below
> shows possibility of null dereference in ovl_get_inode.

I think this is not possible...

>
> (export.c) ovl_lower_fh_to_d
> |_(export.c) ovl_get_dentry(sb, upper, NULL, NULL);

This gets called if ovl_index_upper() succeeded
and ovl_index_upper() can only return d_is_dir().

>  |_(export.c) ovl_obtain_alias (sb, upper, NULL, NULL);

This only gets called for !d_is_dir()

>   |_(inode.c) ovl_get_inode(sb, &oip);
>    |_(in ovl_get_inode) realinode = d_inode(lowerdentry);
>
> Fixes: 09d8b586731bf("ovl: move __upperdentry to ovl_inode")
> Signed-off-by: youngjun <her0gyugyu@gmail.com>
> ---
>  fs/overlayfs/inode.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
> index 8be6cd264f66..53d82ef68ba8 100644
> --- a/fs/overlayfs/inode.c
> +++ b/fs/overlayfs/inode.c
> @@ -958,8 +958,10 @@ struct inode *ovl_get_inode(struct super_block *sb,
>         unsigned long ino = 0;
>         int err = oip->newinode ? -EEXIST : -ENOMEM;
>
> -       if (!realinode)
> +       if (!realinode && lowerdentry)
>                 realinode = d_inode(lowerdentry);
> +       else
> +               return ERR_PTR(-EINVAL);
>

oip->lowerpath and oip->upperdentry should not be both NULL.
If you want you can add a WARN_ON about this and return EINVAL,
because that would be a bug that needs to be fixed, but I saw no
proof that this bug exists.

If the problem was reported by a static analysis tool, maybe you
can re-factor the helpers in export.c to be less entangled.

For example, ovl_obtain_alias() part can be open-coded
in the two call sites of ovl_get_dentry() that care about non-dir
and then we can assert that  ovl_get_dentry() only handles directories.

Thanks,
Amir.