Re: [PATCH] ntfs: change check order in ntfs_attr_find

[Hawkins Jiawei <yin31149@gmail.com>]

On Sat, 27 Aug 2022 at 14:42, Dan Carpenter <dan.carpenter@oracle.com> wrote:
>
> On Fri, Aug 26, 2022 at 11:42:32PM +0800, Hawkins Jiawei wrote:
> > On Fri, 26 Aug 2022 at 23:15, Dan Carpenter <dan.carpenter@oracle.com> wrote:
> > >
> > > On Fri, Aug 26, 2022 at 08:32:57PM +0800, Hawkins Jiawei wrote:
> > > > > syz test https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git   master
> > > > >
> > > > > Looks like it is improper check order that causes this bug.
> > > >
> > > > Sorry for wrong command.
> > > > #syz test https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git   master
> > > >
> > > > diff --git a/fs/ntfs/attrib.c b/fs/ntfs/attrib.c
> > > > index 52615e6090e1..6480cd2d371d 100644
> > > > --- a/fs/ntfs/attrib.c
> > > > +++ b/fs/ntfs/attrib.c
> > > > @@ -594,10 +594,11 @@ static int ntfs_attr_find(const ATTR_TYPE type, const ntfschar *name,
> > > >       for (;; a = (ATTR_RECORD*)((u8*)a + le32_to_cpu(a->length))) {
> > > >               u8 *mrec_end = (u8 *)ctx->mrec +
> > > >                              le32_to_cpu(ctx->mrec->bytes_allocated);
> > > > +             if ((u8*)a < (u8*)ctx->mrec || (u8*)a > mrec_end)
> > > > +                     break;
> > >
> > > This definitely seems like a bug.  But your code won't build.  Syzbot
> > > must have -Werror turned off?
> > Hi Dan,
> > Did you mean we should put the variable declares at the beginning of the function?
> > (Correct me if I understand anything wrong)
>
> You can declare it at the beginning of the block.
OK, I will do like that.

>
> >
> > >
> > > Btw, this was in the original code, but those casts are ugly.  Ideally
> > > there would be some way to get rid of them.  But otherwise at least
> > > put a space after the u8.  "(u8 *)a < (u8 *)ctx->mrec".
> > >
> > > >               u8 *name_end = (u8 *)a + le16_to_cpu(a->name_offset) +
> > > >                              a->name_length * sizeof(ntfschar);
> > > > -             if ((u8*)a < (u8*)ctx->mrec || (u8*)a > mrec_end ||
> > > > -                 name_end > mrec_end)
> > > > +             if (name_end > mrec_end)
> > > >                       break;
> > >
> > > regards,
> > > dan carpenter
> > So maybe I can try to refactor these codes. But I wonder if this can be
> > done in a seperate bug
>
> The kernel has a strict "one thing per patch rule".  Those rules are
> for reviewers and easier backporting.  So the trick is to write the
> commit message to persuade the reviewer that the way you've written the
> patch is the easiest way to review it.  So here is how I would write the
> commit message:
>
> [PATCH] ntfs: fix out of bounds read in ntfs_attr_find()
>
> This code deferences "a" to calculate "name_end" and then it checks to
> ensure that "a" is within bounds.  Move the bounds checks earlier and
> add some comments to make it more clear what they're doing.  Then
> calculate "name_end" and check that.
>
> (Btw, are the wrap around checks really sufficient?  It seems like it
> could wrap to something still within the ->mrec buffer but before the
> current entry so it would end up in a forever loop or something?)
I am not for sure, but it seems that it is OK before.
As for the forever loop, there is a break when a->length is 0 in the loop,
So I think it probably would not end up in a forever loop?(Correct me if
I am wrong)

>
> diff --git a/fs/ntfs/attrib.c b/fs/ntfs/attrib.c
> index 52615e6090e1..90d567acb2a3 100644
> --- a/fs/ntfs/attrib.c
> +++ b/fs/ntfs/attrib.c
> @@ -594,11 +594,20 @@ static int ntfs_attr_find(const ATTR_TYPE type, const ntfschar *name,
>         for (;; a = (ATTR_RECORD*)((u8*)a + le32_to_cpu(a->length))) {
>                 u8 *mrec_end = (u8 *)ctx->mrec +
>                                le32_to_cpu(ctx->mrec->bytes_allocated);
> -               u8 *name_end = (u8 *)a + le16_to_cpu(a->name_offset) +
> -                              a->name_length * sizeof(ntfschar);
> -               if ((u8*)a < (u8*)ctx->mrec || (u8*)a > mrec_end ||
> -                   name_end > mrec_end)
> +               u8 *name_end;
> +
> +               /* check for wrap around */
> +               if ((u8 *)a < (u8 *)ctx->mrec)
> +                       break;
> +               /* check for overflow */
> +               if ((u8 *)a > mrec_end)
>                         break;
> +
> +               name_end = (u8 *)a + le16_to_cpu(a->name_offset) +
> +                          a->name_length * sizeof(ntfschar);
> +               if (name_end > mrec_end)
> +                       break;
> +
>                 ctx->attr = a;
>                 if (unlikely(le32_to_cpu(a->type) > le32_to_cpu(type) ||
>                                 a->type == AT_END))
Thanks for your suggestion, I will refactor these codes in this way.