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SubjectRe: [PATCH] ntfs: change check order in ntfs_attr_find
Date
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.

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