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

[Dan Carpenter <dan.carpenter@oracle.com>]

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.

> 
> >
> > 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?)

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))