Re: [PATCH 1/1] ext4: fix potential negative array index in do_split()

[Lukas Czerner <lczerner@redhat.com>]

On Fri, Jun 19, 2020 at 08:44:19AM -0500, Eric Sandeen wrote:
> On 6/19/20 6:16 AM, Lukas Czerner wrote:
> 
> >> The other possibility is that map[i].size is not right and indeed there
> >> seems to be a bug in dx_make_map()
> >>
> >> map_tail->size = le16_to_cpu(de->rec_len);
> >>
> >> should be
> >>
> >> map_tail->size = ext4_rec_len_from_disk(de->rec_len, blocksize));
> >>
> >> right ? Otherwise with large enough records the size will be smaller
> >> than it really is.
> >>
> >> A quick look at fs/ext4/namei.c reveals couple of places there rec_len
> >> is used without the conversion and we should check whether it needs
> >> fixing.
> >>
> >> -Lukas
> > 
> > And indeed the following patch seems to have fixed the issue we were
> > seeing. Eric I think that this might be a proper fix. But we still need
> > to check the other uses of rec_len to make sure it's ok as well.
> > 
> > diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
> > index 94ec882..5509fdc 100644
> > --- a/fs/ext4/namei.c
> > +++ b/fs/ext4/namei.c
> > @@ -1068,7 +1068,7 @@ static int dx_make_map(struct ext4_dir_entry_2 *de, unsigned blocksize,
> >                         map_tail--;
> >                         map_tail->hash = h.hash;
> >                         map_tail->offs = ((char *) de - base)>>2;
> > -                       map_tail->size = le16_to_cpu(de->rec_len);
> > +                       map_tail->size = ext4_rec_len_from_disk(le16_to_cpu(de->rec_len), blocksize);
> 
> That isn't right, ext4_rec_len_from_disk /takes/ an __le16 :)
> 
> -                       map_tail->size = le16_to_cpu(de->rec_len);
> +                       map_tail->size = ext4_rec_len_from_disk(de->rec_len), blocksize);

Yep, my bad.

> 
> would be more correct, but won't matter for PAGE_SIZE < 65536 right?

True, it's not the problem we're seeing.

-Lukas

> 
> -Eric
>