Re: [PATCHv2 net] sctp: check and update stream->out_curr when allocating stream_out

[Neil Horman <nhorman@tuxdriver.com>]

On Fri, Nov 30, 2018 at 03:22:39PM +0900, Xin Long wrote:
> On Thu, Nov 29, 2018 at 11:39 PM Neil Horman <nhorman@tuxdriver.com> wrote:
> >
> > On Thu, Nov 29, 2018 at 02:42:56PM +0800, Xin Long wrote:
> > > Now when using stream reconfig to add out streams, stream->out
> > > will get re-allocated, and all old streams' information will
> > > be copied to the new ones and the old ones will be freed.
> > >
> > > So without stream->out_curr updated, next time when trying to
> > > send from stream->out_curr stream, a panic would be caused.
> > >
> > > This patch is to check and update stream->out_curr when
> > > allocating stream_out.
> > >
> > > v1->v2:
> > >   - define fa_index() to get elem index from stream->out_curr.
> > >
> > > Fixes: 5bbbbe32a431 ("sctp: introduce stream scheduler foundations")
> > > Reported-by: Ying Xu <yinxu@redhat.com>
> > > Reported-by: syzbot+e33a3a138267ca119c7d@syzkaller.appspotmail.com
> > > Signed-off-by: Xin Long <lucien.xin@gmail.com>
> > > ---
> > >  net/sctp/stream.c | 20 ++++++++++++++++++++
> > >  1 file changed, 20 insertions(+)
> > >
> > > diff --git a/net/sctp/stream.c b/net/sctp/stream.c
> > > index 3892e76..30e7809 100644
> > > --- a/net/sctp/stream.c
> > > +++ b/net/sctp/stream.c
> > > @@ -84,6 +84,19 @@ static void fa_zero(struct flex_array *fa, size_t index, size_t count)
> > >       }
> > >  }
> > >
> > > +static size_t fa_index(struct flex_array *fa, void *elem, size_t count)
> > > +{
> > > +     size_t index = 0;
> > > +
> > > +     while (count--) {
> > > +             if (elem == flex_array_get(fa, index))
> > > +                     break;
> > > +             index++;
> > > +     }
> > > +
> > > +     return index;
> > > +}
> > > +
> > >  /* Migrates chunks from stream queues to new stream queues if needed,
> > >   * but not across associations. Also, removes those chunks to streams
> > >   * higher than the new max.
> > > @@ -147,6 +160,13 @@ static int sctp_stream_alloc_out(struct sctp_stream *stream, __u16 outcnt,
> > >
> > >       if (stream->out) {
> > >               fa_copy(out, stream->out, 0, min(outcnt, stream->outcnt));
> > > +             if (stream->out_curr) {
> > > +                     size_t index = fa_index(stream->out, stream->out_curr,
> > > +                                             stream->outcnt);
> > > +
> > > +                     BUG_ON(index == stream->outcnt);
> > > +                     stream->out_curr = flex_array_get(out, index);
> > > +             }
> > >               fa_free(stream->out);
> > >       }
> > >
> > > --
> > > 2.1.0
> > >
> > >
> >
> > This is the sort of thing I'm talking about. Its a little more code, but if you
> > augment the flex_array api like this, you can preform a resize operation on your
> > existing flex array, and you can avoid all the copying, and need to update
> > pointers maintained outside the array.  Note this code isn't tested at all, but
> > its close to what I think should work.
> >
> >
> > diff --git a/include/linux/flex_array.h b/include/linux/flex_array.h
> > index b94fa61b51fb..7fa1f27a91b5 100644
> > --- a/include/linux/flex_array.h
> > +++ b/include/linux/flex_array.h
> > @@ -73,6 +73,8 @@ struct flex_array {
> >  struct flex_array *flex_array_alloc(int element_size, unsigned int total,
> >                 gfp_t flags);
> >
> > +struct flex_array *flex_array_resize(struct flex_array *fa, unsigned int total, gfp_t flags);
> > +
> >  /**
> >   * flex_array_prealloc() - Ensures that memory for the elements indexed in the
> >   * range defined by start and nr_elements has been allocated.
> > diff --git a/lib/flex_array.c b/lib/flex_array.c
> > index 2eed22fa507c..f8d54af3891b 100644
> > --- a/lib/flex_array.c
> > +++ b/lib/flex_array.c
> > @@ -109,6 +109,7 @@ struct flex_array *flex_array_alloc(int element_size, unsigned int total,
> >         ret->total_nr_elements = total;
> >         ret->elems_per_part = elems_per_part;
> >         ret->reciprocal_elems = reciprocal_elems;
> > +       ret->elements_used = 0;
> >         if (elements_fit_in_base(ret) && !(flags & __GFP_ZERO))
> >                 memset(&ret->parts[0], FLEX_ARRAY_FREE,
> >                                                 FLEX_ARRAY_BASE_BYTES_LEFT);
> > @@ -116,6 +117,53 @@ struct flex_array *flex_array_alloc(int element_size, unsigned int total,
> >  }
> >  EXPORT_SYMBOL(flex_array_alloc);
> >
> > +static int flex_array_last_element_index(struct flex_array *fa)
> > +{
> > +       struct flex_array_part *part;
> > +       int part_nr;
> > +       int i,j;
> > +
> > +       if (elements_fit_in_base(fa)) {
> > +               part = (struct flex_array_part *)&fa->parts[0];
> > +               for (i = fa->elems_per_part; i >= 0; i--)
> > +                       if (part->elements[i] != FLEX_ARRAY_FREE)
> > +                               return i;
> > +       }
> > +
> > +       i = fa->total_nr_elements;
> > +       for (part_nr = 0; part_nr < FLEX_ARRAY_NR_BASE_PTRS; part_nr++) {
> > +               part = fa->parts[part_nr];
> > +               if (!part) {
> > +                       i -= fa->elems_per_part;
> > +                       continue;
> > +               }
> > +               for (j = fa->elems_per_part; j >= 0; j--, i--)
> > +                       if (part->elements[j] != FLEX_ARRAY_FREE)
> > +                               goto out;
> > +       }
> > +out:
> > +       return i;
> > +}
> > +
> > +struct flex_array *flex_array_resize(struct flex_array *fa, unsigned int total, gfp_t flags)
> > +{
> > +       if (total >= fa->total_nr_elements) {
> > +               /* Grow case */
> > +               if (total > max_size)
> > +                       return ERR_PTR(-ETOOBIG);
> > +               fa->total_nr_elements = total;
> > +       } else {
> > +               /* Shrink case */
> > +               /* Drop any pages we don't need*/
> > +               flex_array_shrink(fa);
> > +               if (flex_array_last_element_index(fa) >= total)
> > +                       return ERR_PTR(-ESIZE);
> > +               fa->total_nr_elements = total;
> > +       }
> > +       return fa;
> > +}
> > +EXPORT_SYMBOL(flex_array_resize);
> > +
> >  static int fa_element_to_part_nr(struct flex_array *fa,
> >                                         unsigned int element_nr)
> >  {
> I have a question about how it checks one part is free in
> flex_array_shrink():
>   part_is_free() is doing it by checking:
>   if (part->elements[all] == FLEX_ARRAY_FREE) ...
> 
> What if the data in the array that users put is FLEX_ARRAY_FREE,
> it will be treated as free, but in fact it's still in use?
> 
I noticed that too, I think they're taking an unsafe shortcut honestly, assuming
that the FLEX_ARRAY_FREE poison value in the first byte is indicative of the
entire part being free.  For this use case its not likely a problem, because the
first element of the stream is a pointer, and so its unlikely to start with the
FLEX_ARRAY_FREE value, but it could suffer from aliasing in other cases.  In the
current design they should really be doing a memory comparison of the entire
object length to a buffer filled with the poison value to determine free status. 

I would suggest not worrying about it for the purposes of this patch, but I
think I'll write a fix for the flex_array code that adds a byte to the head of
whatever object is bing stored to be reserved for free / in-use indicators.
Neil