Re: general protection fault in sctp_sched_prio_sched

From: Marcelo Ricardo Leitner
Date: Mon Jun 17 2019 - 09:54:12 EST


Hi,

On Sun, Jun 16, 2019 at 11:38:03PM +0800, Hillf Danton wrote:
>
> Hello Syzbot
>
> On Sat, 15 Jun 2019 16:36:06 -0700 (PDT) syzbot wrote:
> > Hello,
> >
> > syzbot found the following crash on:
> >
...
> Check prio_head and bail out if it is not valid.
>
> Thanks
> Hillf
> ----->8---
> ---
> net/sctp/stream_sched_prio.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/net/sctp/stream_sched_prio.c b/net/sctp/stream_sched_prio.c
> index 2245083..db25a43 100644
> --- a/net/sctp/stream_sched_prio.c
> +++ b/net/sctp/stream_sched_prio.c
> @@ -135,6 +135,8 @@ static void sctp_sched_prio_sched(struct sctp_stream *stream,
> struct sctp_stream_priorities *prio, *prio_head;
>
> prio_head = soute->prio_head;
> + if (!prio_head)
> + return;
>
> /* Nothing to do if already scheduled */
> if (!list_empty(&soute->prio_list))
> --

Thanks but this is not a good fix for this. It will cause the stream
to never be scheduled.

The problem happens because of the fault injection that happened a bit
before the crash, in here:

int sctp_stream_init_ext(struct sctp_stream *stream, __u16 sid)
{
struct sctp_stream_out_ext *soute;

soute = kzalloc(sizeof(*soute), GFP_KERNEL);
if (!soute)
return -ENOMEM;
SCTP_SO(stream, sid)->ext = soute; <---- [A]

return sctp_sched_init_sid(stream, sid, GFP_KERNEL);
^^^^^^^^^^^^---- [B] failed
}

This causes the 1st sendmsg to bail out with the error. When the 2nd
one gets in, it will:

sctp_sendmsg_to_asoc()
{
...
if (unlikely(!SCTP_SO(&asoc->stream, sinfo->sinfo_stream)->ext)) {
^^^^^--- [C]
err = sctp_stream_init_ext(&asoc->stream, sinfo->sinfo_stream);
if (err)
goto err;
}

[A] leaves ext initialized, despite the failed in [B]. Then in [C], it
will not try to initialize again.

We need to either uninitialize ->ext as error handling for [B], or
improve the check on [C].

syzbot++ once again.

Marcelo