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Date:   Mon, 17 Jun 2019 11:43:31 -0300
From:   Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
To:     Hillf Danton <hdanton@sina.com>
Cc:     syzbot <syzbot+c1a380d42b190ad1e559@syzkaller.appspotmail.com>,
        davem@davemloft.net, linux-kernel@vger.kernel.org,
        linux-sctp@vger.kernel.org, lucien.xin@gmail.com,
        netdev@vger.kernel.org, nhorman@tuxdriver.com,
        syzkaller-bugs@googlegroups.com, vyasevich@gmail.com
Subject: Re: general protection fault in sctp_sched_prio_sched
Message-ID: <20190617144331.GE3500@localhost.localdomain>
References: <20190616153804.3604-1-hdanton@sina.com>
 <20190617134913.GL3436@localhost.localdomain>
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On Mon, Jun 17, 2019 at 10:49:13AM -0300, Marcelo Ricardo Leitner wrote:
> 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].

The former one, please. This should be enough (untested):

diff --git a/net/sctp/stream.c b/net/sctp/stream.c
index 93ed07877337..25946604af85 100644
--- a/net/sctp/stream.c
+++ b/net/sctp/stream.c
@@ -153,13 +153,20 @@ int sctp_stream_init(struct sctp_stream *stream, __u16 outcnt, __u16 incnt,
 int sctp_stream_init_ext(struct sctp_stream *stream, __u16 sid)
 {
 	struct sctp_stream_out_ext *soute;
+	int ret;
 
 	soute = kzalloc(sizeof(*soute), GFP_KERNEL);
 	if (!soute)
 		return -ENOMEM;
 	SCTP_SO(stream, sid)->ext = soute;
 
-	return sctp_sched_init_sid(stream, sid, GFP_KERNEL);
+	ret = sctp_sched_init_sid(stream, sid, GFP_KERNEL);
+	if (ret) {
+		kfree(SCTP_SO(stream, sid)->ext);
+		SCTP_SO(stream, sid)->ext = NULL;
+	}
+
+	return ret;
 }
 
 void sctp_stream_free(struct sctp_stream *stream)