Re: general protection fault in sctp_sched_prio_sched

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:

quoted

Hello Syzbot

On Sat, 15 Jun 2019 16:36:06 -0700 (PDT) syzbot wrote:

quoted

Hello,

syzbot found the following crash on:

...

quoted

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)