Re: [PATCH net] sctp: fix the issue that pathmtu may be set lower than MINSEGMENT

[Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>]

On Mon, Jul 02, 2018 at 07:45:12AM -0400, Neil Horman wrote:
> On Mon, Jul 02, 2018 at 02:51:16PM +0800, Xin Long wrote:
> > After commit b6c5734db070 ("sctp: fix the handling of ICMP Frag Needed
> > for too small MTUs"), sctp_transport_update_pmtu would refetch pathmtu
> > from the dst and set it to transport's pathmtu without any check.
> > 
> > The new pathmtu may be lower than MINSEGMENT if the dst is obsolete and
> > updated by .get_dst() in sctp_transport_update_pmtu.

In this case, it could have a smaller MTU as well, and thus we should
validate it against MINSEGMENT instead.

> > 
> > Syzbot reported a warning in sctp_mtu_payload caused by this.
> > 
> > This fix uses the refetched pathmtu only when it's greater than the
> > frag_needed pmtu.
> > 
> > Fixes: b6c5734db070 ("sctp: fix the handling of ICMP Frag Needed for too small MTUs")
> > Reported-by: syzbot+f0d9d7cba052f9344b03@syzkaller.appspotmail.com
> > Signed-off-by: Xin Long <lucien.xin@gmail.com>
> > ---
> >  net/sctp/transport.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> > 
> > diff --git a/net/sctp/transport.c b/net/sctp/transport.c
> > index 445b7ef..ddfb687 100644
> > --- a/net/sctp/transport.c
> > +++ b/net/sctp/transport.c
> > @@ -282,7 +282,10 @@ bool sctp_transport_update_pmtu(struct sctp_transport *t, u32 pmtu)
> >  
> >  	if (dst) {
> >  		/* Re-fetch, as under layers may have a higher minimum size */
> > -		pmtu = SCTP_TRUNC4(dst_mtu(dst));
> > +		u32 mtu = SCTP_TRUNC4(dst_mtu(dst));
> > +
> > +		if (pmtu < mtu)
> > +			pmtu = mtu;
> nit, but why not u32 mtu = min(pmtu, SCTP_TRUNC4(dst_mtu(dst))) here ?

sctp_dst_mtu() is wrapping all that for us :)

-		pmtu = SCTP_TRUNC4(dst_mtu(dst));
+		pmtu = sctp_dst_mtu(dst);

> 
> Neil
> 
> >  		change = t->pathmtu != pmtu;
> >  	}
> >  	t->pathmtu = pmtu;
> > -- 
> > 2.1.0
> > 
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