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 The new pathmtu may be lower than MINSEGMENT if the dst is obsolete and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@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Xin Long <lucien.xin@xxxxxxxxx>
> > ---
> > 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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