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 The new pathmtu may be lower than MINSEGMENT if the dst is obsolete and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.
>
> 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 ?
Neil
> change = t->pathmtu != pmtu;
> }
> t->pathmtu = pmtu;
> --
> 2.1.0
>
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