Re: git: eb18708ec8c7 - main - syncache: accept packet with no SA when TCP_MD5SIG is set
Date: Fri, 14 Jan 2022 18:20:47 UTC
On Wed, Jan 12, 2022 at 1:27 PM Mike Karels <mike@karels.net> wrote:
> Rob wrote:
> > For what it's worth, this behavior is consistent with OpenBSD.
>
> That's a useful data point, thanks.
>
There is value in having the same semantics as OpenBSD for the TCP_MD5SIG
option <https://man.openbsd.org/tcp.4>.
If OpenBSD's semantics of the TCP_MD5SIG option were different, I likely
would have taken a different approach. And also given that FreeBSD and
OpenBSD's TCP_MD5SIG option had the same behavior until we diverged in 2017.
..that's how I ended up going in this direction
-Rob
>
> Mike
>
> > -Rob
>
>
> > >
> > > Mike
> > >
> > > On 8 Jan 2022, at 19:45, Robert Wing wrote:
> > >
> > > > The branch main has been updated by rew:
> > > >
> > > > URL:
> > >
> https://cgit.FreeBSD.org/src/commit/?id=3Deb18708ec8c7e1de6a05aba41971659=
> > 549991b10
> > > >
> > > > commit eb18708ec8c7e1de6a05aba41971659549991b10
> > > > Author: Robert Wing <rew@FreeBSD.org>
> > > > AuthorDate: 2022-01-09 01:07:50 +0000
> > > > Commit: Robert Wing <rew@FreeBSD.org>
> > > > CommitDate: 2022-01-09 01:32:14 +0000
> > > >
> > > > syncache: accept packet with no SA when TCP_MD5SIG is set
> > > >
> > > > When TCP_MD5SIG is set on a socket, all packets are dropped that
> > > don't
> > > > contain an MD5 signature. Relax this behavior to accept a
> non-signe=
> > d
> > > > packet when a security association doesn't exist with the peer.
> > > >
> > > > This is useful when a listen socket set with TCP_MD5SIG wants to
> > > handle
> > > > connections protected with and without MD5 signatures.
> > > >
> > > > Reviewed by: bz (previous version)
> > > > Sponsored by: nepustil.net
> > > > Sponsored by: Klara Inc.
> > > > Differential Revision: https://reviews.freebsd.org/D33227
> > > > ---
> > > > share/man/man4/tcp.4 | 6 +++++-
> > > > sys/netinet/tcp_syncache.c | 30 ++++++++++++++++++------------
> > > > sys/netipsec/xform_tcp.c | 5 +++++
> > > > 3 files changed, 28 insertions(+), 13 deletions(-)
> > > >
> > > > diff --git a/share/man/man4/tcp.4 b/share/man/man4/tcp.4
> > > > index 17138fa224ba..d103293132ba 100644
> > > > --- a/share/man/man4/tcp.4
> > > > +++ b/share/man/man4/tcp.4
> > > > @@ -34,7 +34,7 @@
> > > > .\" From: @(#)tcp.4 8.1 (Berkeley) 6/5/93
> > > > .\" $FreeBSD$
> > > > .\"
> > > > -.Dd June 27, 2021
> > > > +.Dd January 8, 2022
> > > > .Dt TCP 4
> > > > .Os
> > > > .Sh NAME
> > > > @@ -339,6 +339,10 @@ This entry can only be specified on a per-host
> > > basis at this time.
> > > > .Pp
> > > > If an SADB entry cannot be found for the destination,
> > > > the system does not send any outgoing segments and drops any inbound
> > > segments.
> > > > +However, during connection negotiation, a non-signed segment will be
> > > accepted if
> > > > +an SADB entry does not exist between hosts.
> > > > +When a non-signed segment is accepted, the established connection
> is n=
> > ot
> > > > +protected with MD5 digests.
> > > > .It Dv TCP_STATS
> > > > Manage collection of connection level statistics using the
> > > > .Xr stats 3
> > > > diff --git a/sys/netinet/tcp_syncache.c b/sys/netinet/tcp_syncache.c
> > > > index 7dd8443cad65..32ca3bc2209b 100644
> > > > --- a/sys/netinet/tcp_syncache.c
> > > > +++ b/sys/netinet/tcp_syncache.c
> > > > @@ -1514,19 +1514,25 @@ syncache_add(struct in_conninfo *inc, struct
> > > tcpopt *to, struct tcphdr *th,
> > > >
> > > > #if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
> > > > /*
> > > > - * If listening socket requested TCP digests, check that
> received
> > > > - * SYN has signature and it is correct. If signature doesn't
> matc=
> > h
> > > > - * or TCP_SIGNATURE support isn't enabled, drop the packet.
> > > > + * When the socket is TCP-MD5 enabled check that,
> > > > + * - a signed packet is valid
> > > > + * - a non-signed packet does not have a security association
> > > > + *
> > > > + * If a signed packet fails validation or a non-signed packet
> ha=
> > s
> > > a
> > > > + * security association, the packet will be dropped.
> > > > */
> > > > if (ltflags & TF_SIGNATURE) {
> > > > - if ((to->to_flags & TOF_SIGNATURE) =3D=3D 0) {
> > > > - TCPSTAT_INC(tcps_sig_err_nosigopt);
> > > > - goto done;
> > > > + if (to->to_flags & TOF_SIGNATURE) {
> > > > + if (!TCPMD5_ENABLED() ||
> > > > + TCPMD5_INPUT(m, th, to->to_signature) !=3D
> 0)
> > > > + goto done;
> > > > + } else {
> > > > + if (TCPMD5_ENABLED() &&
> > > > + TCPMD5_INPUT(m, NULL, NULL) !=3D ENOENT)
> > > > + goto done;
> > > > }
> > > > - if (!TCPMD5_ENABLED() ||
> > > > - TCPMD5_INPUT(m, th, to->to_signature) !=3D 0)
> > > > - goto done;
> > > > - }
> > > > + } else if (to->to_flags & TOF_SIGNATURE)
> > > > + goto done;
> > > > #endif /* TCP_SIGNATURE */
> > > > /*
> > > > * See if we already have an entry for this connection.
> > > > @@ -1724,11 +1730,11 @@ skip_alloc:
> > > > }
> > > > #if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
> > > > /*
> > > > - * If listening socket requested TCP digests, flag this in the
> > > > + * If incoming packet has an MD5 signature, flag this in the
> > > > * syncache so that syncache_respond() will do the right thing
> > > > * with the SYN+ACK.
> > > > */
> > > > - if (ltflags & TF_SIGNATURE)
> > > > + if (to->to_flags & TOF_SIGNATURE)
> > > > sc->sc_flags |=3D SCF_SIGNATURE;
> > > > #endif /* TCP_SIGNATURE */
> > > > if (to->to_flags & TOF_SACKPERM)
> > > > diff --git a/sys/netipsec/xform_tcp.c b/sys/netipsec/xform_tcp.c
> > > > index b53544cd00fb..ce2552f0a205 100644
> > > > --- a/sys/netipsec/xform_tcp.c
> > > > +++ b/sys/netipsec/xform_tcp.c
> > > > @@ -269,6 +269,11 @@ tcp_ipsec_input(struct mbuf *m, struct tcphdr
> *th,
> > > u_char *buf)
> > > > KMOD_TCPSTAT_INC(tcps_sig_err_buildsig);
> > > > return (ENOENT);
> > > > }
> > > > + if (buf =3D=3D NULL) {
> > > > + key_freesav(&sav);
> > > > + KMOD_TCPSTAT_INC(tcps_sig_err_nosigopt);
> > > > + return (EACCES);
> > > > + }
> > > > /*
> > > > * tcp_input() operates with TCP header fields in host
> > > > * byte order. We expect them in network byte order.
> > >
>