Re: git: eb18708ec8c7 - main - syncache: accept packet with no SA when TCP_MD5SIG is set
Date: Wed, 12 Jan 2022 17:35:50 UTC
On Wed, Jan 12, 2022 at 6:58 AM Mike Karels <mike@karels.net> wrote:
> Sorry for the delayed response; I’ve been out.
>
No worries.
This change seems wrong to me. The TCP_MD5SIG option clearly required MD5
> signatures on all connections in the past, for many years,
You're right, since 2017. Before that, TCP_MD5SIG did not require MD5
signature when establishing a connection either.
> That meant that connections were limited to peers in the Security
> Association Database.
A program like a routing daemon could then be sure that it was talking to a
> known peer using
> signed packets
This is still the case, if you've configured a peer to use MD5, it will use
MD5.
> Redefining the option at this late date seems unsafe and unwise.
The option hasn't been redefined.
> If there is a use case for a socket that requires MD5SIG for known
> peers and not for others, it seems to me that it would be better to add a
> new option with those semantics.
>
The use case is a bgp daemon socket that wants to handle/establish
connections with MD5 and non-MD5 peers.
For peers configured with MD5, the established connection will be protected
by MD5 signatures (i.e., the TCP_MD5SIG option will be set). If a peer is
not configured for MD5, then the established connection will not be
protected with MD5 signatures (i.e., TCP_MD5SIG will not be set).
For what it's worth, this behavior is consistent with OpenBSD.
-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=eb18708ec8c7e1de6a05aba41971659549991b10
> >
> > 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-signed
> > 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 not
> > +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 match
> > - * 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 has
> a
> > + * security association, the packet will be dropped.
> > */
> > if (ltflags & TF_SIGNATURE) {
> > - if ((to->to_flags & TOF_SIGNATURE) == 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) != 0)
> > + goto done;
> > + } else {
> > + if (TCPMD5_ENABLED() &&
> > + TCPMD5_INPUT(m, NULL, NULL) != ENOENT)
> > + goto done;
> > }
> > - if (!TCPMD5_ENABLED() ||
> > - TCPMD5_INPUT(m, th, to->to_signature) != 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 |= 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 == 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.
>