Re: [PATCH] mptcp: Do TCP fallback on early DSS checksum failure

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, May 25, 2022 at 02:36:10PM +0200, Matthieu Baerts wrote:
> On 25/05/2022 14:25, Greg KH wrote:
> > On Wed, May 25, 2022 at 12:32:04PM +0200, Matthieu Baerts wrote:
> >> Hi Greg, Mat,
> >>
> >> On 25/05/2022 09:51, Greg KH wrote:
> >>> On Tue, May 24, 2022 at 11:10:41AM -0700, Mat Martineau wrote:
> >>>> [ Upstream commit ae66fb2ba6c3dcaf8b9612b65aa949a1a4bed150 ]
> >>>>
> >>>> RFC 8684 section 3.7 describes several opportunities for a MPTCP
> >>>> connection to "fall back" to regular TCP early in the connection
> >>>> process, before it has been confirmed that MPTCP options can be
> >>>> successfully propagated on all SYN, SYN/ACK, and data packets. If a peer
> >>>> acknowledges the first received data packet with a regular TCP header
> >>>> (no MPTCP options), fallback is allowed.
> >>>>
> >>>> If the recipient of that first data packet finds a MPTCP DSS checksum
> >>>> error, this provides an opportunity to fail gracefully with a TCP
> >>>> fallback rather than resetting the connection (as might happen if a
> >>>> checksum failure were detected later).
> >>>>
> >>>> This commit modifies the checksum failure code to attempt fallback on
> >>>> the initial subflow of a MPTCP connection, only if it's a failure in the
> >>>> first data mapping. In cases where the peer initiates the connection,
> >>>> requests checksums, is the first to send data, and the peer is sending
> >>>> incorrect checksums (see
> >>>> https://github.com/multipath-tcp/mptcp_net-next/issues/275), this allows
> >>>> the connection to proceed as TCP rather than reset.
> >>>>
> >>>> Cc: <stable@vger.kernel.org> # 5.17.x
> >>>> Cc: <stable@vger.kernel.org> # 5.15.x
> >>>> Fixes: dd8bcd1768ff ("mptcp: validate the data checksum")
> >>>> Acked-by: Paolo Abeni <pabeni@redhat.com>
> >>>> Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
> >>>> Signed-off-by: David S. Miller <davem@davemloft.net>
> >>>> [mathew.j.martineau: backport: Resolved bitfield conflict in protocol.h]
> >>>> Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
> >>>> ---
> >>>>
> >>>> This patch is already in 5.17.10-rc1 and 5.15.42-rc1, but involves a
> >>>> context dependency on upstream commit 4cf86ae84c71 which I have
> >>>> requested to be dropped from the stable queues.
> >>>>
> >>>> I'm posting this backport without the protocol.h conflict to
> >>>> (hopefully?) make it easier for the stable maintainers to drop
> >>>> 4cf86ae84c71.
> >>>>
> >>>> For context see https://lore.kernel.org/stable/fa953ec-288f-7715-c6fb-47a222e85270@linux.intel.com/
> >>>
> >>> THanks, will take this after this round of releases.
> >>
> >> It might already be too late but is it possible to have this patch
> >> ("mptcp: Do TCP fallback on early DSS checksum failure") and "mptcp: fix
> >> checksum byte order" in the same stable release?
> >>
> >> Note that "mptcp: fix checksum byte order" patch has been recently
> >> queued by Sasha at the same time as "mptcp: Do TCP fallback on early DSS
> >> checksum failure".
> >>
> >> A bit of context: "mptcp: fix checksum byte order" fixes an important
> >> encoding issue but it also breaks the interoperability with previous
> >> Linux versions not having this patch.
> >>
> >> The patch from Mat ("mptcp: Do TCP fallback on early DSS checksum
> >> failure") improves the situation when there is this interoperability
> >> issue with a previous Linux versions not implementing the RFC properly.
> >> The improvement is there to make the MPTCP connections falling back to
> >> TCP instead of resetting them: at least there is a connection.
> >>
> >> In other words, that would be really nice to have these two commits
> >> backported together. If it is easier, it looks best to me to delay the
> >> main fix ("mptcp: fix checksum byte order") than having the two patches
> >> in different stable versions. But I understand it was not clear and
> >> maybe too late to do these modifications.
> >>
> >> Anyway, thank you for your work maintaining these stable versions! :)
> > 
> > I have already done a release with the first change in it, sorry, but
> > have queued this up now.  Given that this is fixing a problem with that
> > commit, I'll go do a release right now for 5.17 and 5.15.
> 
> I'm sorry for the troubles this is causing you but thank you for doing that!
> 
> Please note that on the bright side, the DSS Checksum feature is
> disabled by default so hopefully, this interoperability issue should not
> affect too many people. It is hard to quantify but I guess there is no
> need to rush if you prefer to wait before doing this release.

No worries, it was easy to do a new release, now done!

greg k-h