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

[Greg KH <gregkh@linuxfoundation.org>]

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.

thanks,

greg k-h