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

[Greg KH <gregkh@linuxfoundation.org>]

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.

greg k-h