Re: FAILED: patch "[PATCH] mptcp: make fallback action and fallback decision atomic" failed to apply to 5.15-stable tree

[Matthieu Baerts <matttbe@kernel.org>]

Hi Greg,

On 21/07/2025 12:55, gregkh@linuxfoundation.org wrote:
> 
> The patch below does not apply to the 5.15-stable tree.
> If someone wants it applied there, or to any other stable or longterm
> tree, then please email the backport, including the original git commit
> id to <stable@vger.kernel.org>.

(...)

> ------------------ original commit in Linus's tree ------------------
> 
> From f8a1d9b18c5efc76784f5a326e905f641f839894 Mon Sep 17 00:00:00 2001
> From: Paolo Abeni <pabeni@redhat.com>
> Date: Mon, 14 Jul 2025 18:41:44 +0200
> Subject: [PATCH] mptcp: make fallback action and fallback decision atomic
> 
> Syzkaller reported the following splat:

(...)

> Since we need to track the 'fallback is possible' condition and the
> fallback status separately, there are a few possible races open between
> the check and the actual fallback action.
> 
> Add a spinlock to protect the fallback related information and use it
> close all the possible related races. While at it also remove the
> too-early clearing of allow_infinite_fallback in __mptcp_subflow_connect():
> the field will be correctly cleared by subflow_finish_connect() if/when
> the connection will complete successfully.
> 
> If fallback is not possible, as per RFC, reset the current subflow.
> 
> Since the fallback operation can now fail and return value should be
> checked, rename the helper accordingly.
> 
> Fixes: 0530020a7c8f ("mptcp: track and update contiguous data status")

FYI, I don't think backporting this commit f8a1d9b18c5e ("mptcp: make
fallback action and fallback decision atomic") in v5.15 and v5.10 is
worth it: triggering this warning is difficult and required admin
rights. Plus the fix is quite invasive, and causes many conflicts in
these versions.

Same for commit def5b7b2643e ("mptcp: plug races between subflow fail
and subflow creation") for v5.15 only which depends on the other one.

Cheers,
Matt
-- 
Sponsored by the NGI0 Core fund.