netdev - Re: [PATCH net v2] tcp: drop secpath at the same time as we currently drop dst

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Message-ID: <CANn89iLZ9SuWnKD1cVu_3cvVYD9jzziq6P=AJy=nUyQUOe4T4g@mail.gmail.com>
Date: Mon, 17 Feb 2025 12:58:44 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Sabrina Dubroca <sd@...asysnail.net>
Cc: netdev@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Neal Cardwell <ncardwell@...gle.com>, 
	Kuniyuki Iwashima <kuniyu@...zon.com>, David Ahern <dsahern@...nel.org>, 
	linux-security-module@...r.kernel.org, Paul Moore <paul@...l-moore.com>, 
	Xiumei Mu <xmu@...hat.com>
Subject: Re: [PATCH net v2] tcp: drop secpath at the same time as we currently
 drop dst

On Mon, Feb 17, 2025 at 11:23 AM Sabrina Dubroca <sd@...asysnail.net> wrote:
>
> Xiumei reported hitting the WARN in xfrm6_tunnel_net_exit while
> running tests that boil down to:
>  - create a pair of netns
>  - run a basic TCP test over ipcomp6
>  - delete the pair of netns
>
> The xfrm_state found on spi_byaddr was not deleted at the time we
> delete the netns, because we still have a reference on it. This
> lingering reference comes from a secpath (which holds a ref on the
> xfrm_state), which is still attached to an skb. This skb is not
> leaked, it ends up on sk_receive_queue and then gets defer-free'd by
> skb_attempt_defer_free.
>
> The problem happens when we defer freeing an skb (push it on one CPU's
> defer_list), and don't flush that list before the netns is deleted. In
> that case, we still have a reference on the xfrm_state that we don't
> expect at this point.
>
> We already drop the skb's dst in the TCP receive path when it's no
> longer needed, so let's also drop the secpath. At this point,
> tcp_filter has already called into the LSM hooks that may require the
> secpath, so it should not be needed anymore. However, in some of those
> places, the MPTCP extension has just been attached to the skb, so we
> cannot simply drop all extensions.
>
> Fixes: 68822bdf76f1 ("net: generalize skb freeing deferral to per-cpu lists")
> Reported-by: Xiumei Mu <xmu@...hat.com>
> Signed-off-by: Sabrina Dubroca <sd@...asysnail.net>
> ---
> v1: drop all extensions just before calling skb_attempt_defer_free
>     https://lore.kernel.org/netdev/879a4592e4e4bd0c30dbe29ca189e224ec1739a5.1739201151.git.sd@queasysnail.net/
> v2: - drop only secpath, as soon as possible - per Eric's feedback
>     - add debug warns if trying to add to sk_receive_queue an skb with
>       a dst or a secpath
>
> @Eric feel free to add some tags (Suggested-by? sign-off?) for the
> code I adapted from
> https://lore.kernel.org/netdev/CANn89i+JdDukwEhZ%3D41FxY-w63eER6JVixkwL+s2eSOjo6aWEQ@mail.gmail.com/

Reviewed-by: Eric Dumazet <edumazet@...gle.com>

Thanks !