linux-kernel - Re: [PATCH net v3] net/tcp: Disable TCP-AO static key after RCU grace period

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Message-ID: <CANn89iKiDDQu9A8URr6ZtYuBL6uSmtGxYhw7-TOUgGz5cp9OnQ@mail.gmail.com>
Date: Thu, 1 Aug 2024 08:46:08 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: 0x7f454c46@...il.com
Cc: "David S. Miller" <davem@...emloft.net>, David Ahern <dsahern@...nel.org>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org, stable@...nel.org
Subject: Re: [PATCH net v3] net/tcp: Disable TCP-AO static key after RCU grace period

On Thu, Aug 1, 2024 at 2:13 AM Dmitry Safonov via B4 Relay
<devnull+0x7f454c46.gmail.com@...nel.org> wrote:
>
> From: Dmitry Safonov <0x7f454c46@...il.com>
>
> The lifetime of TCP-AO static_key is the same as the last
> tcp_ao_info. On the socket destruction tcp_ao_info ceases to be
> with RCU grace period, while tcp-ao static branch is currently deferred
> destructed. The static key definition is
> : DEFINE_STATIC_KEY_DEFERRED_FALSE(tcp_ao_needed, HZ);
>
> which means that if RCU grace period is delayed by more than a second
> and tcp_ao_needed is in the process of disablement, other CPUs may
> yet see tcp_ao_info which atent dead, but soon-to-be.
> And that breaks the assumption of static_key_fast_inc_not_disabled().
>
> See the comment near the definition:
> > * The caller must make sure that the static key can't get disabled while
> > * in this function. It doesn't patch jump labels, only adds a user to
> > * an already enabled static key.
>
> Originally it was introduced in commit eb8c507296f6 ("jump_label:
> Prevent key->enabled int overflow"), which is needed for the atomic
> contexts, one of which would be the creation of a full socket from a
> request socket. In that atomic context, it's known by the presence
> of the key (md5/ao) that the static branch is already enabled.
> So, the ref counter for that static branch is just incremented
> instead of holding the proper mutex.
> static_key_fast_inc_not_disabled() is just a helper for such usage
> case. But it must not be used if the static branch could get disabled
> in parallel as it's not protected by jump_label_mutex and as a result,
> races with jump_label_update() implementation details.
>
> Happened on netdev test-bot[1], so not a theoretical issue:
>
>
> [1]: https://netdev-3.bots.linux.dev/vmksft-tcp-ao-dbg/results/696681/5-connect-deny-ipv6/stderr
>
> Cc: stable@...nel.org
> Fixes: 67fa83f7c86a ("net/tcp: Add static_key for TCP-AO")
> Signed-off-by: Dmitry Safonov <0x7f454c46@...il.com>

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