linux-kernel - Re: [PATCH] net: Fix suspicious RCU usage in bpf_sk_reuseport

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Message-ID: <20220817004319.fd7dekpqeumbvmsh@kafai-mbp>
Date:   Tue, 16 Aug 2022 17:43:19 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     David Howells <dhowells@...hat.com>,
        Hawkins Jiawei <yin31149@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        bpf@...r.kernel.org, Jakub Sitnicki <jakub@...udflare.com>
Subject: Re: [PATCH] net: Fix suspicious RCU usage in
 bpf_sk_reuseport_detach()

On Tue, Aug 16, 2022 at 04:44:35PM -0700, Jakub Kicinski wrote:
> On Tue, 16 Aug 2022 22:16:46 +0100 David Howells wrote:
> > So either __rcu_dereference_sk_user_data_with_flags_check() has to be a macro,
> > or we need to go with something like the first version of my patch where I
> > don't pass the condition through.  Do you have a preference?
> 
> I like your version because it documents what the lock protecting this
> field is.
> 
> In fact should we also add && sock_owned_by_user(). Martin, WDYT? Would
> that work for reuseport? Jakub S is fixing l2tp to hold the socket lock
> while setting this field, yet most places take the callback lock...
It needs to take a closer look at where the lock_sock() has already
been acquired and also need to consider the lock ordering with reuseport_lock.
It probably should work but may need a separate patch to discuss those
considerations ?

> 
> One the naming - maybe just drop the _with_flags() ? There's no version
> of locked helper which does not take the flags. And not underscores?
I am also good with a shorter name.

Could a comment be added to bpf_sk_reuseport_detach() mentioning
sk_user_data access is protected by the sk_callback_lock alone (or the lock
sock in the future) while reusing __locked_read_sk_user_data() with
a rcu_dereference().  It will be easier to understand if there is
actually any rcu reader in the future code reading.