linux-kernel - Re: Missing a write memory barrier in tls

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Message-ID: <ZU4Ecx2qbdqGfRVw@dragonet>
Date:   Fri, 10 Nov 2023 19:22:43 +0900
From:   "Dae R. Jeong" <threeearcat@...il.com>
To:     Sabrina Dubroca <sd@...asysnail.net>
Cc:     Jakub Kicinski <kuba@...nel.org>, borisp@...dia.com,
        john.fastabend@...il.com, davem@...emloft.net, edumazet@...gle.com,
        pabeni@...hat.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, ywchoi@...ys.kaist.ac.kr
Subject: Re: Missing a write memory barrier in tls_init()

On Wed, Nov 08, 2023 at 10:07:58AM +0100, Sabrina Dubroca wrote:
> 2023-11-07, 18:53:24 -0800, Jakub Kicinski wrote:
> > On Tue, 7 Nov 2023 23:45:46 +0100 Sabrina Dubroca wrote:
> > > Wouldn't it be enough to just move the rcu_assign_pointer after ctx is
> > > fully initialized, ie just before update_sk_prot? also clearer wrt
> > > RCU.
> > 
> > I'm not sure, IIUC rcu_assign_pointer() is equivalent to
> > WRITE_ONCE() on any sane architecture, it depends on address
> > dependencies to provide ordering.
> 
> Not what the doc says:
> 
>     /**
>      * rcu_assign_pointer() - assign to RCU-protected pointer
>      [...]
>      * Inserts memory barriers on architectures that require them
>      * (which is most of them), and also prevents the compiler from
>      * reordering the code that initializes the structure after the pointer
>      * assignment.
>      [...]
>      */
> 
> And it uses smp_store_release (unless writing NULL).
> 

I think Sabrina is right. We can rely on the release semantic implied
in rcu_assign_pointer(). Simply moving rcu_assign_pointer() to the end
of tls_ctx_create() should prevent a scenario what I thought (ie.,
store-store reordering between ctx->sk_proto and sk->sk_prot).

diff --git a/net/tls/tls_main.c b/net/tls/tls_main.c
index 1c2c6800949d..d20b823c68d4 100644
--- a/net/tls/tls_main.c
+++ b/net/tls/tls_main.c
@@ -816,9 +816,9 @@ struct tls_context *tls_ctx_create(struct sock *sk)
                return NULL;
 
        mutex_init(&ctx->tx_lock);
-       rcu_assign_pointer(icsk->icsk_ulp_data, ctx);
        ctx->sk_proto = READ_ONCE(sk->sk_prot);
        ctx->sk = sk;
+       rcu_assign_pointer(icsk->icsk_ulp_data, ctx);
        return ctx;
 }

But what I also wonder is that, do we need to ensure that
ctx->{tx,rx}_conf is visible before updating sk->sk_prot? If so, as
Sabrina suggested, we may want to move rcu_assign_pointer() right
before update_sk_prot().


Best regards,
Dae R. Jeong