linux-kernel - Re: [PATCH] net: Fix potential RCU dereference issue in tcp_assign_congestion

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Message-ID: <CANn89i+E4RP+gJghHZujmKUJbCgYY_L20ssVmvmRUT4a8FvunQ@mail.gmail.com>
Date: Fri, 20 Sep 2024 16:11:13 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Florian Westphal <fw@...len.de>
Cc: Jiawei Ye <jiawei.ye@...mail.com>, davem@...emloft.net, dsahern@...nel.org, 
	kuba@...nel.org, pabeni@...hat.com, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: Fix potential RCU dereference issue in tcp_assign_congestion_control

On Fri, Sep 20, 2024 at 11:35 AM Florian Westphal <fw@...len.de> wrote:
>
> Jiawei Ye <jiawei.ye@...mail.com> wrote:
> > In the `tcp_assign_congestion_control` function, the `ca->flags` is
> > accessed after the RCU read-side critical section is unlocked. According
> > to RCU usage rules, this is illegal. Reusing this pointer can lead to
> > unpredictable behavior, including accessing memory that has been updated
> > or causing use-after-free issues.
> >
> > This possible bug was identified using a static analysis tool developed
> > by myself, specifically designed to detect RCU-related issues.
> >
> > To resolve this issue, the `rcu_read_unlock` call has been moved to the
> > end of the function.
> >
> > Signed-off-by: Jiawei Ye <jiawei.ye@...mail.com>
> > ---
> > In another part of the file, `tcp_set_congestion_control` calls
> > `tcp_reinit_congestion_control`, ensuring that the congestion control
> > reinitialization process is protected by RCU. The
> > `tcp_reinit_congestion_control` function contains operations almost
> > identical to those in `tcp_assign_congestion_control`, but the former
> > operates under full RCU protection, whereas the latter is only partially
> > protected. The differing protection strategies between the two may
> > warrant further unification.
> > ---
> >  net/ipv4/tcp_cong.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/net/ipv4/tcp_cong.c b/net/ipv4/tcp_cong.c
> > index 0306d257fa64..356a59d316e3 100644
> > --- a/net/ipv4/tcp_cong.c
> > +++ b/net/ipv4/tcp_cong.c
> > @@ -223,13 +223,13 @@ void tcp_assign_congestion_control(struct sock *sk)
> >       if (unlikely(!bpf_try_module_get(ca, ca->owner)))
> >               ca = &tcp_reno;
>
> After this, ca either has module refcount incremented, so it can't
> go away anymore, or reno fallback was enabled (always bultin).
>
> >       icsk->icsk_ca_ops = ca;
> > -     rcu_read_unlock();
>
> Therefore its ok to rcu unlock here.

I agree, there is no bug here.

Jiawei Ye, I guess your static analysis tool is not ready yet.