Re: [PATCH] net: Fix potential RCU dereference issue in tcp_assign_congestion_control

[Florian Westphal]

Jiawei Ye <jiawei.ye@xxxxxxxxxxx> 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@xxxxxxxxxxx>
> ---
> 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.