Subject: Re: [PATCH-next v2 2/2] ipvs: Guard access of HK_TYPE_KTHREAD cpumask with RCU

From: Frederic Weisbecker <[email-masked]>
Date: Wed, 1 Apr 2026 14:54:20 +0200

Le Tue, Mar 31, 2026 at 12:50:15PM -0400, Waiman Long a écrit :
> The ip_vs_ctl.c file and the associated ip_vs.h file are the only places
> in the kernel where HK_TYPE_KTHREAD cpumask is being retrieved and used.
> Now that HK_TYPE_KTHREAD/HK_TYPE_DOMAIN cpumask can be changed at run
> time. We need to use RCU to guard access to this cpumask to avoid a
> potential UAF problem as the returned cpumask may be freed before it
> is being used.
> 
> We can replace HK_TYPE_KTHREAD by HK_TYPE_DOMAIN as they are aliases
> of each other, but keeping the HK_TYPE_KTHREAD name can highlight the
> fact that it is the kthread initiated by ipvs that is being controlled.
> 
> Signed-off-by: Waiman Long <[email-masked]>

Oh I see you're handling a few concerns here. But it's too late, the previous
patch broke bisection.

> ---
>  include/net/ip_vs.h            | 20 ++++++++++++++++----
>  net/netfilter/ipvs/ip_vs_ctl.c | 13 ++++++++-----
>  2 files changed, 24 insertions(+), 9 deletions(-)
> 
> diff --git a/include/net/ip_vs.h b/include/net/ip_vs.h
> index 72d325c81313..7bda92fd3fe6 100644
> --- a/include/net/ip_vs.h
> +++ b/include/net/ip_vs.h
> @@ -1411,7 +1411,7 @@ static inline int sysctl_run_estimation(struct netns_ipvs *ipvs)
>  	return ipvs->sysctl_run_estimation;
>  }
>  
> -static inline const struct cpumask *sysctl_est_cpulist(struct netns_ipvs *ipvs)
> +static inline const struct cpumask *__sysctl_est_cpulist(struct netns_ipvs *ipvs)
>  {
>  	if (ipvs->est_cpulist_valid)
>  		return ipvs->sysctl_est_cpulist;
> @@ -1529,7 +1529,7 @@ static inline int sysctl_run_estimation(struct netns_ipvs *ipvs)
>  	return 1;
>  }
>  
> -static inline const struct cpumask *sysctl_est_cpulist(struct netns_ipvs *ipvs)
> +static inline const struct cpumask *__sysctl_est_cpulist(struct netns_ipvs *ipvs)
>  {
>  	return housekeeping_cpumask(HK_TYPE_KTHREAD);
>  }
> @@ -1564,6 +1564,18 @@ static inline int sysctl_svc_lfactor(struct netns_ipvs *ipvs)
>  	return READ_ONCE(ipvs->sysctl_svc_lfactor);
>  }
>  
> +static inline bool sysctl_est_cpulist_empty(struct netns_ipvs *ipvs)
> +{
> +	guard(rcu)();
> +	return cpumask_empty(__sysctl_est_cpulist(ipvs));
> +}
> +
> +static inline unsigned int sysctl_est_cpulist_weight(struct netns_ipvs *ipvs)
> +{
> +	guard(rcu)();
> +	return cpumask_weight(__sysctl_est_cpulist(ipvs));
> +}
> +
>  /* IPVS core functions
>   * (from ip_vs_core.c)
>   */
> @@ -1895,7 +1907,7 @@ static inline void ip_vs_est_stopped_recalc(struct netns_ipvs *ipvs)
>  	/* Stop tasks while cpulist is empty or if disabled with flag */
>  	ipvs->est_stopped = !sysctl_run_estimation(ipvs) ||
>  			    (ipvs->est_cpulist_valid &&
> -			     cpumask_empty(sysctl_est_cpulist(ipvs)));
> +			     sysctl_est_cpulist_empty(ipvs));

It's not needed, if ipvs->est_cpulist_valid, sysctl_est_cpulist() doesn't
refer to housekeeping.

>  #endif
>  }
>  
> @@ -1911,7 +1923,7 @@ static inline bool ip_vs_est_stopped(struct netns_ipvs *ipvs)
>  static inline int ip_vs_est_max_threads(struct netns_ipvs *ipvs)
>  {
>  	unsigned int limit = IPVS_EST_CPU_KTHREADS *
> -			     cpumask_weight(sysctl_est_cpulist(ipvs));
> +			     sysctl_est_cpulist_weight(ipvs);

That probably works for callers ip_vs_start_estimator().

But this is not handling the core issue that related kthreads should be updated,
as is done in ipvs_proc_est_cpumask_set(), when HK_TYPE_DOMAIN mask changes.

>  
>  	return max(1U, limit);
>  }
> diff --git a/net/netfilter/ipvs/ip_vs_ctl.c b/net/netfilter/ipvs/ip_vs_ctl.c
> index 032425025d88..e253a1ceef48 100644
> --- a/net/netfilter/ipvs/ip_vs_ctl.c
> +++ b/net/netfilter/ipvs/ip_vs_ctl.c
> @@ -2338,11 +2338,14 @@ static int ipvs_proc_est_cpumask_get(const struct ctl_table *table,
>  
>  	mutex_lock(&ipvs->est_mutex);
>  
> -	if (ipvs->est_cpulist_valid)
> -		mask = *valp;
> -	else
> -		mask = (struct cpumask *)housekeeping_cpumask(HK_TYPE_KTHREAD);
> -	ret = scnprintf(buffer, size, "%*pbl\n", cpumask_pr_args(mask));
> +	/* HK_TYPE_KTHREAD cpumask needs RCU protection */
> +	scoped_guard(rcu) {
> +		if (ipvs->est_cpulist_valid)
> +			mask = *valp;
> +		else
> +			mask = (struct cpumask *)housekeeping_cpumask(HK_TYPE_KTHREAD);
> +		ret = scnprintf(buffer, size, "%*pbl\n", cpumask_pr_args(mask));
> +	}

And that works.

Thanks.

>  
>  	mutex_unlock(&ipvs->est_mutex);
>  
> -- 
> 2.53.0
> 

-- 
Frederic Weisbecker
SUSE Labs