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

[Waiman Long <longman@redhat.com>]

On 4/1/26 8:54 AM, Frederic Weisbecker wrote:
> 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 <longman@redhat.com>
> Oh I see you're handling a few concerns here. But it's too late, the previous
> patch broke bisection.
Good point. So I have to either reverse the patch order, or just change 
HK_TYPE_KTHREAD to HK_TYPE_DOMAIN & drop the first one.
>
>> ---
>>   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.
Right.
>>   #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.

If ipvs_proc_est_cpumask_set() has been called, the real affinity should 
be the intersection of the given cpumask and HK_TYPE_DOMAIN cpumask. Is 
that what you are referring to?

Cheers,
Longman

>
>>   
>>   	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
>>