linux-kernel - Re: [PATCH] cpuset: prevent freeing unallocated cpumask in hotplug handling

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Message-ID: <927f1afc-4fd4-4d42-948b-5da355443a4a@redhat.com>
Date: Tue, 2 Sep 2025 14:21:25 -0400
From: Waiman Long <llong@...hat.com>
To: Ashay Jaiswal <quic_ashayj@...cinc.com>
Cc: cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
 stable@...r.kernel.org, Michal Koutný <mkoutny@...e.com>,
 Tejun Heo <tj@...nel.org>, "Peter Zijlstra (Intel)" <peterz@...radead.org>,
 Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH] cpuset: prevent freeing unallocated cpumask in hotplug
 handling

On 9/2/25 1:14 PM, Waiman Long wrote:
>
> On 9/2/25 12:26 AM, Ashay Jaiswal wrote:
>> In cpuset hotplug handling, temporary cpumasks are allocated only when
>> running under cgroup v2. The current code unconditionally frees these
>> masks, which can lead to a crash on cgroup v1 case.
>>
>> Free the temporary cpumasks only when they were actually allocated.
>>
>> Fixes: 4b842da276a8 ("cpuset: Make CPU hotplug work with partition")
>> Cc: stable@...r.kernel.org
>> Signed-off-by: Ashay Jaiswal <quic_ashayj@...cinc.com>
>> ---
>>   kernel/cgroup/cpuset.c | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>> index 
>> a78ccd11ce9b43c2e8b0e2c454a8ee845ebdc808..a4f908024f3c0a22628a32f8a5b0ae96c7dccbb9 
>> 100644
>> --- a/kernel/cgroup/cpuset.c
>> +++ b/kernel/cgroup/cpuset.c
>> @@ -4019,7 +4019,8 @@ static void cpuset_handle_hotplug(void)
>>       if (force_sd_rebuild)
>>           rebuild_sched_domains_cpuslocked();
>>   -    free_tmpmasks(ptmp);
>> +    if (on_dfl && ptmp)
>> +        free_tmpmasks(ptmp);
>>   }
>>     void cpuset_update_active_cpus(void)
> The patch that introduces the bug is actually commit 5806b3d05165 
> ("cpuset: decouple tmpmasks and cpumasks freeing in cgroup") which 
> removes the NULL check. The on_dfl check is not necessary and I would 
> suggest adding the NULL check in free_tmpmasks().

As this email was bounced back from your email account because it is 
full, I decide to send out another patch on your behalf. Note that this 
affects only the linux-next tree as the commit to be fixed isn't merged 
into the mainline yet. There is no need for stable branch backport.

Cheers,
Longman