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

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Message-ID: <ef0e91b7-be44-4d5d-a556-240709c80fcb@redhat.com>
Date: Tue, 2 Sep 2025 13:06:04 -0400
From: Waiman Long <llong@...hat.com>
To: Ashay Jaiswal <quic_ashayj@...cinc.com>, Tejun Heo <tj@...nel.org>,
 Johannes Weiner <hannes@...xchg.org>, Michal Koutný
 <mkoutny@...e.com>, "Peter Zijlstra (Intel)" <peterz@...radead.org>
Cc: cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
 stable@...r.kernel.org
Subject: Re: [PATCH] cpuset: prevent freeing unallocated cpumask in hotplug
 handling

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().

Cheers,
Longman