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

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Message-ID: <wndxi6qoq6bq6lsovlpfutgx7jfummvipx7hhuu4khrdm35ls2@65nymtjs2q7w>
Date: Tue, 2 Sep 2025 11:51:52 +0200
From: Michal Koutný <mkoutny@...e.com>
To: Ashay Jaiswal <quic_ashayj@...cinc.com>
Cc: Waiman Long <longman@...hat.com>, Tejun Heo <tj@...nel.org>, 
	Johannes Weiner <hannes@...xchg.org>, "Peter Zijlstra (Intel)" <peterz@...radead.org>, 
	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 Tue, Sep 02, 2025 at 09:56:17AM +0530, Ashay Jaiswal <quic_ashayj@...cinc.com> 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);
>  }

Can you do 
	if (ptmp)
		free_tmpmasks(ptmp);

so that v2 check in concentrated in one place only?

Thanks,
Michal

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