linux-kernel - Re: [PATCH] powercap: dtpm_cpu: Fix NULL pointer dereference race during CPU offlining

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Message-ID: <CAJZ5v0jXPten8QwruK_Dqzvc0izLCfecMPBWMMoP8cnG4mDY0Q@mail.gmail.com>
Date: Fri, 27 Jun 2025 22:06:37 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Sivan Zohar-Kotzer <sivany32@...il.com>
Cc: Daniel Lezcano <daniel.lezcano@...nel.org>, "Rafael J . Wysocki" <rafael@...nel.org>, elazarl@...il.com, 
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powercap: dtpm_cpu: Fix NULL pointer dereference race
 during CPU offlining

On Thu, Jun 19, 2025 at 1:16 AM Sivan Zohar-Kotzer <sivany32@...il.com> wrote:
>
> The get_pd_power_uw() function contains a race condition during CPU
> offlining:
>
> * DTPM power calculations are triggered (e.g., via sysfs reads) while CPU is online
> * The CPU goes offline during the calculation, before em_cpu_get() is called
> * em_cpu_get() now returns NULL since the energy model was unregistered

But energy models for CPUs are never unregistered.

> * em_span_cpus() dereferences the NULL pointer, causing a crash
>
> Commit eb82bace8931 introduced the call to em_span_cpus(pd) without
> checking if pd is NULL.
>
> Add a NULL check after em_cpu_get() and return 0 power if no energy model
> is available, matching the existing fallback behavior.
>
> Fixes: eb82bace8931 ("powercap/drivers/dtpm: Scale the power with the load")
> Signed-off-by: Sivan Zohar-Kotzer <sivany32@...il.com>
> ---
>  drivers/powercap/dtpm_cpu.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/drivers/powercap/dtpm_cpu.c b/drivers/powercap/dtpm_cpu.c
> index 6b6f51b21550..80d93ab4dc54 100644
> --- a/drivers/powercap/dtpm_cpu.c
> +++ b/drivers/powercap/dtpm_cpu.c
> @@ -97,6 +97,11 @@ static u64 get_pd_power_uw(struct dtpm *dtpm)
>
>         pd = em_cpu_get(dtpm_cpu->cpu);
>
> +       if (!pd) {
> +               pr_warn("DTPM: No energy model available for CPU%d\n", dtpm_cpu->cpu);
> +               return 0;
> +       }
> +
>         pd_mask = em_span_cpus(pd);
>
>         freq = cpufreq_quick_get(dtpm_cpu->cpu);
> @@ -207,6 +212,7 @@ static int __dtpm_cpu_setup(int cpu, struct dtpm *parent)
>         pd = em_cpu_get(cpu);
>         if (!pd || em_is_artificial(pd)) {
>                 ret = -EINVAL;
> +
>                 goto release_policy;
>         }
>
> --