Re: [PATCH v5 2/5] arm64: amu: Rule out potential use after free

[Sudeep Holla]

On Wed, Apr 17, 2024 at 10:38:45AM +0100, Beata Michalska wrote:
> For the time being, the amu_fie_cpus cpumask is being exclusively used
> by the AMU-related internals of FIE support and is guaranteed to be
> valid on every access currently made. Still the mask is not being
> invalidated on one of the error handling code paths, which leaves
> a soft spot with potential risk of uaf for CPUMASK_OFFSTACK cases.
> To make things sound, set the cpumaks pointer explicitly to NULL upon
> failing to register the cpufreq notifier.
> Note that, due to the quirks of CPUMASK_OFFSTACK, this change needs to
> be wrapped with grim ifdefing (it would be better served by
> incorporating this into free_cpumask_var ...)
>

Yes it doesn't look neat.

> Signed-off-by: Beata Michalska <beata.michalska@xxxxxxx>
> ---
>  arch/arm64/kernel/topology.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
> index 1a2c72f3e7f8..3c814a278534 100644
> --- a/arch/arm64/kernel/topology.c
> +++ b/arch/arm64/kernel/topology.c
> @@ -244,8 +244,12 @@ static int __init init_amu_fie(void)
>
>  	ret = cpufreq_register_notifier(&init_amu_fie_notifier,
>  					CPUFREQ_POLICY_NOTIFIER);
> -	if (ret)
> +	if (ret) {
>  		free_cpumask_var(amu_fie_cpus);
> +#ifdef CONFIG_CPUMASK_OFFSTACK
> +		amu_fie_cpus = NULL;
> +#endif
> +	}

Instead of this #ifdeffery, I was wondering if we can actually do the
allocation in init_amu_fie_callback() the first time it gets called
checking if amu_fie_cpus is NULL. init_amu_fie_callback() must get called
only if the cpufreq_register_notifier() succeeds right ?

Also I don't see anyone calling amu_fie_setup(), so where do you think
the possible use after free could occur for amu_fie_cpus. Just thinking
out loud to check if I missed anything.

--
Regards,
Sudeep