Re: [PATCH 1/2] cpumask: Introduce possible_cpu_safe()

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Apr 04, 2019 at 01:02:19PM +0300, Dan Carpenter wrote:
> There have been two cases recently where we pass user a controlled "cpu"
> to possible_cpus().  That's not allowed.  If it's invalid, it will
> trigger a WARN_ONCE() and an out of bounds read which could result in an
> Oops.

> +/**
> + * cpumask_test_cpu_safe - test for a cpu in a cpumask
> + * @cpu: cpu number
> + * @cpumask: the cpumask pointer
> + *
> + * Returns 1 if @cpu is valid and set in @cpumask, else returns 0
> + */
> +static inline int cpumask_test_cpu_safe(int cpu, const struct cpumask *cpumask)
> +{
> +	if ((unsigned int)cpu >= nr_cpu_ids)
> +		return 0;
> +	cpu = array_index_nospec(cpu, NR_CPUS);

That should be:

	cpu = array_index_nospec(cpu, nr_cpu_ids);

NR_CPUS might still be out-of-bounds for dynamically allocated cpumasks.

> +	return test_bit(cpu, cpumask_bits(cpumask));
> +}

That said; I don't particularly like this interface not its naming, how
about something like:

static inline unsigned int cpumask_validate_cpu(unsigned int cpu)
{
	if (cpu >= nr_cpumask_bits)
		return nr_cpumask_bits;
	return array_index_nospec(cpu, nr_cpumask_bits);
}

Which you can then use like:

	cpu = cpumask_validate_cpu(user_cpu);
	if (cpu >= nr_cpu_ids)
		return -ENORANGE;

	/* @cpu is valid, do what needs doing */