[PATCH v4 1/1] x86: cpuinfo: Ensure inputs to cpumask_next are valid

[Yury Norov]

On Thu, Nov 03, 2022 at 03:25:04PM +0100, Andrew Jones wrote:
> The valid cpumask range is [0, nr_cpu_ids) and cpumask_next()
> currently calls find_next_bit() with its input CPU ID number plus one
> for the bit number, giving cpumask_next() the range [-1, nr_cpu_ids - 1).
> seq_read_iter() and cpuinfo's start and next seq operations implement a
> pattern like
> 
>   n = cpumask_next(n - 1, mask);
>   show(n);
>   while (1) {
>       ++n;
>       n = cpumask_next(n - 1, mask);
>       if (n >= nr_cpu_ids)
>           break;
>       show(n);
>   }
> 
> which will eventually result in cpumask_next() being called with
> nr_cpu_ids - 1. A kernel compiled with commit 78e5a3399421 ("cpumask:
> fix checking valid cpu range"), but not its revert, commit
> 80493877d7d0 ("Revert "cpumask: fix checking valid cpu range"."),
> will generate a warning when DEBUG_PER_CPU_MAPS is enabled each time
> /proc/cpuinfo is read. Future-proof cpuinfo by checking its input to
> cpumask_next() is valid.
> 
> Signed-off-by: Andrew Jones <ajones at ventanamicro.com>
> Cc: Yury Norov <yury.norov at gmail.com>

Reviewed-by: Yury Norov <yury.norov at gmail.com>

> ---
>  arch/x86/kernel/cpu/proc.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/x86/kernel/cpu/proc.c b/arch/x86/kernel/cpu/proc.c
> index 099b6f0d96bd..de3f93ac6e49 100644
> --- a/arch/x86/kernel/cpu/proc.c
> +++ b/arch/x86/kernel/cpu/proc.c
> @@ -153,6 +153,9 @@ static int show_cpuinfo(struct seq_file *m, void *v)
>  
>  static void *c_start(struct seq_file *m, loff_t *pos)
>  {
> +	if (*pos == nr_cpu_ids)
> +		return NULL;
> +
>  	*pos = cpumask_next(*pos - 1, cpu_online_mask);
>  	if ((*pos) < nr_cpu_ids)
>  		return &cpu_data(*pos);
> -- 
> 2.37.3