Subject: Re: [PATCH V2 1/2] tools/perf: Fix out of bound access to affinity "sched_cpus"

From: Arnaldo Carvalho de Melo <[email-masked]>
Date: Tue, 6 Sep 2022 09:27:08 -0300

Em Mon, Sep 05, 2022 at 07:49:28PM +0530, Athira Rajeev escreveu:
> The affinity code in "affinity_set" function access array
> named "sched_cpus". The size for this array is allocated in
> affinity_setup function which is nothing but value from
> get_cpu_set_size. This is used to contain the cpumask value
> for each cpu. While setting bit for each cpu, it calls
> "set_bit" function which access index in sched_cpus array.
> If we provide a command-line option to -C which is more than
> the number of CPU's present in the system, the set_bit could
> access an array member which is out-of the array size. This
> is because currently, there is no boundary check for the CPU.
> This will result in seg fault:

Thanks, tested, reproduced the problem before, and the fix after,
applied.

- Arnaldo

> <<>>
>  ./perf stat -C 12323431 ls
> Perf can support 2048 CPUs. Consider raising MAX_NR_CPUS
> Segmentation fault (core dumped)
> <<>>
> 
> Fix this by adding boundary check for the array.
> 
> After the fix from powerpc system:
> 
> <<>>
> ./perf stat -C 12323431 ls 1>out
> Perf can support 2048 CPUs. Consider raising MAX_NR_CPUS
> 
>  Performance counter stats for 'CPU(s) 12323431':
> 
>    <not supported> msec cpu-clock
>    <not supported>      context-switches
>    <not supported>      cpu-migrations
>    <not supported>      page-faults
>    <not supported>      cycles
>    <not supported>      instructions
>    <not supported>      branches
>    <not supported>      branch-misses
> 
>        0.001192373 seconds time elapsed
> <<>>
> 
> Reported-by: Nageswara Sastry <[email-masked]>
> Tested-by: Nageswara Sastry <[email-masked]>
> Signed-off-by: Athira Rajeev <[email-masked]>
> ---
> Changelog:
>  From v1 -> v2:
>  Addressed review comment from Jiri Olsa by changing condition
>  check to directly use "cpu_set_size * 8" for comparing with the
>  cpu number.
> 
>  tools/perf/util/affinity.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/perf/util/affinity.c b/tools/perf/util/affinity.c
> index 4d216c0dc425..4ee96b3c755b 100644
> --- a/tools/perf/util/affinity.c
> +++ b/tools/perf/util/affinity.c
> @@ -49,8 +49,14 @@ void affinity__set(struct affinity *a, int cpu)
>  {
>  	int cpu_set_size = get_cpu_set_size();
>  
> -	if (cpu == -1)
> +	/*
> +	 * Return:
> +	 * - if cpu is -1
> +	 * - restrict out of bound access to sched_cpus
> +	 */
> +	if (cpu == -1 || ((cpu >= (cpu_set_size * 8))))
>  		return;
> +
>  	a->changed = true;
>  	set_bit(cpu, a->sched_cpus);
>  	/*
> -- 
> 2.35.1

-- 

- Arnaldo