linux-kernel - Re: [PATCH v3] smp: Fix a potential usage of stale nr

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Date:   Mon, 27 Jul 2020 23:34:44 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Muchun Song <songmuchun@...edance.com>, peterz@...radead.org,
        bigeasy@...utronix.de, namit@...are.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] smp: Fix a potential usage of stale nr_cpus

* Thomas Gleixner <tglx@...utronix.de> wrote:

> Ingo Molnar <mingo@...nel.org> writes:
> >> -	get_option(&str, &nr_cpus);
> >> +	if (get_option(&str, &nr_cpus) != 1)
> >> +		return -EINVAL;
> >> +
> >>  	if (nr_cpus > 0 && nr_cpus < nr_cpu_ids)
> >>  		nr_cpu_ids = nr_cpus;
> >> +	else
> >> +		return -EINVAL;
> >
> > Exactly what does 'not valid' mean, and why doesn't get_option() 
> > return -EINVAL in that case?
> 
> What's unclear about invalid? If you specify nr_cpus=-1 or
> nr_cpus=2000000 the its obviously invalid.

So this was the old (buggy) code:

>  {
>       int nr_cpus;
>
>       get_option(&str, &nr_cpus);
>       if (nr_cpus > 0 && nr_cpus < nr_cpu_ids)
>               nr_cpu_ids = nr_cpus;

And this was the explanation given in the changelog:

>> When the cmdline of "nr_cpus" is not valid, the @nr_cpu_ids is 
>> assigned a stale value. The nr_cpus is only valid when get_option() 
>> return 1. So check the return value to prevent this.

The answer to my question is that the bug is that the return value of 
get_option() wasn't checked properly, and if get_option() returns an 
error then the nr_cpus local variable is not set - but we used it in 
the old code, which can result in essentially a random value for 
nr_cpu_ids.

> How should get_option() know that this is invalid? get_option() is a 
> number parser and does not know about any restrictions on the parsed 
> value obviously.

But that's apparently not the bug here, 'invalid' here was meant as 
per the parser's syntax. If nr_cpus is out of range (like the 2000000 
example you gave), then nr_cpu_ids might not be set at all, and 
remains at the 0 initialized value. Which isn't good but not 'stale' 
either.

This is why I was puzzled where a 'stale' value might come from, at 
first sight I was assuming that some large value was written, like 
your 200000 example. The "stale value" happens if it's invalid syntax 
and get_option() returns an error, in which case 'nr_cpus' remains 
uninitialized.

And this is the explanation I didn't find at first reading, and which 
explanation future changelogs should perhaps include.

The new code does this:

        int nr_cpus;

        if (get_option(&str, &nr_cpus) != 1)
                return -EINVAL;

        if (nr_cpus > 0 && nr_cpus < nr_cpu_ids)
                nr_cpu_ids = nr_cpus;
        else
                return -EINVAL;

Which does all the proper error handling and fixes the uninitialized 
'nr_cpus' local variable usage. So I agree with the fix:

Reviewed-by: Ingo Molnar <mingo@...nel.org>

Thanks,

	Ingo