linux-kernel - Re: [PATCH] drivers/base/cpu: Use scnprintf() for avoiding potential buffer overflow

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Date:   Wed, 11 Mar 2020 08:24:51 +0100
From:   Takashi Iwai <tiwai@...e.de>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     "Rafael J . Wysocki" <rafael@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drivers/base/cpu: Use scnprintf() for avoiding potential buffer overflow

On Wed, 11 Mar 2020 08:19:35 +0100,
Greg Kroah-Hartman wrote:
> 
> On Wed, Mar 11, 2020 at 08:12:00AM +0100, Takashi Iwai wrote:
> > Since snprintf() returns the would-be-output size instead of the
> > actual output size, the succeeding calls may go beyond the given
> > buffer limit.  Fix it by replacing with scnprintf().
> > 
> > Signed-off-by: Takashi Iwai <tiwai@...e.de>
> > ---
> >  drivers/base/cpu.c | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
> > index 6265871a4af2..0abcd9d68714 100644
> > --- a/drivers/base/cpu.c
> > +++ b/drivers/base/cpu.c
> > @@ -231,7 +231,7 @@ static struct cpu_attr cpu_attrs[] = {
> >  static ssize_t print_cpus_kernel_max(struct device *dev,
> >  				     struct device_attribute *attr, char *buf)
> >  {
> > -	int n = snprintf(buf, PAGE_SIZE-2, "%d\n", NR_CPUS - 1);
> > +	int n = scnprintf(buf, PAGE_SIZE-2, "%d\n", NR_CPUS - 1);
> 
> This should just be "sprintf()" as we "know" that fitting a single
> number will work.
> 
> >  	return n;
> >  }
> >  static DEVICE_ATTR(kernel_max, 0444, print_cpus_kernel_max, NULL);
> > @@ -258,13 +258,13 @@ static ssize_t print_cpus_offline(struct device *dev,
> >  			buf[n++] = ',';
> >  
> >  		if (nr_cpu_ids == total_cpus-1)
> > -			n += snprintf(&buf[n], len - n, "%u", nr_cpu_ids);
> > +			n += scnprintf(&buf[n], len - n, "%u", nr_cpu_ids);
> >  		else
> > -			n += snprintf(&buf[n], len - n, "%u-%d",
> > +			n += scnprintf(&buf[n], len - n, "%u-%d",
> >  						      nr_cpu_ids, total_cpus-1);
> >  	}
> >  
> > -	n += snprintf(&buf[n], len - n, "\n");
> > +	n += scnprintf(&buf[n], len - n, "\n");
> 
> this part looks sane, can you respin this?

OK, will do it.

Thanks for a quick review!


Takashi