[PATCH 3/3] Use get_online_cpus to avoid races involving for_each_online_cpu

[PATCH 3/3] Use get_online_cpus to avoid races involving for_each_online_cpu

[Silas Boyd-Wickizer]

via_cputemp_init in drivers/hwmon/via-cputemp.c loops with
for_each_online_cpu, adding platform_devices, then calls
register_hotcpu_notifier.  If a CPU is offlined between the loop and
register_hotcpu_notifier, then later onlined, via_cputemp_device_add
will attempt to platform devices with the same ID.

This fix surrounds for_each_online_cpu and register_hotcpu_notifier
with get_online_cpus+put_online_cpus.

Build tested.

Signed-off-by: Silas Boyd-Wickizer <sbw@mit.edu>
---
 drivers/hwmon/via-cputemp.c |    3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/hwmon/via-cputemp.c b/drivers/hwmon/via-cputemp.c
index 8689664..9ad07c3 100644
--- a/drivers/hwmon/via-cputemp.c
+++ b/drivers/hwmon/via-cputemp.c
@@ -328,6 +328,7 @@ static int __init via_cputemp_init(void)
 	if (err)
 		goto exit;
 
+	get_online_cpus();
 	for_each_online_cpu(i) {
 		struct cpuinfo_x86 *c = &cpu_data(i);
 
@@ -347,12 +348,14 @@ static int __init via_cputemp_init(void)
 
 #ifndef CONFIG_HOTPLUG_CPU
 	if (list_empty(&pdev_list)) {
+		put_online_cpus();
 		err = -ENODEV;
 		goto exit_driver_unreg;
 	}
 #endif
 
 	register_hotcpu_notifier(&via_cputemp_cpu_notifier);
+	put_online_cpus();
 	return 0;
 
 #ifndef CONFIG_HOTPLUG_CPU
-- 
1.7.10.4

Re: [PATCH 3/3] Use get_online_cpus to avoid races involving for_each_online_cpu

[Harald Welte]

Hi Silas,

thanks a lot for the finding and addressing the issue.

On Thu, Aug 02, 2012 at 05:07:08PM -0700, Silas Boyd-Wickizer wrote:
> Signed-off-by: Silas Boyd-Wickizer <sbw@mit.edu>

Acked-by: Harald Welte <laforge@gnumonks.org>

-- 
- Harald Welte <laforge@gnumonks.org>           http://laforge.gnumonks.org/
============================================================================
"Privacy in residential applications is a desirable marketing option."
                                                  (ETSI EN 300 175-7 Ch. A6)

Re: [PATCH 3/3] Use get_online_cpus to avoid races involving for_each_online_cpu

[Jean Delvare]

Hi Silas,

On Thu, 2 Aug 2012 17:07:08 -0700, Silas Boyd-Wickizer wrote:
> via_cputemp_init in drivers/hwmon/via-cputemp.c loops with
> for_each_online_cpu, adding platform_devices, then calls
> register_hotcpu_notifier.  If a CPU is offlined between the loop and
> register_hotcpu_notifier, then later onlined, via_cputemp_device_add
> will attempt to platform devices with the same ID.

Missing word in this last sentence.

> 
> This fix surrounds for_each_online_cpu and register_hotcpu_notifier
> with get_online_cpus+put_online_cpus.
> 
> Build tested.

Thanks for reporting and for the fix. Two questions:

What about via_cputemp_exit()? While less obvious, I suspect it is racy
too. The notifier is unregistered first. If a CPU gets offline before
the devices are removed, we will have a device pointing to a
non-existent CPU for a short time. I think we should play it safe and
use get/put_online_cpus() there too, as I seem to understand it
guarantees CPUs can't go offline when we wouldn't handle that event
properly. Alternatively, unregistering the platform driver first might
close the race.

Secondly, drivers/hwmon/coretemp.c is very similar to via-cputemp.c, so
I think it needs the exact same fix(es).

> 
> Signed-off-by: Silas Boyd-Wickizer <sbw@mit.edu>
> ---
>  drivers/hwmon/via-cputemp.c |    3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/hwmon/via-cputemp.c b/drivers/hwmon/via-cputemp.c
> index 8689664..9ad07c3 100644
> --- a/drivers/hwmon/via-cputemp.c
> +++ b/drivers/hwmon/via-cputemp.c
> @@ -328,6 +328,7 @@ static int __init via_cputemp_init(void)
>  	if (err)
>  		goto exit;
>  
> +	get_online_cpus();
>  	for_each_online_cpu(i) {
>  		struct cpuinfo_x86 *c = &cpu_data(i);
>  
> @@ -347,12 +348,14 @@ static int __init via_cputemp_init(void)
>  
>  #ifndef CONFIG_HOTPLUG_CPU
>  	if (list_empty(&pdev_list)) {
> +		put_online_cpus();
>  		err = -ENODEV;
>  		goto exit_driver_unreg;
>  	}
>  #endif
>  
>  	register_hotcpu_notifier(&via_cputemp_cpu_notifier);
> +	put_online_cpus();
>  	return 0;
>  
>  #ifndef CONFIG_HOTPLUG_CPU


-- 
Jean Delvare

Re: [PATCH 3/3] Use get_online_cpus to avoid races involving for_each_online_cpu

[Silas Boyd-Wickizer]

On Fri, Aug 03, 2012 at 03:56:40PM +0200, Jean Delvare wrote:
> Hi Silas,
> 
> On Thu, 2 Aug 2012 17:07:08 -0700, Silas Boyd-Wickizer wrote:
> > via_cputemp_init in drivers/hwmon/via-cputemp.c loops with
> > for_each_online_cpu, adding platform_devices, then calls
> > register_hotcpu_notifier.  If a CPU is offlined between the loop and
> > register_hotcpu_notifier, then later onlined, via_cputemp_device_add
> > will attempt to platform devices with the same ID.
> 
> Missing word in this last sentence.

Fixed.

> 
> > 
> > This fix surrounds for_each_online_cpu and register_hotcpu_notifier
> > with get_online_cpus+put_online_cpus.
> > 
> > Build tested.
> 
> Thanks for reporting and for the fix. Two questions:
> 
> What about via_cputemp_exit()? While less obvious, I suspect it is racy
> too. The notifier is unregistered first. If a CPU gets offline before
> the devices are removed, we will have a device pointing to a
> non-existent CPU for a short time. I think we should play it safe and
> use get/put_online_cpus() there too, as I seem to understand it
> guarantees CPUs can't go offline when we wouldn't handle that event
> properly. Alternatively, unregistering the platform driver first might
> close the race.

I added get/put_online_cpus().

> 
> Secondly, drivers/hwmon/coretemp.c is very similar to via-cputemp.c, so
> I think it needs the exact same fix(es).

Indeed -- I'll submit a fix for this.

Silas

> 
> > 
> > Signed-off-by: Silas Boyd-Wickizer <sbw@mit.edu>
> > ---
> >  drivers/hwmon/via-cputemp.c |    3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/drivers/hwmon/via-cputemp.c b/drivers/hwmon/via-cputemp.c
> > index 8689664..9ad07c3 100644
> > --- a/drivers/hwmon/via-cputemp.c
> > +++ b/drivers/hwmon/via-cputemp.c
> > @@ -328,6 +328,7 @@ static int __init via_cputemp_init(void)
> >  	if (err)
> >  		goto exit;
> >  
> > +	get_online_cpus();
> >  	for_each_online_cpu(i) {
> >  		struct cpuinfo_x86 *c = &cpu_data(i);
> >  
> > @@ -347,12 +348,14 @@ static int __init via_cputemp_init(void)
> >  
> >  #ifndef CONFIG_HOTPLUG_CPU
> >  	if (list_empty(&pdev_list)) {
> > +		put_online_cpus();
> >  		err = -ENODEV;
> >  		goto exit_driver_unreg;
> >  	}
> >  #endif
> >  
> >  	register_hotcpu_notifier(&via_cputemp_cpu_notifier);
> > +	put_online_cpus();
> >  	return 0;
> >  
> >  #ifndef CONFIG_HOTPLUG_CPU
> 
> 
> -- 
> Jean Delvare