Re: [PATCH v2] powernv/elog: Fix the race while processing OPAL error log event.

[Vasant Hegde <hegdevasant@linux.vnet.ibm.com>]

On 10/6/20 5:55 AM, Oliver O'Halloran wrote:
> On Mon, Oct 5, 2020 at 3:12 PM Mahesh Salgaonkar <mahesh@linux.ibm.com> wrote:
>>
>> Every error log reported by OPAL is exported to userspace through a sysfs
>> interface and notified using kobject_uevent(). The userspace daemon
>> (opal_errd) then reads the error log and acknowledges it error log is saved
>> safely to disk. Once acknowledged the kernel removes the respective sysfs
>> file entry causing respective resources getting released including kobject.
>>
>> However there are chances where user daemon may already be scanning elog
>> entries while new sysfs elog entry is being created by kernel. User daemon
>> may read this new entry and ack it even before kernel can notify userspace
>> about it through kobject_uevent() call. If that happens then we have a
>> potential race between elog_ack_store->kobject_put() and kobject_uevent
>> which can lead to use-after-free issue of a kernfs object resulting into a
>> kernel crash. This patch fixes this race by protecting a sysfs file
>> creation/notification by holding an additional reference count on kobject
>> until we safely send kobject_uevent().
>>
>> Reported-by: Oliver O'Halloran <oohall@gmail.com>
>> Signed-off-by: Mahesh Salgaonkar <mahesh@linux.ibm.com>
>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
>> ---
>> Change in v2:
>> - Instead of mutex and use extra reference count on kobject to avoid the
>>    race.
>> ---
>>   arch/powerpc/platforms/powernv/opal-elog.c |   15 +++++++++++++++
>>   1 file changed, 15 insertions(+)
>>
>> diff --git a/arch/powerpc/platforms/powernv/opal-elog.c b/arch/powerpc/platforms/powernv/opal-elog.c
>> index 62ef7ad995da..230f102e87c0 100644
>> --- a/arch/powerpc/platforms/powernv/opal-elog.c
>> +++ b/arch/powerpc/platforms/powernv/opal-elog.c
>> @@ -222,13 +222,28 @@ static struct elog_obj *create_elog_obj(uint64_t id, size_t size, uint64_t type)
>>                  return NULL;
>>          }
>>
>> +       /*
>> +        * As soon as sysfs file for this elog is created/activated there is
>> +        * chance opal_errd daemon might read and acknowledge this elog before
>> +        * kobject_uevent() is called. If that happens then we have a potential
>> +        * race between elog_ack_store->kobject_put() and kobject_uevent which
>> +        * leads to use-after-free issue of a kernfs object resulting into
>> +        * kernel crash. To avoid this race take an additional reference count
>> +        * on kobject until we safely send kobject_uevent().
>> +        */
>> +
>> +       kobject_get(&elog->kobj);  /* extra reference count */
>>          rc = sysfs_create_bin_file(&elog->kobj, &elog->raw_attr);
>>          if (rc) {
>> +               kobject_put(&elog->kobj);
>> +               /* Drop the extra reference count  */
>>                  kobject_put(&elog->kobj);
>>                  return NULL;
>>          }
>>
>>          kobject_uevent(&elog->kobj, KOBJ_ADD);
>> +       /* Drop the extra reference count  */
>> +       kobject_put(&elog->kobj);
> 
> Makes sense,
> 
> Reviewed-by: Oliver O'Halloran <oohall@gmail.com>

Reviewed-by: Vasant Hegde <hegdevasant@linux.vnet.ibm.com>

> 
>>
>>          return elog;
> 
> Does the returned value actually get used anywhere? We'd have a
> similar use-after-free problem if it does. This should probably return
> an error code rather than the object itself.

No one is using it today. May be we should just make it void function.

-Vasant