Re: [PATCH] block: make sure last_lookup set as NULL after part deleted

[Hou Tao <houtao1@huawei.com>]

Hi,

On 2020/1/2 9:23, Ming Lei wrote:
> On Tue, Dec 31, 2019 at 10:55:47PM +0800, Hou Tao wrote:
>> Hi,
>>
>> On 2019/12/31 19:09, Yufen Yu wrote:
>>> When delete partition executes concurrently with IOs issue,
>>> it may cause use-after-free on part in disk_map_sector_rcu()
>>> as following:
>> snip
>>
>>>
>>> diff --git a/block/genhd.c b/block/genhd.c
>>> index ff6268970ddc..39fa8999905f 100644
>>> --- a/block/genhd.c
>>> +++ b/block/genhd.c
>>> @@ -293,7 +293,23 @@ struct hd_struct *disk_map_sector_rcu(struct gendisk *disk, sector_t sector)
>>>  		part = rcu_dereference(ptbl->part[i]);
>>>  
>>>  		if (part && sector_in_part(part, sector)) {
>> snip
>>
>>>  			rcu_assign_pointer(ptbl->last_lookup, part);
>>> +			part = rcu_dereference(ptbl->part[i]);
>>> +			if (part == NULL) {
>>> +				rcu_assign_pointer(ptbl->last_lookup, NULL);
>>> +				break;
>>> +			}
>>>  			return part;
>>>  		}
>>>  	}
>>
>> Not ensure whether the re-read can handle the following case or not:
>>
We have written a similar test case for the following case and found out that
process C still may got the freed hd_struct pointer from process A. So
the re-read will not resolve the problem.

>> process A                                 process B                          process C
>>
>> disk_map_sector_rcu():                    delete_partition():               disk_map_sector_rcu():
>>
>> rcu_read_lock
>>
>>   // need to iterate partition table
>>   part[i] != NULL   (1)                   part[i] = NULL (2)
>>                                           smp_mb()
>>                                           last_lookup = NULL (3)
>>                                           call_rcu()  (4)
>>     last_lookup = part[i] (5)
>>
>>
>>                                                                              rcu_read_lock()
>>                                                                              read last_lookup return part[i] (6)
>>                                                                              sector_in_part() is OK (7)
>>                                                                              return part[i] (8)
>>
>>   part[i] == NULL (9)
>>       last_lookup = NULL (10)
>>   rcu_read_unlock() (11)
>>                                            one RCU grace period completes
>>                                            __delete_partition() (12)
>>                                            free hd_partition (13)
>>                                                                              // use-after-free
>>                                                                              hd_struct_try_get(part[i])  (14)
>>
>> * the number in the parenthesis is the sequence of events.
>>



>> Maybe RCU experts can shed some light on this problem, so cc +paulmck@kernel.org, +joel@joelfernandes.org and +RCU maillist.
>>
>> If the above case is possible, maybe we can fix the problem by pinning last_lookup through increasing its ref-count
>> (the following patch is only compile tested):
>>
>> diff --git a/block/genhd.c b/block/genhd.c
>> index 6e8543ca6912..179e0056fae1 100644
>> --- a/block/genhd.c
>> +++ b/block/genhd.c
>> @@ -279,7 +279,14 @@ struct hd_struct *disk_map_sector_rcu(struct gendisk *disk, sector_t sector)
>>  		part = rcu_dereference(ptbl->part[i]);
>>
>>  		if (part && sector_in_part(part, sector)) {
>> -			rcu_assign_pointer(ptbl->last_lookup, part);
>> +			struct hd_struct *old;
>> +
>> +			if (!hd_struct_try_get(part))
>> +				break;
>> +
>> +			old = xchg(&ptbl->last_lookup, part);
>> +			if (old)
>> +				hd_struct_put(old);
>>  			return part;
>>  		}
>>  	}
>> @@ -1231,7 +1238,11 @@ static void disk_replace_part_tbl(struct gendisk *disk,
>>  	rcu_assign_pointer(disk->part_tbl, new_ptbl);
>>
>>  	if (old_ptbl) {
>> -		rcu_assign_pointer(old_ptbl->last_lookup, NULL);
>> +		struct hd_struct *part;
>> +
>> +		part = xchg(&old_ptbl->last_lookup, NULL);
>> +		if (part)
>> +			hd_struct_put(part);
>>  		kfree_rcu(old_ptbl, rcu_head);
>>  	}
>>  }
>> diff --git a/block/partition-generic.c b/block/partition-generic.c
>> index 98d60a59b843..441c1c591c04 100644
>> --- a/block/partition-generic.c
>> +++ b/block/partition-generic.c
>> @@ -285,7 +285,8 @@ void delete_partition(struct gendisk *disk, int partno)
>>  		return;
>>
>>  	rcu_assign_pointer(ptbl->part[partno], NULL);
>> -	rcu_assign_pointer(ptbl->last_lookup, NULL);
>> +	if (cmpxchg(&ptbl->last_lookup, part, NULL) == part)
>> +		hd_struct_put(part);
>>  	kobject_put(part->holder_dir);
>>  	device_del(part_to_dev(part));
> 
> IMO this approach looks good.
>
Not sure about the overhead when there are concurrent IOs on different partitions,
we will measure that.

We have got a seemingly better solution: caching the index of last_lookup in tbl->part[]
instead of caching the pointer itself, so we can ensure the validity of returned pointer
by ensuring it's not NULL in tbl->part[] as does when last_lookup is NULL or 0.

> Given partition is actually protected by percpu-refcount now, I guess the
> RCU annotation for referencing ->part[partno] and ->last_lookup may not
> be necessary, together with the part->rcu_work.
> 
So we will depends on the invocation of of call_rcu() on __percpu_ref_switch_mode() to
ensure the RCU readers will find part[i] is NULL before trying to increasing
the atomic ref-counter of part[i], right ?

Regards,
Tao

> 
> Thanks,
> Ming
> 
> 
> .
>