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

[Hou Tao <houtao1@huawei.com>]

Hi Joel,

On 2020/1/4 7:45, Joel Fernandes wrote:
> On Tue, Dec 31, 2019 at 03:11:58PM -0800, Paul E. McKenney 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:
>>>
>>> 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)
>>
>> Just for the record...
>>
>> Use of RCU needs to ensure that readers cannot access the to-be-freed
>> structure -before- invoking call_rcu().  Which does look to happen here
>> with the "last_lookup = NULL".  But in addition, the callback needs to
>> get access to the to-be-freed structure via some sideband (usually the
>> structure passed to call_rcu()), not from the reader-accessible structure.
>>
>> Or am I misinterpreting this sequence of events?
> 
> If I understand correctly, the issue described above is there are 2 threads
> setting last_lookup pointer simultaneously, one of them is NULLing it and
> waiting for a GP before freeing it (process B above), while the other is
> assigning to it concurrently after it was just NULLed (process A). Meanwhile
> process C starts a reader section *after* the GP by process B already started
> and accesses the reassigned pointer causing use-after-free.
> 
> Did I miss something?
> 
No. It's exactly the same as you have summarized. And thanks for that.

> I believe the fix is what Tao already posted which is to use refcounts so
> that the destructor does not free it while references are already held. Is
> that what the final fix is going to be? That other thread is pretty long so I
> lost track a bit..
> We are just trying to find a better solution (e.g. more readable or understandable).

Regards,
Tao

> thanks,
> 
>  - Joel
> 
> 
> 
>> 							Thanx, Paul
>>
>>>   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));
>>>
>>> -- 
>>> 2.22.0
>>>
>>> Regards,
>>> Tao
>>>
>>>
>>>> diff --git a/block/partition-generic.c b/block/partition-generic.c
>>>> index 1d20c9cf213f..1e0065ed6f02 100644
>>>> --- a/block/partition-generic.c
>>>> +++ b/block/partition-generic.c
>>>> @@ -284,6 +284,13 @@ void delete_partition(struct gendisk *disk, int partno)
>>>>  		return;
>>>>  
>>>>  	rcu_assign_pointer(ptbl->part[partno], NULL);
>>>> +	/*
>>>> +	 * Without the memory barrier, disk_map_sector_rcu()
>>>> +	 * may read the old value after overwriting the
>>>> +	 * last_lookup. Then it can not clear last_lookup,
>>>> +	 * which may cause use-after-free.
>>>> +	 */
>>>> +	smp_mb();
>>>>  	rcu_assign_pointer(ptbl->last_lookup, NULL);
>>>>  	kobject_put(part->holder_dir);
>>>>  	device_del(part_to_dev(part));
>>>>
>>>
> 
> .
>