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@xxxxxxxxxx, +joel@xxxxxxxxxxxxxxxxx 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
>
>
> .
>
