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@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));
>>>
>>> --
>>> 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));
>>>>
>>>
>
> .
>
