Re: [PATCH 1/2] lockdep: check the depth of subclass

[Hitoshi Mitake]

On 10/12/10 19:27, Peter Zijlstra wrote:
> On Tue, 2010-10-05 at 18:01 +0900, Hitoshi Mitake wrote:
>> Current look_up_lock_class() doesn't check the parameter "subclass".
>> This rarely rises problems because the main caller of this function,
>> register_lock_class(), checks it.
>> But register_lock_class() is not the only function which calls
>> look_up_lock_class(). lock_set_class() and its callees also call it.
>> And lock_set_class() doesn't check this parameter.
>>
>> This will rise problems when the the value of subclass is larger
>> MAX_LOCKDEP_SUBCLASSES. Because the address (used as the key of class)
>> caliculated with too large subclass has a possibility to point
>> another key in different lock_class_key.
>> Of course this problem depends on the memory layout and
>> occurs with really low possibility.
>>
>> And mousedev_create() calles lockdep_set_subclass() and
>> sets class of mousedev->mutex as MOUSEDEV_MIX(== 31).
>> And if my understanding is correct,
>> this subclass doesn't have to be MOUSEDEV_MIX,
>> so I modified this value to SINGLE_DEPTH_NESTING.
>>
>> Signed-off-by: Hitoshi Mitake<mitake@xxxxxxxxxxxxxxxxxxxxx>
>> Cc: Dmitry Torokhov<dtor@xxxxxxx>
>> Cc: Vojtech Pavlik<vojtech@xxxxxx>
>> Cc: Peter Zijlstra<peterz@xxxxxxxxxxxxx>
>> Cc: Frederic Weisbecker<fweisbec@xxxxxxxxx>
>> ---
>>   drivers/input/mousedev.c |    2 +-
>>   kernel/lockdep.c         |   15 +++++++++++++++
>>   2 files changed, 16 insertions(+), 1 deletions(-)
>>
>> diff --git a/drivers/input/mousedev.c b/drivers/input/mousedev.c
>> index d528a2d..9897334 100644
>> --- a/drivers/input/mousedev.c
>> +++ b/drivers/input/mousedev.c
>> @@ -866,7 +866,7 @@ static struct mousedev *mousedev_create(struct 
input_dev *dev,
>>   	spin_lock_init(&mousedev->client_lock);
>>   	mutex_init(&mousedev->mutex);
>>   	lockdep_set_subclass(&mousedev->mutex,
>> -			     minor == MOUSEDEV_MIX ? MOUSEDEV_MIX : 0);
>> +			     minor == MOUSEDEV_MIX ? SINGLE_DEPTH_NESTING : 0);
>
> Ah good find.
>
>>   	init_waitqueue_head(&mousedev->wait);
>>
>>   	if (minor == MOUSEDEV_MIX)
>> diff --git a/kernel/lockdep.c b/kernel/lockdep.c
>> index 84baa71..c4c13ae 100644
>> --- a/kernel/lockdep.c
>> +++ b/kernel/lockdep.c
>> @@ -639,6 +639,21 @@ look_up_lock_class(struct lockdep_map *lock, 
unsigned int subclass)
>>   	}
>>   #endif
>>
>> +	if (unlikely(subclass>= MAX_LOCKDEP_SUBCLASSES)) {
>> +		/*
>> +		 * This check should be done not only in __lock_acquire()
>> +		 * but also here. Because register_lock_class() is also called
>> +		 * by lock_set_class(). Callers of lock_set_class() can
>> +		 * pass invalid value as subclass.
>> +		 */
>> +
>> +		debug_locks_off();
>> +		printk(KERN_ERR "BUG: looking up invalid subclass: %u\n", subclass);
>> +		printk(KERN_ERR "turning off the locking correctness validator.\n");
>> +		dump_stack();
>> +		return NULL;
>> +	}
>
> Would we catch all cases if we moved this check from __lock_acquire()
> into register_lock_class()? It would result in only a single instance of
> this logic.
>

I think that __lock_acquire() should also check the value of subclass.
Because it access to the lock->class_cache as array
before calling look_up_lock_class() after applying this patch.

So if the check isn't done in __lock_acquire(),
the invalid addresses might be interpreted as the addresses of
struct lock_class.
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