linux-kernel - Re: [PATCH v3 16/24] locking/lockdep: Retain the class key and name while freeing a lock class

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Date:   Fri, 7 Dec 2018 09:50:49 -0500
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Bart Van Assche <bvanassche@....org>
Cc:     mingo@...hat.com, tj@...nel.org, johannes.berg@...el.com,
        linux-kernel@...r.kernel.org,
        Johannes Berg <johannes@...solutions.net>
Subject: Re: [PATCH v3 16/24] locking/lockdep: Retain the class key and name
 while freeing a lock class

On 12/07/2018 05:21 AM, Peter Zijlstra wrote:
> On Thu, Dec 06, 2018 at 05:11:40PM -0800, Bart Van Assche wrote:
>> The next patch in this series uses the class name in code that
>> detects calls to lock_acquire() while a class key is being freed. Hence
>> retain the class name for lock classes that are being freed.
> From readin the discussion with v2; you meant to say: 'uses the class
> name pointer', right? You're not actually ever going to dereference it,
> since that would be a UaF.
>
>> Cc: Peter Zijlstra <peterz@...radead.org>
>> Cc: Waiman Long <longman@...hat.com>
>> Cc: Johannes Berg <johannes@...solutions.net>
>> Signed-off-by: Bart Van Assche <bvanassche@....org>
>> ---
>>  kernel/locking/lockdep.c | 4 +---
>>  1 file changed, 1 insertion(+), 3 deletions(-)
>>
>> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
>> index ecd92969674c..92bdb187987f 100644
>> --- a/kernel/locking/lockdep.c
>> +++ b/kernel/locking/lockdep.c
>> @@ -4147,10 +4147,8 @@ static void zap_class(struct lock_class *class)
>>  	 * Unhash the class and remove it from the all_lock_classes list:
>>  	 */
>>  	hlist_del_rcu(&class->hash_entry);
>> +	class->hash_entry.pprev = NULL;
>>  	list_del(&class->lock_entry);
>> -
>> -	RCU_INIT_POINTER(class->key, NULL);
>> -	RCU_INIT_POINTER(class->name, NULL);
>>  }
>>  
>>  static inline int within(const void *addr, void *start, unsigned long size)
>> -- 
>> 2.20.0.rc2.403.gdbc3b29805-goog
>>
I still prefer keeping the clearing statements. Leaving key behind
should be OK as it is just a long value to be compared against. The
variable name is a different story as it is a pointer that will
reference bytes that it points to unless you save the name in a safe
storage area and change the pointer to point there.

Cheers,
Longman