Re: [PATCH v5 05/12] keys: add call key_put_sync() to flush key_gc_work when doing a key_put().

[Dave Jiang <dave.jiang@intel.com>]

On 07/17/2018 04:53 PM, Eric Biggers wrote:
> On Tue, Jul 17, 2018 at 01:54:32PM -0700, Dave Jiang wrote:
>> Adding key_put_sync() that calls flush_work on key_gc_work to ensure the
>> key we want to remove is gone.
>>
>> Signed-off-by: Dave Jiang <dave.jiang@intel.com>
> 
> Why is this needed?  Isn't key_unlink() or key_invalidate() enough?

Without this, I was getting -EKEYREVOKED when calling request_key() and
it wasn't pulling from userspace, even with key_invalidate() called on
the key. I assume it has something to do with the garbage collector not
run yet and it's still finding the old key? With this I don't hit that
anymore. Please let me know if I'm doing something wrong with the sequence.

> 
> - Eric
> 
>> ---
>>  include/linux/key.h |    1 +
>>  security/keys/key.c |   35 +++++++++++++++++++++++++++++------
>>  2 files changed, 30 insertions(+), 6 deletions(-)
>>
>> diff --git a/include/linux/key.h b/include/linux/key.h
>> index e58ee10f6e58..4dffbd23d4e4 100644
>> --- a/include/linux/key.h
>> +++ b/include/linux/key.h
>> @@ -253,6 +253,7 @@ extern struct key *key_alloc(struct key_type *type,
>>  extern void key_revoke(struct key *key);
>>  extern void key_invalidate(struct key *key);
>>  extern void key_put(struct key *key);
>> +extern void key_put_sync(struct key *key);
>>  
>>  static inline struct key *__key_get(struct key *key)
>>  {
>> diff --git a/security/keys/key.c b/security/keys/key.c
>> index d97c9394b5dd..b6f3ec19ab0f 100644
>> --- a/security/keys/key.c
>> +++ b/security/keys/key.c
>> @@ -622,6 +622,19 @@ int key_reject_and_link(struct key *key,
>>  }
>>  EXPORT_SYMBOL(key_reject_and_link);
>>  
>> +static void __key_put(struct key *key, bool sync)
>> +{
>> +	if (key) {
>> +		key_check(key);
>> +
>> +		if (refcount_dec_and_test(&key->usage)) {
>> +			schedule_work(&key_gc_work);
>> +			if (sync)
>> +				flush_work(&key_gc_work);
>> +		}
>> +	}
>> +}
>> +
>>  /**
>>   * key_put - Discard a reference to a key.
>>   * @key: The key to discard a reference from.
>> @@ -632,15 +645,25 @@ EXPORT_SYMBOL(key_reject_and_link);
>>   */
>>  void key_put(struct key *key)
>>  {
>> -	if (key) {
>> -		key_check(key);
>> -
>> -		if (refcount_dec_and_test(&key->usage))
>> -			schedule_work(&key_gc_work);
>> -	}
>> +	__key_put(key, false);
>>  }
>>  EXPORT_SYMBOL(key_put);
>>  
>> +/**
>> + * key_put - Discard a reference to a key and flush the key gc work item.
>> + * @key: The key to discard a reference from.
>> + *
>> + * Discard a reference to a key, and when all the references are gone, we
>> + * schedule the cleanup task to come and pull it out of the tree in process
>> + * context at some later time. This call flushes the clean up so we are sure
>> + * that the key is gone.
>> + */
>> +void key_put_sync(struct key *key)
>> +{
>> +	__key_put(key, true);
>> +}
>> +EXPORT_SYMBOL(key_put_sync);
>> +
>>  /*
>>   * Find a key by its serial number.
>>   */
>>
>> --
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