netdev - Re: [PATCH bpf] bpf: fix memory leak in lpm_trie map

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Date:   Tue, 13 Feb 2018 17:46:23 -0800
From:   Yonghong Song <yhs@...com>
To:     Daniel Borkmann <daniel@...earbox.net>, <ast@...com>,
        <malat@...ian.org>, <netdev@...r.kernel.org>
CC:     <kernel-team@...com>
Subject: Re: [PATCH bpf] bpf: fix memory leak in lpm_trie map_free callback
 function



On 2/13/18 5:11 PM, Daniel Borkmann wrote:
> Hi Yonghong,
> 
> On 02/12/2018 10:58 PM, Yonghong Song wrote:
>> There is a memory leak happening in lpm_trie map_free callback
>> function trie_free. The trie structure itself does not get freed.
>>
>> Also, trie_free function did not do synchronize_rcu before freeing
>> various data structures. This is incorrect as some rcu_read_lock
>> region(s) for lookup, update, delete or get_next_key may not complete yet.
>> The fix is to add synchronize_rcu in the beginning of trie_free.
>> The useless spin_lock is removed from this function as well.
>>
>> Fixes: b95a5c4db09b ("bpf: add a longest prefix match trie map implementation")
>> Reported-by: Mathieu Malaterre <malat@...ian.org>
>> Reported-by: Alexei Starovoitov <ast@...nel.org>
>> Tested-by: Mathieu Malaterre <malat@...ian.org>
>> Signed-off-by: Yonghong Song <yhs@...com>
>> ---
>>   kernel/bpf/lpm_trie.c | 9 +++++++--
>>   1 file changed, 7 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/bpf/lpm_trie.c b/kernel/bpf/lpm_trie.c
>> index 7b469d1..9b41ea4 100644
>> --- a/kernel/bpf/lpm_trie.c
>> +++ b/kernel/bpf/lpm_trie.c
>> @@ -555,7 +555,12 @@ static void trie_free(struct bpf_map *map)
>>   	struct lpm_trie_node __rcu **slot;
>>   	struct lpm_trie_node *node;
>>   
>> -	raw_spin_lock(&trie->lock);
>> +	/* at this point bpf_prog->aux->refcnt == 0 and this map->refcnt == 0,
>> +	 * so the programs (can be more than one that used this map) were
>> +	 * disconnected from events. Wait for outstanding programs to complete
>> +	 * update/lookup/delete/get_next_key and free the trie.
>> +	 */
> 
> I think the first part of the comment is slightly misleading, e.g. map refcount
> could drop to zero also for various other reasons, not strictly due to prog
> refcnt dropping to 0, so I would just keep the second part with 'Wait for

Oh, yes. Make sense. Copy-paste without thinking.
We have similar comments in virtually all other places of using
synchronize_rcu under kernel/bpf. This can be cleaned up later though.

> outstanding [...]' which is okay since this is eventually what is relevant in
> this context.

Will do.

> 
>> +	synchronize_rcu();
>>   
>>   	/* Always start at the root and walk down to a node that has no
>>   	 * children. Then free that node, nullify its reference in the parent
>> @@ -588,7 +593,7 @@ static void trie_free(struct bpf_map *map)
>>   	}
>>   
>>   unlock:
>> -	raw_spin_unlock(&trie->lock);
> 
> Could you do a minor change here: since we get rid of the locking as there's
> no user left anymore after grace period, it would be great if you could also
> change the 'unlock' label name above.

Will do.

> 
> Other than that, good to go, thanks!
> 
>> +	kfree(trie);
>>   }
>>   
>>   static int trie_get_next_key(struct bpf_map *map, void *_key, void *_next_key)
>>
> 
> Thanks,
> Daniel
>