Re: [PATCH v3 1/2] nfc: llcp: avoid double release/put on LLCP_CLOSED in nfc_llcp_recv_disc()

[Krzysztof Kozlowski <krzk@kernel.org>]

On 28/12/2025 10:16, Paolo Abeni wrote:
> On 12/28/25 10:02 AM, Paolo Abeni wrote:
>> On 12/18/25 3:59 AM, Qianchang Zhao wrote:
>>> nfc_llcp_sock_get() takes a reference on the LLCP socket via sock_hold().
>>>
>>> In nfc_llcp_recv_disc(), when the socket is already in LLCP_CLOSED state,
>>> the code used to perform release_sock() and nfc_llcp_sock_put() in the
>>> CLOSED branch but then continued execution and later performed the same
>>> cleanup again on the common exit path. This results in refcount imbalance
>>> (double put) and unbalanced lock release.
>>>
>>> Remove the redundant CLOSED-branch cleanup so that release_sock() and
>>> nfc_llcp_sock_put() are performed exactly once via the common exit path, 
>>> while keeping the existing DM_DISC reply behavior.
>>>
>>> Fixes: d646960f7986 ("NFC: Initial LLCP support")
>>> Cc: stable@vger.kernel.org
>>> Signed-off-by: Qianchang Zhao <pioooooooooip@gmail.com>
>>> ---
>>>  net/nfc/llcp_core.c | 5 -----
>>>  1 file changed, 5 deletions(-)
>>>
>>> diff --git a/net/nfc/llcp_core.c b/net/nfc/llcp_core.c
>>> index beeb3b4d2..ed37604ed 100644
>>> --- a/net/nfc/llcp_core.c
>>> +++ b/net/nfc/llcp_core.c
>>> @@ -1177,11 +1177,6 @@ static void nfc_llcp_recv_disc(struct nfc_llcp_local *local,
>>>  
>>>  	nfc_llcp_socket_purge(llcp_sock);
>>>  
>>> -	if (sk->sk_state == LLCP_CLOSED) {
>>> -		release_sock(sk);
>>> -		nfc_llcp_sock_put(llcp_sock);
>>
>> To rephrase Krzysztof concernt, this does not looks like the correct
>> fix: later on nfc_llcp_recv_disc() will try a send over a closed socket,
>> which looks wrong. Instead you could just return after
>> nfc_llcp_sock_put(), or do something alike:
>>
>> 	if (sk->sk_state == LLCP_CLOSED)
>> 		goto cleanup;
>>
>> 	// ...
>>
>>
>> cleanup:
>> 	release_sock(sk);
>> 	nfc_llcp_sock_put(llcp_sock);
>> }
> 
> I'm sorry for the confusing feedback above.
> 
> I read the comments on patch 2/2 only after processing this one.
> 
> Indeed following the half-interrupted discussion on old revision, with
> bad patch splitting is quite difficult.
> 
> @Qianchang Zhao: my _guess_ is that on LLCP_CLOSED the code has to
> release the final sk reference... In any case discussion an a patch
> series revision is not concluded until the reviewer agrees on that.

I would expect the code to return on LLCP_CLOSED, instead of proceeding
to sending nfc_llcp_send_dm() disconnect, because nfc_llcp_send_dm()
should happen earlier (before marking LLCP socket as closed), but that's
more of my assumption than actual knowledge.

> 
> @Krzysztof: ... but still it looks like in the current code there is a
> double release on the sk socket lock, which looks wrong, what am I
> missing here?

Author focused only on get/put and of course from that point of view
there is imbalance. But I asked at v2, for which there was still no
answer, what about releasing the initial reference from
nfc_llcp_sock_from_sn(). Maybe that was the intention here?

Best regards,
Krzysztof