Re: [PATCH] nfc: llcp: Fix socket reference leak in nfc_llcp_recv_ui()

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- [High] Incomplete fix: the identical nfc_llcp_sock_get() reference leak
  remains at the sibling call site in nfc_llcp_recv_connect()…

Pre-existing issues:
- [Critical] Post-handoff SKB access / use-after-free race in
  nfc_llcp_recv_ui() (net/nfc/llcp_core.c:902-907): the extra SKB…

--

nfc: llcp: Fix socket reference leak in nfc_llcp_recv_ui()

This splits the combined NULL/socket-type test in nfc_llcp_recv_ui() so that
the reference taken by nfc_llcp_sock_get() is dropped with
nfc_llcp_sock_put() when the socket is not a SOCK_DGRAM socket.  Previously
that branch returned with the reference still held.

> diff --git a/net/nfc/llcp_core.c b/net/nfc/llcp_core.c
> index cac1b5487064d..f83196e49d2a6 100644
> --- a/net/nfc/llcp_core.c
> +++ b/net/nfc/llcp_core.c
> @@ -890,8 +890,12 @@ static void nfc_llcp_recv_ui(struct nfc_llcp_local *local,
>  
>  	/* We're looking for a bound socket, not a client one */
>  	llcp_sock = nfc_llcp_sock_get(local, dsap, LLCP_SAP_SDP);
> -	if (llcp_sock == NULL || llcp_sock->sk.sk_type != SOCK_DGRAM)
> +	if (!llcp_sock)
>  		return;
> +	if (llcp_sock->sk.sk_type != SOCK_DGRAM) {
> +		nfc_llcp_sock_put(llcp_sock);
> +		return;
> +	}

[Severity: High]
Does the same nfc_llcp_sock_get() reference get leaked in
nfc_llcp_recv_connect(), which is a few lines below this in the same file?
It has the identical condition shape that is being split apart here:

net/nfc/llcp_core.c:nfc_llcp_recv_connect() {
	if (dsap != LLCP_SAP_SDP) {
		sock = nfc_llcp_sock_get(local, dsap, LLCP_SAP_SDP);
		if (sock == NULL || sock->sk.sk_state != LLCP_LISTEN) {
			reason = LLCP_DM_NOBOUND;
			goto fail;
		}
	...
fail:
	/* Send DM */
	nfc_llcp_send_dm(local, dsap, ssap, reason);
}

The fail: label only sends the DM PDU and returns, so when sock is non-NULL
but sk_state != LLCP_LISTEN, the reference taken by
nfc_llcp_sock_get()'s sock_hold(&llcp_sock->sk) is never released.  Every
other exit of that function (acceptq full, LLCP_SAP_MAX, nfc_llcp_sock_alloc
failure, nfc_llcp_local_get failure, and the success path) does call
sock_put(&sock->sk).

Is that branch reachable from a remote peer?  nfc_llcp_sock_alloc() sets

	llcp_sock->ssap = 0;
	llcp_sock->dsap = LLCP_SAP_SDP;

and llcp_sock_bind() publishes the socket with

	nfc_llcp_sock_link(&local->sockets, sk);
	...
	sk->sk_state = LLCP_BOUND;

so a bound-but-not-listening socket (any bound SOCK_DGRAM socket, or a stream
socket bound before listen()) matches the lookup key used by
nfc_llcp_recv_connect(), and a CONNECT PDU aimed at its ssap would leak one
sk_refcnt per frame received.  Since the socket would then never reach
llcp_sock_destruct(), does the nfc device reference that destructor releases
leak as well?  Would it make sense to fix both call sites in one patch?

>  
>  	/* There is no sequence with UI frames */
>  	skb_pull(skb, LLCP_HEADER_SIZE);

[Severity: Critical]
This isn't a bug introduced by this patch, but while looking at
nfc_llcp_recv_ui() the ordering of the skb reference just after the quoted
hunk looks racy:

net/nfc/llcp_core.c:nfc_llcp_recv_ui() {
	skb_pull(skb, LLCP_HEADER_SIZE);
	if (!sock_queue_rcv_skb(&llcp_sock->sk, skb)) {
		/*
		 * UI frames will be freed from the socket layer, so we
		 * need to keep them alive until someone receives them.
		 */
		skb_get(skb);
	...
}

Can the skb be freed before skb_get() runs?  sock_queue_rcv_skb() links the
buffer into sk->sk_receive_queue, drops sk_receive_queue.lock and then calls
sk->sk_data_ready(), so at that point skb->users is still 1 and the buffer is
already visible to a reader on another cpu:

net/nfc/llcp_sock.c:llcp_sock_recvmsg() {
	skb = skb_recv_datagram(sk, flags, &err);
	...
		kfree_skb(skb);
}

If that reader dequeues and frees the buffer first, does skb_get() then do
refcount_inc() on freed memory?

And would nfc_llcp_rx_work() free the same buffer a second time afterwards?

net/nfc/llcp_core.c:nfc_llcp_rx_work() {
	...
	kfree_skb(local->rx_pending);
	local->rx_pending = NULL;
}

No lock appears to span the handoff here: nfc_llcp_recv_ui() does not hold
the socket lock, the queue spinlock is released inside
__sock_queue_rcv_skb() before the wakeup, and the nfc_llcp_sock_get()
reference only keeps the socket alive, not the skb.  Would taking the
reference before sock_queue_rcv_skb() and dropping it when queueing fails be
the safer order?

-- 
Sashiko AI review · https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260916073424.1971784-1-vulab%40iscas.ac.cn