[PATCH net 3/5] nfc: nci: consume the loopback response exactly once

nci_nfcc_loopback_cb() stores the received data packet in
conn_info->rx_skb and completes the request, and nci_nfcc_loopback()
returns conn_info->rx_skb to the caller, which frees it.  The pointer
is never cleared.

Any NCI response with a zero status also completes the request, as
nci_req_complete() only checks NCI_REQ_PEND.  If the NFCC sends one
instead of the data packet, nci_nfcc_loopback() returns the skb of the
previous loopback, which was already freed, or NULL.  st_nci_loopback()
then copies skb->data into its vendor command reply and frees the skb
again.

With an emulated NFCC on virtual_ncidev answering the second loopback
with an unsolicited CORE_SET_CONFIG_RSP, nci_nfcc_loopback() returns 0
and the skb of the first loopback, and KASAN reports a
slab-use-after-free as soon as the caller reads it.

Detach the skb from conn_info when consuming it, free it when the
caller does not want it, and return -EPROTO when the request was
completed without a data packet.

Fixes: 1c53855f6be2 ("nfc: nci: Add nci_nfcc_loopback to the nci core")
Cc: <redacted>
Signed-off-by: Joas Antonio dos Santos <redacted>
Assisted-by: Yaga Agent HackerSec Claude:claude-opus-5-5
---
 net/nfc/nci/core.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/net/nfc/nci/core.c b/net/nfc/nci/core.c
index 73e3a9647..8e2a755ce 100644
--- a/net/nfc/nci/core.c
+++ b/net/nfc/nci/core.c

@@ -428,7 +428,7 @@ int nci_nfcc_loopback(struct nci_dev *ndev, const void *data, size_t data_len,
 	int r;
 	struct nci_loopback_data loopback_data;
 	struct nci_conn_info *conn_info;
-	struct sk_buff *skb;
+	struct sk_buff *skb, *rx_skb;
 	int conn_id = nci_get_conn_info_by_dest_type_params(ndev,
 					NCI_DESTINATION_NFCC_LOOPBACK, NULL);
 

@@ -464,8 +464,20 @@ int nci_nfcc_loopback(struct nci_dev *ndev, const void *data, size_t data_len,
 	ndev->cur_conn_id = conn_id;
 	r = nci_request(ndev, nci_send_data_req, &loopback_data,
 			msecs_to_jiffies(NCI_DATA_TIMEOUT));
-	if (r == NCI_STATUS_OK && resp)
-		*resp = conn_info->rx_skb;
+	if (r == NCI_STATUS_OK) {
+		/* Consume the stored data packet exactly once, see
+		 * nci_nfcc_loopback_cb().
+		 */
+		rx_skb = conn_info->rx_skb;
+		conn_info->rx_skb = NULL;
+		if (!rx_skb)
+			return -EPROTO;
+
+		if (resp)
+			*resp = rx_skb;
+		else
+			kfree_skb(rx_skb);
+	}
 
 	return r;
 }

-- 
2.43.0