linux-kernel - [PATCH] staging: r8188eu: fix NULL check for rcu pointer

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Message-Id: <20230131090057.241779-1-martin@kaiser.cx>
Date:   Tue, 31 Jan 2023 10:00:57 +0100
From:   Martin Kaiser <martin@...ser.cx>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Larry Finger <Larry.Finger@...inger.net>,
        Phillip Potter <phil@...lpotter.co.uk>,
        Michael Straube <straube.linux@...il.com>,
        Pavel Skripkin <paskripkin@...il.com>,
        linux-staging@...ts.linux.dev, linux-kernel@...r.kernel.org,
        Martin Kaiser <martin@...ser.cx>
Subject: [PATCH] staging: r8188eu: fix NULL check for rcu pointer

Fix the NULL check for padapter->pnetdev->rx_handler_data.

The current code calls rcu_dereference while it holds the rcu read lock
and checks the pointer after releasing the lock. An rcu pointer may only be
used between calls to rcu_read_lock and rcu_read_unlock.

Replace the check with rcu_access_pointer. My understanding is that this
function returns the value of the pointer and needs no locking. We can
then check the pointer but we must not dereference it.

Signed-off-by: Martin Kaiser <martin@...ser.cx>
---
 drivers/staging/r8188eu/core/rtw_xmit.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/drivers/staging/r8188eu/core/rtw_xmit.c b/drivers/staging/r8188eu/core/rtw_xmit.c
index 91f92ec5ef69..18941320e70e 100644
--- a/drivers/staging/r8188eu/core/rtw_xmit.c
+++ b/drivers/staging/r8188eu/core/rtw_xmit.c
@@ -1631,18 +1631,14 @@ s32 rtw_xmit(struct adapter *padapter, struct sk_buff **ppkt)
 	struct xmit_priv *pxmitpriv = &padapter->xmitpriv;
 	struct xmit_frame *pxmitframe = NULL;
 	struct mlme_priv	*pmlmepriv = &padapter->mlmepriv;
-	void *br_port = NULL;
 	s32 res;
 
 	pxmitframe = rtw_alloc_xmitframe(pxmitpriv);
 	if (!pxmitframe)
 		return -1;
 
-	rcu_read_lock();
-	br_port = rcu_dereference(padapter->pnetdev->rx_handler_data);
-	rcu_read_unlock();
-
-	if (br_port && check_fwstate(pmlmepriv, WIFI_STATION_STATE | WIFI_ADHOC_STATE)) {
+	if (rcu_access_pointer(padapter->pnetdev->rx_handler_data) &&
+	    check_fwstate(pmlmepriv, WIFI_STATION_STATE | WIFI_ADHOC_STATE)) {
 		res = rtw_br_client_tx(padapter, ppkt);
 		if (res == -1) {
 			rtw_free_xmitframe(pxmitpriv, pxmitframe);
-- 
2.30.2