staging: rtl8712: unterminated string leads to read overflow

commit d660f4f upstream.

The memdup_user() function does not necessarily return a NUL terminated
string so this can lead to a read overflow.  Switch from memdup_user()
to strndup_user() to fix this bug.

Fixes: c6dc001 ("staging: r8712u: Merging Realtek's latest (v2.6.6). Various fixes.")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/YDYSR+1rj26NRhvb@mwanda
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/rtl8712/rtl871x_ioctl_linux.c

@@ -924,7 +924,7 @@ static int r871x_wx_set_priv(struct net_device *dev,
 	struct iw_point *dwrq = (struct iw_point *)awrq;
 
 	len = dwrq->length;
-	ext = memdup_user(dwrq->pointer, len);
+	ext = strndup_user(dwrq->pointer, len);
 	if (IS_ERR(ext))
 		return PTR_ERR(ext);