slcan: Don't transmit uninitialized stack data in padding

CVE-2020-11494

struct can_frame contains some padding which is not explicitly zeroed in
slc_bump. This uninitialized data will then be transmitted if the stack
initialization hardening feature is not enabled (CONFIG_INIT_STACK_ALL).

This commit just zeroes the whole struct including the padding.
Signed-off-by: Richard Palethorpe <rpalethorpe@suse.com>
Fixes: a1044e36 ("can: add slcan driver for serial/USB-serial CAN adapters")
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: linux-can@vger.kernel.org
Cc: netdev@vger.kernel.org
Cc: security@kernel.org
Cc: wg@grandegger.com
Cc: mkl@pengutronix.de
Cc: davem@davemloft.net
Acked-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit b9258a2c)
Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Acked-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Andrea Righi <andrea.righi@canonical.com>
Signed-off-by: Kelsey Skunberg <kelsey.skunberg@canonical.com>

drivers/net/can/slcan.c

@@ -147,7 +147,7 @@ static void slc_bump(struct slcan *sl)
 	u32 tmpid;
 	char *cmd = sl->rbuff;
 
-	cf.can_id = 0;
+	memset(&cf, 0, sizeof(cf));
 
 	switch (*cmd) {
 	case 'r':
@@ -186,8 +186,6 @@ static void slc_bump(struct slcan *sl)
 	else
 		return;
 
-	*(u64 *) (&cf.data) = 0; /* clear payload */
-
 	/* RTR frames may have a dlc > 0 but they never have any data bytes */
 	if (!(cf.can_id & CAN_RTR_FLAG)) {
 		for (i = 0; i < cf.can_dlc; i++) {