can: af_can: prevent potential access of uninitialized member in canfd_rcv()

In canfd_rcv(), cfd->len is uninitialized when skb->len = 0, and this
uninitialized cfd->len is accessed nonetheless by pr_warn_once().

Fix this uninitialized variable access by checking cfd->len's validity
condition (cfd->len > CANFD_MAX_DLEN) separately after the skb->len's
condition is checked, and appropriately modify the log messages that
are generated as well.
In case either of the required conditions fail, the skb is freed and
NET_RX_DROP is returned, same as before.

Fixes: d4689846881d ("can: af_can: canfd_rcv(): replace WARN_ONCE by pr_warn_once")
Reported-by: syzbot+9bcb0c9409066696d3aa@syzkaller.appspotmail.com
Tested-by: Anant Thazhemadam <anant.thazhemadam@gmail.com>
Signed-off-by: Anant Thazhemadam <anant.thazhemadam@gmail.com>
Link: https://lore.kernel.org/r/20201103213906.24219-3-anant.thazhemadam@gmail.com
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

diff --git a/net/can/af_can.c b/net/can/af_can.c
index e8d4e3e..5d124c1 100644 (file)
--- a/net/can/af_can.c
+++ b/net/can/af_can.c
@@ -703,16 +703,25 @@ static int canfd_rcv(struct sk_buff *skb, struct net_device *dev,
 {
        struct canfd_frame *cfd = (struct canfd_frame *)skb->data;
 
-       if (unlikely(dev->type != ARPHRD_CAN || skb->len != CANFD_MTU ||
-                    cfd->len > CANFD_MAX_DLEN)) {
-               pr_warn_once("PF_CAN: dropped non conform CAN FD skbuf: dev type %d, len %d, datalen %d\n",
+       if (unlikely(dev->type != ARPHRD_CAN || skb->len != CANFD_MTU)) {
+               pr_warn_once("PF_CAN: dropped non conform CAN FD skbuff: dev type %d, len %d\n",
+                            dev->type, skb->len);
+               goto free_skb;
+       }
+
+       /* This check is made separately since cfd->len would be uninitialized if skb->len = 0. */
+       if (unlikely(cfd->len > CANFD_MAX_DLEN)) {
+               pr_warn_once("PF_CAN: dropped non conform CAN FD skbuff: dev type %d, len %d, datalen %d\n",
                             dev->type, skb->len, cfd->len);
-               kfree_skb(skb);
-               return NET_RX_DROP;
+               goto free_skb;
        }
 
        can_receive(skb, dev);
        return NET_RX_SUCCESS;
+
+free_skb:
+       kfree_skb(skb);
+       return NET_RX_DROP;
 }
 
 /* af_can protocol functions */