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SubjectRe: [PATCH] net: can: prevent potential access of uninitialized value in canfd_rcv()
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On 11/2/20 4:13 AM, Anant Thazhemadam wrote:
> 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.
>
> Reported-by: syzbot+9bcb0c9409066696d3aa@syzkaller.appspotmail.com
> Tested-by: Anant Thazhemadam <anant.thazhemadam@gmail.com>
> Signed-off-by: Anant Thazhemadam <anant.thazhemadam@gmail.com>
> ---
> This patch was locally tested using the reproducer and .config file
> generated by syzbot.
>
> net/can/af_can.c | 19 ++++++++++++++-----
> 1 file changed, 14 insertions(+), 5 deletions(-)
>
> diff --git a/net/can/af_can.c b/net/can/af_can.c
> index ea29a6d97ef5..1b9f2e50f065 100644
> --- a/net/can/af_can.c
> +++ b/net/can/af_can.c
> @@ -694,16 +694,25 @@ static int canfd_rcv(struct sk_buff *skb, struct net_device *dev,

Can you create a similar patch for "can_rcv()"?

> {
> 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.

Please don't use C++ comment style in the kernel.

> + else if (unlikely(cfd->len > CANFD_MAX_DLEN)) {

Please move the "else" right after the closing curly bracket: "} else if () {"
or convert it into an "if () {"

> + 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 */
>

regards,
Marc

--
Pengutronix e.K. | Marc Kleine-Budde |
Embedded Linux | https://www.pengutronix.de |
Vertretung West/Dortmund | Phone: +49-231-2826-924 |
Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 |

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