Re: [PATCH v2] net/sched: em_canid: fix uninit-value in em_canid_match

[Oliver Hartkopp]

Hello Shaurya,

many thanks that you picked up this KMSAN issue!

On 26.11.25 08:06, ssrane_b23@xxxxxxxxxxxxx wrote:
> From: Shaurya Rane <ssrane_b23@xxxxxxxxxxxxx>
>
> Use pskb_may_pull() to ensure the CAN ID is accessible in the linear
> data buffer before reading it. A simple skb->len check is insufficient
> because it only verifies the total data length but does not guarantee
> the data is present in skb->data (it could be in fragments).
>
> pskb_may_pull() both validates the length and pulls fragmented data
> into the linear buffer if necessary, making it safe to directly
> access skb->data.
>
> Reported-by: syzbot+5d8269a1e099279152bc@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=5d8269a1e099279152bc
> Fixes: f057bbb6f9ed ("net: em_canid: Ematch rule to match CAN frames according to their identifiers")
> Signed-off-by: Shaurya Rane <ssrane_b23@xxxxxxxxxxxxx>
> ---
> v2: Use pskb_may_pull() instead of skb->len check to properly
>      handle fragmented skbs (Eric Dumazet)
> ---
>   net/sched/em_canid.c | 3 +++
>   1 file changed, 3 insertions(+)
>
> diff --git a/net/sched/em_canid.c b/net/sched/em_canid.c
> index 5337bc462755..2214b548fab8 100644
> --- a/net/sched/em_canid.c
> +++ b/net/sched/em_canid.c
> @@ -99,6 +99,9 @@ static int em_canid_match(struct sk_buff *skb, struct tcf_ematch *m,
>   	int i;
>   	const struct can_filter *lp;
>   
> +	if (!pskb_may_pull(skb, sizeof(canid_t)))

The EM CANID handles struct CAN frames in skb->data.

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/uapi/linux/can.h#n221

The smallest type of CAN frame that can be properly handled with EM 
CANID is a Classical CAN frame which has a length of 16 bytes.

Therefore I would suggest

	if (!pskb_may_pull(skb, CAN_MTU))

instead of only checking for the first element in struct can_frame.

Many thanks and best regards,
Oliver


> +		return 0;
> +
>   	can_id = em_canid_get_id(skb);
>   
>   	if (can_id & CAN_EFF_FLAG) {