Re: [PATCH v3] net/sched: em_canid: fix uninit-value in em_canid_match

[Eric Dumazet]

On Wed, Nov 26, 2025 at 12:57 AM <ssrane_b23@xxxxxxxxxxxxx> wrote:
>
> From: Shaurya Rane <ssrane_b23@xxxxxxxxxxxxx>
>
> Use pskb_may_pull() to ensure a complete CAN frame is present in the
> linear data buffer before reading the CAN ID. 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>
> ---
> v3: Use CAN_MTU to validate a complete CAN frame is present
> v2: Use pskb_may_pull() instead of skb->len check to properly
>     handle fragmented skbs
> ---
>  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..2d27f91d8441 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, CAN_MTU))
> +               return 0;
> +
>         can_id = em_canid_get_id(skb);
>
>         if (can_id & CAN_EFF_FLAG) {

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