Re: [PATCH] ieee802154: socket: fix shared skb BUG_ON and double header subtraction in dgram_ioctl()

[Miquel Raynal <miquel.raynal@bootlin.com>]

On 19/09/2026 at 21:28:55 GMT, Hui Peng <benquike@gmail.com> wrote:

> In `dgram_ioctl()` (`SIOCINQ`), the queued skb at the head of
> `sk->sk_receive_queue` has already had its MAC header stripped by
> `ieee802154_subif_frame()` prior to `dgram_deliver()`, so `skb->len` is
> already the exact payload length that `dgram_recvmsg()` will return to
> userspace.
>
> However, `dgram_ioctl()` computes `*karg = skb->len -
> ieee802154_hdr_length(skb)`. Calling `ieee802154_hdr_length(skb)`
> re-parses the payload bytes as an 802.15.4 MAC header and calls
> `pskb_may_pull(skb, ...)` while holding `sk->sk_receive_queue.lock`.
> When multiple `SOCK_DGRAM` sockets receive a cloned copy of the frame
> (`skb_shared(skb)` is true) and the payload is non-linear,
> `pskb_may_pull()` calls `pskb_expand_head()`, which triggers
> `BUG_ON(skb_shared(skb))`. Even on unshared linear skbs, subtracting
> `ieee802154_hdr_length(skb)` a second time underflows `*karg` to a
> negative value when the payload is shorter than the bogus parsed header
> length.
>
> Set `*karg = skb->len` directly in `dgram_ioctl()`.
>
> Fixes: e1d001fa5b47 ("net: ioctl: Use kernel memory on protocol ioctl callbacks")
> Assisted-by: LLM
> Signed-off-by: Hui Peng <benquike@gmail.com>

Looks legitimate.

Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>

Thanks,
Miquèl