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

[Hui Peng <benquike@gmail.com>]

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>

---
 net/ieee802154/socket.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ieee802154/socket.c b/net/ieee802154/socket.c
index 5a36e87893f6..8d660fe87d4e 100644
--- a/net/ieee802154/socket.c
+++ b/net/ieee802154/socket.c
@@ -552,7 +552,7 @@ static int dgram_ioctl(struct sock *sk, int cmd, int *karg)
 			 * of this packet since that is all
 			 * that will be read.
 			 */
-			*karg = skb->len - ieee802154_hdr_length(skb);
+			*karg = skb->len;
 		}
 		spin_unlock_bh(&sk->sk_receive_queue.lock);
 		return 0;
-- 
2.55.0.1082.g2b9226bbc0-goog