net: hamradio: bpqether: validate frame length in bpq_rcv()

The BPQ length field is decoded as:

len = skb->data[0] + skb->data[1] * 256 - 5;

If the sender sets bytes [0..1] to values whose combined value is
less than 5, len becomes negative. Passing a negative int to
skb_trim() silently converts to a huge unsigned value, causing the
function to be a no-op. The frame is then passed up to AX.25 with
its original (untrimmed) payload, delivering garbage beyond the
declared frame boundary.

Additionally, a negative len corrupts the 64-bit rx_bytes counter
through implicit sign-extension.

Add a bounds check before pulling the length bytes: reject frames
where len is negative or exceeds the remaining skb data.

Acked-by: Joerg Reuter <[e-mail]>
Signed-off-by: Mashiro Chen <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/hamradio/bpqether.c

···
 
 	len = skb->data[0] + skb->data[1] * 256 - 5;
 
+	if (len < 0 || len > skb->len - 2)
+		goto drop_unlock;
+
 	skb_pull(skb, 2);	/* Remove the length bytes */
 	skb_trim(skb, len);	/* Set the length of the data */