[PATCH net 1/2] net: hamradio: bpqether: validate frame length in bpq_rcv()

[Mashiro Chen <mashiro.chen@mailbox.org>]

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.

Cc: stable@vger.kernel.org
Cc: linux-hams@vger.kernel.org
Signed-off-by: Mashiro Chen <mashiro.chen@mailbox.org>
---
 drivers/net/hamradio/bpqether.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/net/hamradio/bpqether.c b/drivers/net/hamradio/bpqether.c
index 045c5177262eaf..214fd1f819a1bb 100644
--- a/drivers/net/hamradio/bpqether.c
+++ b/drivers/net/hamradio/bpqether.c
@@ -187,6 +187,9 @@ static int bpq_rcv(struct sk_buff *skb, struct net_device *dev, struct packet_ty
 
 	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 */
 
-- 
2.53.0