[PATCH v2 net 1/2] net: hamradio: bpqether: validate frame length in bpq_rcv()

Mashiro Chen
2026-04-09T02:50:00Z

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
Acked-by: Joerg Reuter <jreuter@yaina.de>
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