[PATCH v3 net] ax25: fix OOB read after address header strip in ax25_rcv()

[Ashutosh Desai <ashutoshdesai993@gmail.com>]

A remote station can send a crafted KISS frame that is just long enough
to pass ax25_addr_parse() (minimum 14 address bytes) but carries no
control or PID bytes. After ax25_kiss_rcv() strips the KISS framing
byte and ax25_rcv() strips the address header with skb_pull(), skb->len
drops to zero. The subsequent reads of skb->data[0] (control byte) and
skb->data[1] (PID byte) are then out of bounds, which can crash the
kernel or leak heap memory to a remote attacker.

Use pskb_may_pull(skb, 2) after the skb_pull() to ensure both bytes
are in the linear area before reading them. Discard malformed frames
that carry no control/PID pair.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Ashutosh Desai <ashutoshdesai993@gmail.com>
---
V2 -> V3: remove incorrect Suggested-by; add symptom, Fixes, Cc stable
V1 -> V2: use pskb_may_pull(skb, 2) instead of skb->len < 2

v2: https://lore.kernel.org/netdev/20260409152400.2219716-1-ashutoshdesai993@gmail.com/
v1: https://lore.kernel.org/netdev/20260409012235.2049389-1-ashutoshdesai993@gmail.com/

 net/ax25/ax25_in.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/net/ax25/ax25_in.c b/net/ax25/ax25_in.c
index d75b3e9ed93d..6a71dea876a1 100644
--- a/net/ax25/ax25_in.c
+++ b/net/ax25/ax25_in.c
@@ -217,6 +217,11 @@ static int ax25_rcv(struct sk_buff *skb, struct net_device *dev,
 	 */
 	skb_pull(skb, ax25_addr_size(&dp));
 
+	if (!pskb_may_pull(skb, 2)) {
+		kfree_skb(skb);
+		return 0;
+	}
+
 	/* For our port addresses ? */
 	if (ax25cmp(&dest, dev_addr) == 0 && dp.lastrepeat + 1 == dp.ndigi)
 		mine = 1;
-- 
2.34.1