[PATCH v4 net] ax25: fix OOB read after address header strip in ax25_rcv()

[Ashutosh Desai <ashutoshdesai993@gmail.com>]

A crafted AX.25 frame with a valid address header but no control or PID
bytes causes skb->len to drop to zero after skb_pull() strips the
address header. The subsequent reads of skb->data[0] and skb->data[1]
are then out of bounds.

Linearize the skb at entry to ax25_rcv() so all subsequent accesses to
skb->data are safe. Then check skb->len before reading the control and
PID bytes, discarding frames that are too short.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Ashutosh Desai <ashutoshdesai993@gmail.com>
---
v4: linearize skb at entry to ax25_rcv(); replace pskb_may_pull() with
    skb->len check
v3: remove incorrect Suggested-by; add symptom, Fixes, Cc stable
v2: use pskb_may_pull(skb, 2) instead of skb->len < 2

Link to v3: https://lore.kernel.org/netdev/20260415063654.3831353-1-ashutoshdesai993@gmail.com/
Link to v2: https://lore.kernel.org/netdev/20260409152400.2219716-1-ashutoshdesai993@gmail.com/
Link to v1: https://lore.kernel.org/netdev/20260409012235.2049389-1-ashutoshdesai993@gmail.com/

 net/ax25/ax25_in.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/ax25/ax25_in.c b/net/ax25/ax25_in.c
index d75b3e9ed93d..d14ccebf9cdd 100644
--- a/net/ax25/ax25_in.c
+++ b/net/ax25/ax25_in.c
@@ -190,6 +190,9 @@ static int ax25_rcv(struct sk_buff *skb, struct net_device *dev,
 	ax25_cb *ax25;
 	ax25_dev *ax25_dev;
 
+	if (skb_linearize(skb))
+		goto free;
+
 	/*
 	 *	Process the AX.25/LAPB frame.
 	 */
@@ -217,6 +220,9 @@ static int ax25_rcv(struct sk_buff *skb, struct net_device *dev,
 	 */
 	skb_pull(skb, ax25_addr_size(&dp));
 
+	if (skb->len < 2)
+		goto free;
+
 	/* For our port addresses ? */
 	if (ax25cmp(&dest, dev_addr) == 0 && dp.lastrepeat + 1 == dp.ndigi)
 		mine = 1;
-- 
2.34.1