[PATCH v5 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 byte
causes skb->len to reach zero after skb_pull() strips the header.
The subsequent reads of skb->data[0] (control) and skb->data[1] (PID)
are then out of bounds.

Linearize the skb after confirming the device is an AX.25 interface.
Guard with skb->len < 1 after the pull - one byte suffices for LAPB
control frames which have no PID byte. Add a separate skb->len < 2
check inside the UI branch before accessing the PID byte.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Ashutosh Desai <ashutoshdesai993@gmail.com>
---
v5:
- Move skb_linearize() to after ax25_dev_ax25dev() check; avoids
  unnecessary allocation for frames on non-AX.25 interfaces
- Lower general guard from skb->len < 2 to skb->len < 1; the stricter
  limit incorrectly dropped valid 1-byte LAPB control frames (SABM,
  DISC, UA, DM, RR) which carry no PID byte
- Add explicit skb->len < 2 check inside UI branch before the PID
  byte (skb->data[1]) access
v4:
- Linearize skb at entry to ax25_rcv(); replace pskb_may_pull() with
  skb->len < 2 check (per David Laight review)
v3:
- Remove incorrect Suggested-by; add Fixes:, Cc: stable@
v2:
- Replace skb->len check with pskb_may_pull(skb, 2)

Link to v4: https://lore.kernel.org/netdev/20260417065407.206499-1-ashutoshdesai993@gmail.com/
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 | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/net/ax25/ax25_in.c b/net/ax25/ax25_in.c
index d75b3e9ed93d..c81d6830af48 100644
--- a/net/ax25/ax25_in.c
+++ b/net/ax25/ax25_in.c
@@ -199,6 +199,9 @@ static int ax25_rcv(struct sk_buff *skb, struct net_device *dev,
 	if ((ax25_dev = ax25_dev_ax25dev(dev)) == NULL)
 		goto free;
 
+	if (skb_linearize(skb))
+		goto free;
+
 	/*
 	 *	Parse the address header.
 	 */
@@ -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 < 1)
+		goto free;
+
 	/* For our port addresses ? */
 	if (ax25cmp(&dest, dev_addr) == 0 && dp.lastrepeat + 1 == dp.ndigi)
 		mine = 1;
@@ -227,6 +233,9 @@ static int ax25_rcv(struct sk_buff *skb, struct net_device *dev,
 
 	/* UI frame - bypass LAPB processing */
 	if ((*skb->data & ~0x10) == AX25_UI && dp.lastrepeat + 1 == dp.ndigi) {
+		if (skb->len < 2)
+			goto free;
+
 		skb_set_transport_header(skb, 2); /* skip control and pid */
 
 		ax25_send_to_raw(&dest, skb, skb->data[1]);
-- 
2.34.1