[PATCH v2] ax25: fix OOB read after address header strip in ax25_rcv().

[Ashutosh Desai <ashutoshdesai993@gmail.com>]

ax25_rcv() calls skb_pull(skb, ax25_addr_size(&dp)) to strip the
address header, then reads skb->data[0] (control byte) and skb->data[1]
(PID byte) without verifying those bytes are in the linear sk_buff area.

The original fix checked skb->len < 2, but as Eric Dumazet pointed out,
skb->len counts bytes across the linear head and any non-linear
fragments. If the two bytes needed sit in a fragment, the check passes
but the direct skb->data access is still out of bounds.

Use pskb_may_pull(skb, 2) instead, which ensures both bytes are present
and pulls them into the linear area if needed before we read them.

Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Ashutosh Desai <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 d75b3e9ed..6a71dea87 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