[PATCH] 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 immediately reads skb->data[0] and skb->data[1]
without verifying the buffer still contains at least 2 bytes.

A crafted 15-byte KISS frame (1 KISS byte + 14 address bytes with
EBIT set in the source address, no control/PID bytes) passes
ax25_addr_parse() which only requires len >= 14, and passes the KISS
byte check (low nibble == 0). After skb_pull(1) in ax25_kiss_rcv()
and skb_pull(14) in ax25_rcv(), skb->len is 0 and the subsequent
reads of skb->data[0] (control byte) and skb->data[1] (PID byte)
are out of bounds.

Add a check that at least 2 bytes remain after stripping the address
header, freeing the skb and returning on malformed input.

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..92baac77f 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 (skb->len < 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