[PATCH net-next v6 4/5] net: hdlc_fr: Improve the initial checks when we receive an skb

[Xie He <xie.he.0141@gmail.com>]

1.
Change the skb->len check from "<= 4" to "< 4".
At first we only need to ensure a 4-byte header is present. We indeed
normally need the 5th byte, too, but it'd be more logical and cleaner
to check its existence when we actually need it.

2.
Add an fh->ea2 check to the initial checks in fr_rx. fh->ea2 == 1 means
the second address byte is the final address byte. We only support the
case where the address length is 2 bytes. If the address length is not
2 bytes, the control field and the protocol field would not be the 3rd
and 4th byte as we assume. (Say it is 3 bytes, then the control field
and the protocol field would be the 4th and 5th byte instead.)

Cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Cc: Krzysztof Halasa <khc@pm.waw.pl>
Signed-off-by: Xie He <xie.he.0141@gmail.com>
---
 drivers/net/wan/hdlc_fr.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/wan/hdlc_fr.c b/drivers/net/wan/hdlc_fr.c
index eb83116aa9df..98444f1d8cc3 100644
--- a/drivers/net/wan/hdlc_fr.c
+++ b/drivers/net/wan/hdlc_fr.c
@@ -882,7 +882,7 @@ static int fr_rx(struct sk_buff *skb)
 	struct pvc_device *pvc;
 	struct net_device *dev;
 
-	if (skb->len <= 4 || fh->ea1 || data[2] != FR_UI)
+	if (skb->len < 4 || fh->ea1 || !fh->ea2 || data[2] != FR_UI)
 		goto rx_error;
 
 	dlci = q922_to_dlci(skb->data);
-- 
2.27.0