[patch v2] wan/x25_asy: integer overflow in x25_asy_change_mtu()

If "newmtu * 2 + 4" is too large then it can cause an integer overflow
leading to memory corruption.  Eric Dumazet suggests that 65534 is a
reasonable upper limit.

Btw, "newmtu" is not allowed to be a negative number because of the
check in dev_set_mtu(), so that's ok.

Signed-off-by: Dan Carpenter <redacted>
---
v2: Cap it at 65534 instead of just testing for integer overflows.
    Thanks David and Eric!

diff --git a/drivers/net/wan/x25_asy.c b/drivers/net/wan/x25_asy.c
index df6c073..5c47b01 100644
--- a/drivers/net/wan/x25_asy.c
+++ b/drivers/net/wan/x25_asy.c

@@ -122,8 +122,12 @@ static int x25_asy_change_mtu(struct net_device *dev, int newmtu)
 {
 	struct x25_asy *sl = netdev_priv(dev);
 	unsigned char *xbuff, *rbuff;
-	int len = 2 * newmtu;
+	int len;
 
+	if (newmtu > 65534)
+		return -EINVAL;
+
+	len = 2 * newmtu;
 	xbuff = kmalloc(len + 4, GFP_ATOMIC);
 	rbuff = kmalloc(len + 4, GFP_ATOMIC);