netdev - [PATCH net,stable] net: cdc_ether: fix divide by 0 on bad descriptors

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Message-Id: <20171106143722.3171-1-bjorn@mork.no>
Date:   Mon,  6 Nov 2017 15:37:22 +0100
From:   Bjørn Mork <bjorn@...k.no>
To:     netdev@...r.kernel.org
Cc:     linux-usb@...r.kernel.org, Oliver Neukum <oneukum@...e.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Kostya Serebryany <kcc@...gle.com>, syzkaller@...glegroups.com,
        Bjørn Mork <bjorn@...k.no>
Subject: [PATCH net,stable] net: cdc_ether: fix divide by 0 on bad descriptors

Setting dev->hard_mtu to 0 will cause a divide error in
usbnet_probe. Protect against devices with bogus CDC Ethernet
functional descriptors by ignoring a zero wMaxSegmentSize.

Signed-off-by: Bjørn Mork <bjorn@...k.no>
---
I believe the problem found by syzcaller in qmi_wwan also applies
to cdc_ether.  We cannot allow the .bind callback to set
dev->hard_mtu to 0.

 drivers/net/usb/cdc_ether.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/usb/cdc_ether.c b/drivers/net/usb/cdc_ether.c
index 3e7a3ac3a362..05dca3e5c93d 100644
--- a/drivers/net/usb/cdc_ether.c
+++ b/drivers/net/usb/cdc_ether.c
@@ -230,7 +230,7 @@ int usbnet_generic_cdc_bind(struct usbnet *dev, struct usb_interface *intf)
 			goto bad_desc;
 	}
 
-	if (header.usb_cdc_ether_desc) {
+	if (header.usb_cdc_ether_desc && info->ether->wMaxSegmentSize) {
 		dev->hard_mtu = le16_to_cpu(info->ether->wMaxSegmentSize);
 		/* because of Zaurus, we may be ignoring the host
 		 * side link address we were given.
-- 
2.11.0