netdev - Re: malicious devices causing unaligned accesses [v2]

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Date:   Thu, 17 Feb 2022 11:27:39 +0100
From:   Bjørn Mork <bjorn@...k.no>
To:     Oliver Neukum <oneukum@...e.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stern@...land.harvard.edu, USB list <linux-usb@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Giuliano Belinassi <giuliano.belinassi@...e.com>
Subject: Re: malicious devices causing unaligned accesses [v2]

Oliver Neukum <oneukum@...e.com> writes:

> Hi,
>
> going through the USB network drivers looking for ways
> a malicious device could do us harm I found drivers taking
> the alignment coming from the device for granted.
>
> An example can be seen in qmi_wwan:
>
> while (offset + qmimux_hdr_sz < skb->len) {
>     hdr = (struct qmimux_hdr*)(skb->data + offset);
>     len = be16_to_cpu(hdr->pkt_len);
>
> As you can see the driver accesses stuff coming from the device with the
> expectation
> that it keep to natural alignment. On some architectures that is a way a
> device could use to do bad things to a host. What is to be done about
> that?

We can deal with this the same way we deal with hostile hot-plugged CPUs
or memory modules.

Yes, the aligment should probably be verified.  But there are so many
ways a hostile network adapter can mess with us than I don't buy the
"malicious device" argument...

FWIW, the more recent rmnet demuxing implementation from Qualcomm seems
to suffer from the same problem.


struct sk_buff *rmnet_map_deaggregate(struct sk_buff *skb,
				      struct rmnet_port *port)
{
	struct rmnet_map_header *maph;
	struct sk_buff *skbn;
	u32 packet_len;

	if (skb->len == 0)
		return NULL;

	maph = (struct rmnet_map_header *)skb->data;
	packet_len = ntohs(maph->pkt_len) + sizeof(struct rmnet_map_header);


(this implementation moves skb->data by packet_len instead of doing the
offset calculation, but I don't think that makes any difference?)

I guess there is no alignment guarantee here, whether the device is
malicious or not. So we probably have to deal with unaligned accesses to
maph/hdr->pkt_len?



Bjørn