netdev - Re: [Patch net] ip_gre: set dev->hard_header

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Message-ID: <CAM_iQpWCR84sD6dZBforgt4cg-Jya91D6EynDo2y2sC7vi-vMg@mail.gmail.com>
Date:   Thu, 8 Oct 2020 10:33:43 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        syzbot <syzbot+4a2c52677a8a1aa283cb@...kaller.appspotmail.com>,
        William Tu <u9012063@...il.com>, Xie He <xie.he.0141@...il.com>
Subject: Re: [Patch net] ip_gre: set dev->hard_header_len properly

On Thu, Oct 8, 2020 at 4:49 AM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> On Wed, Oct 7, 2020 at 9:22 PM Cong Wang <xiyou.wangcong@...il.com> wrote:
> >
> > GRE tunnel has its own header_ops, ipgre_header_ops, and sets it
> > conditionally. When it is set, it assumes the outer IP header is
> > already created before ipgre_xmit().
> >
> > This is not true when we send packets through a raw packet socket,
> > where L2 headers are supposed to be constructed by user. Packet
> > socket calls dev_validate_header() to validate the header. But
> > GRE tunnel does not set dev->hard_header_len, so that check can
> > be simply bypassed, therefore uninit memory could be passed down
> > to ipgre_xmit().
>
> If dev->hard_header_len is zero, the packet socket will not reserve
> room for the link layer header, so skb->data points to network_header.
> But I don't see any uninitialized packet data?

The uninit data is allocated by packet_alloc_skb(), if dev->hard_header_len
is 0 and 'len' is anything between [0, tunnel->hlen + sizeof(struct iphdr)),
dev_validate_header() still returns true obviously but only 'len'
bytes are copied
from user-space by skb_copy_datagram_from_iter(). Therefore, those bytes
within range (len, tunnel->hlen + sizeof(struct iphdr)] are uninitialized.

Thanks.