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

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Date:   Thu, 8 Oct 2020 12:50:55 -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 12:18 PM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> On Thu, Oct 8, 2020 at 3:04 PM Willem de Bruijn
> <willemdebruijn.kernel@...il.com> wrote:
> >
> > On Thu, Oct 8, 2020 at 1:34 PM Cong Wang <xiyou.wangcong@...il.com> wrote:
> > >
> > > 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.
>
> Oh, do you mean that ipgre_xmit will process undefined data because it
> calls skb_pull(skb, tunnel->hlen + sizeof(struct iphdr)) ?

The syzbot report has the information for both of your questions:
https://syzkaller.appspot.com/text?tag=CrashReport&x=11845568500000

It clearly shows packet_snd() and ipgre_xmit().

Thanks.