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Date:   Wed, 9 May 2018 00:37:54 -0700
From:   Eric Biggers <ebiggers3@gmail.com>
To:     Eric Dumazet <eric.dumazet@gmail.com>
Cc:     Eric Dumazet <edumazet@google.com>,
        syzbot <syzbot+71d74a5406d02057d559@syzkaller.appspotmail.com>,
        alexander.deucher@amd.com,
        Andrey Konovalov <andreyknvl@google.com>,
        Anoob Soman <anoob.soman@citrix.com>, chris@chris-wilson.co.uk,
        David Miller <davem@davemloft.net>, elena.reshetova@intel.com,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Kees Cook <keescook@chromium.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Mike Maloney <maloney@google.com>, mchehab@kernel.org,
        netdev <netdev@vger.kernel.org>, rami.rosen@intel.com,
        Sowmini Varadhan <sowmini.varadhan@oracle.com>,
        syzkaller-bugs@googlegroups.com,
        Willem de Bruijn <willemb@google.com>
Subject: Re: KASAN: use-after-free Read in __dev_queue_xmit
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On Wed, Jan 03, 2018 at 10:53:14PM -0800, Eric Dumazet wrote:
> On Wed, 2018-01-03 at 21:13 -0800, Eric Dumazet wrote:
> > Note: all commands must start from beginning of the line in the email body.
> > 
> > I guess skb_probe_transport_header() should be hardened to reject malicious
> > packets given by user space, instead of being gentle.
> 
> Although bug triggered for this particular repro is in flow dissector
> :/
> 
> I will test :
> 
> diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
> index 15ce300637650e17fcab7e378b20fe7972686d46..544bddf08e13c7f6e47aadc737244c9ba5af56b2 100644
> --- a/net/core/flow_dissector.c
> +++ b/net/core/flow_dissector.c
> @@ -976,8 +976,8 @@ bool __skb_flow_dissect(const struct sk_buff *skb,
> ?out_good:
> ????????ret = true;
> ?
> -???????key_control->thoff = (u16)nhoff;
> ?out:
> +???????key_control->thoff = min_t(u16, nhoff, skb ? skb->len : hlen);
> ????????key_basic->n_proto = proto;
> ????????key_basic->ip_proto = ip_proto;
> ?
> @@ -985,7 +985,6 @@ bool __skb_flow_dissect(const struct sk_buff *skb,
> ?
> ?out_bad:
> ????????ret = false;
> -???????key_control->thoff = min_t(u16, nhoff, skb ? skb->len : hlen);
> ????????goto out;
> ?}
> ?EXPORT_SYMBOL(__skb_flow_dissect);

Fix for this was commit d0c081b49137cd:

#syz fix: flow_dissector: properly cap thoff field

But a crash with the same signature is still occurring, so it should eventually
get reported again.  C reproducer is here, it works on Linus' tree (commit
036db8bd963): https://syzkaller.appspot.com/text?tag=ReproC&x=105b1ae7800000

- Eric