netdev - Re: [PATCH net-next] xdp: implement xdp_redirect

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Date:   Wed, 6 Sep 2017 18:01:39 +0200
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Andy Gospodarek <andy@...yhouse.net>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        John Fastabend <john.fastabend@...il.com>, brouer@...hat.com
Subject: Re: [PATCH net-next] xdp: implement xdp_redirect_map for generic
 XDP

On Wed, 6 Sep 2017 11:44:18 -0400
Andy Gospodarek <andy@...yhouse.net> wrote:

> On Wed, Sep 6, 2017 at 11:26 AM, Jesper Dangaard Brouer
> <brouer@...hat.com> wrote:
> > Using bpf_redirect_map is allowed for generic XDP programs, but the
> > appropriate map lookup was never performed in xdp_do_generic_redirect().
> >
> > Instead the map-index is directly used as the ifindex.  For the
> > xdp_redirect_map sample in SKB-mode '-S', this resulted in trying
> > sending on ifindex 0 which isn't valid, resulting in getting SKB
> > packets dropped.  Thus, the reported performance numbers are wrong in
> > commit 24251c264798 ("samples/bpf: add option for native and skb mode
> > for redirect apps") for the 'xdp_redirect_map -S' case.
> >
> > It might seem innocent this was lacking, but it can actually crash the
> > kernel.  The potential crash is caused by not consuming redirect_info->map.
> > The bpf_redirect_map helper will set this_cpu_ptr(&redirect_info)->map
> > pointer, which will survive even after unloading the xdp bpf_prog and
> > deallocating the devmap data-structure.  This leaves a dead map
> > pointer around.  The kernel will crash when loading the xdp_redirect
> > sample (in native XDP mode) as it doesn't reset map (via bpf_redirect)
> > and returns XDP_REDIRECT, which will cause it to dereference the map
> > pointer.  
> 
> Nice catch!
> 
> Since 'net-next' is closed and this is a bugfix it seems like this is
> a good candidate for 'net' right?

Yes, I know 'net-next' is closed, but 'net' doesn't contain the
XDP_REDIRECT code yet... thus I had to base it on net-next ;-)


> >
> > Fixes: 6103aa96ec07 ("net: implement XDP_REDIRECT for xdp generic")
> > Fixes: 24251c264798 ("samples/bpf: add option for native and skb mode for redirect apps")
> > Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>  
> 
> Acked-by: Andy Gospodarek <andy@...yhouse.net>

Thanks
-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer