On Thu, 07 Sep 2017 16:09:56 +0200
Daniel Borkmann <dan...@iogearbox.net> wrote:
> On 09/07/2017 02:33 PM, Jesper Dangaard Brouer 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.
> >
> > 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 <bro...@redhat.com>
> > ---
> > include/trace/events/xdp.h | 4 ++--
> > net/core/filter.c | 14 +++++++++++---
> > 2 files changed, 13 insertions(+), 5 deletions(-)
> >
> > diff --git a/include/trace/events/xdp.h b/include/trace/events/xdp.h
> > index 862575ac8da9..4e16c43fba10 100644
> > --- a/include/trace/events/xdp.h
> > +++ b/include/trace/events/xdp.h
> > @@ -138,11 +138,11 @@ DEFINE_EVENT_PRINT(xdp_redirect_template,
> > xdp_redirect_map_err,
> >
> > #define _trace_xdp_redirect_map(dev, xdp, fwd, map, idx) \
> > trace_xdp_redirect_map(dev, xdp, fwd ? fwd->ifindex : 0, \
> > - 0, map, idx);
> > + 0, map, idx)
> >
> > #define _trace_xdp_redirect_map_err(dev, xdp, fwd, map, idx, err) \
> > trace_xdp_redirect_map_err(dev, xdp, fwd ? fwd->ifindex : 0, \
> > - err, map, idx);
> > + err, map, idx)
> >
> > #endif /* _TRACE_XDP_H */
> >
> > diff --git a/net/core/filter.c b/net/core/filter.c
> > index 5912c738a7b2..3767470cab6c 100644
> > --- a/net/core/filter.c
> > +++ b/net/core/filter.c
> > @@ -2566,13 +2566,19 @@ int xdp_do_generic_redirect(struct net_device *dev,
> > struct sk_buff *skb,
> > struct bpf_prog *xdp_prog)
> > {
> > struct redirect_info *ri = this_cpu_ptr(&redirect_info);
> > + struct bpf_map *map = ri->map;
> > u32 index = ri->ifindex;
> > struct net_device *fwd;
> > unsigned int len;
> > int err = 0;
> >
> > - fwd = dev_get_by_index_rcu(dev_net(dev), index);
> > ri->ifindex = 0;
> > + ri->map = NULL;
> > +
> > + if (map)
> > + fwd = __dev_map_lookup_elem(map, index);
> > + else
> > + fwd = dev_get_by_index_rcu(dev_net(dev), index);
> > if (unlikely(!fwd)) {
> > err = -EINVAL;
> > goto err;
> > @@ -2590,10 +2596,12 @@ int xdp_do_generic_redirect(struct net_device *dev,
> > struct sk_buff *skb,
> > }
> >
> > skb->dev = fwd;
>
> Looks much better above, thanks!
>
> > - _trace_xdp_redirect(dev, xdp_prog, index);
> > + map ? _trace_xdp_redirect_map(dev, xdp_prog, fwd, map, index)
> > + : _trace_xdp_redirect(dev, xdp_prog, index);
>
> Could we rather make this in a way such that when the two
> tracepoints are disabled and thus patched out, that we can
> also omit the extra conditional which has no purpose then?
First of all I don't think it make much of a difference, I measured the
impact of the full patch to "cost" 1.62 nanosec (which is arguably
below the accuracy level of the system under test)
Secondly, I plan to optimize the map case for generic XDP later, where
I would naturally split this into two functions (as V1, and as
native-XDP), thus this extra conditional would go away. As I've shown
offlist (to you, John and Andy) I demonstrated a 24% speedup via a
xmit_more hack for generic XDP.
> Perhaps just a consolidated _trace_xdp_generic_redirect_map()
> would be better to avoid this altogether given we have twice
> the same anyway, here and in err path.
I do want separate tracepoints for xdp_redirect and xdp_redirect_map,
as it makes it more clear for users of the tracepoint (and attached
bpf_prog's can be faster, knowing the context).
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer
