netdev - Re: [PATCH net 1/2] vxlan: do not modify the shared tunnel info when PMTU triggers an ICMP reply

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Date:   Thu, 20 Jan 2022 11:27:51 +0100
From:   Antoine Tenart <atenart@...nel.org>
To:     Vlad Buslov <vladbu@...dia.com>
Cc:     davem@...emloft.net, kuba@...nel.org, echaudro@...hat.com,
        sbrivio@...hat.com, netdev@...r.kernel.org, pshelar@....org
Subject: Re: [PATCH net 1/2] vxlan: do not modify the shared tunnel info when PMTU triggers an ICMP reply

Hello Vlad,

Quoting Vlad Buslov (2022-01-20 08:38:05)
> On Thu 25 Mar 2021 at 17:35, Antoine Tenart <atenart@...nel.org> wrote:
> >
> > diff --git a/drivers/net/vxlan.c b/drivers/net/vxlan.c
> > index 666dd201c3d5..53dbc67e8a34 100644
> > --- a/drivers/net/vxlan.c
> > +++ b/drivers/net/vxlan.c
> > @@ -2725,12 +2725,17 @@ static void vxlan_xmit_one(struct sk_buff *skb, struct net_device *dev,
> >                       goto tx_error;
> >               } else if (err) {
> >                       if (info) {
> > +                             struct ip_tunnel_info *unclone;
> >                               struct in_addr src, dst;
> >  
> > +                             unclone = skb_tunnel_info_unclone(skb);
> > +                             if (unlikely(!unclone))
> > +                                     goto tx_error;
> > +
> 
> We have been getting memleaks in one of our tests that point to this
> code (test deletes vxlan device while running traffic redirected by OvS
> TC at the same time):
> 
> unreferenced object 0xffff8882d0114200 (size 256):
>   comm "softirq", pid 0, jiffies 4296140292 (age 1435.992s)
>   hex dump (first 32 bytes):
>     00 00 00 00 00 00 00 00 00 3b 85 84 ff ff ff ff  .........;......
>     a1 26 b7 83 ff ff ff ff 00 00 00 00 00 00 00 00  .&..............
>   backtrace:
>     [<0000000097659d47>] metadata_dst_alloc+0x1f/0x470
>     [<000000007571c30f>] tun_dst_unclone+0xee/0x360 [vxlan]
>     [<00000000d2dcfd00>] vxlan_xmit_one+0x131d/0x2a00 [vxlan]
>     [<00000000281572b6>] vxlan_xmit+0x8e6/0x4cd0 [vxlan]
>     [<00000000d49d33fe>] dev_hard_start_xmit+0x1ba/0x710
>     [<00000000eac444f5>] __dev_queue_xmit+0x17c5/0x25f0
>     [<000000005fbd8585>] tcf_mirred_act+0xb1d/0xf70 [act_mirred]
>     [<0000000064b6eb2d>] tcf_action_exec+0x10e/0x350
>     [<00000000352821e8>] fl_classify+0x4e3/0x610 [cls_flower]
>     [<0000000011d3f765>] tcf_classify+0x33d/0x800
>     [<000000006c69b225>] __netif_receive_skb_core+0x18d6/0x2ae0
>     [<00000000dd256fe3>] __netif_receive_skb_one_core+0xaf/0x180
>     [<0000000065d43bd6>] process_backlog+0x2e3/0x710
>     [<00000000964357ae>] __napi_poll+0x9f/0x560
>     [<0000000059a93cf6>] net_rx_action+0x357/0xa60
>     [<00000000766481bc>] __do_softirq+0x282/0x94e
> 
> Looking at the code the potential issue seems to be that
> tun_dst_unclone() creates new metadata_dst instance with refcount==1,
> increments the refcount with dst_hold() to value 2, then returns it.
> This seems to imply that caller is expected to release one of the
> references (second one if for skb), but none of the callers (including
> original dev_fill_metadata_dst()) do that, so I guess I'm
> misunderstanding something here.
> 
> Any tips or suggestions?

I'd say there is no need to increase the dst refcount here after calling
metadata_dst_alloc, as the metadata is local to the skb and the dst
refcount was already initialized to 1. This might be an issue with
commit fc4099f17240 ("openvswitch: Fix egress tunnel info."); I CCed
Pravin, he might recall if there was a reason to increase the refcount.

Thanks,
Antoine