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:   Fri, 28 Jan 2022 18:01:33 +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

Hi Vlad,

Quoting Vlad Buslov (2022-01-20 13:58:18)
> On Thu 20 Jan 2022 at 12:27, Antoine Tenart <atenart@...nel.org> wrote:
> > Quoting Vlad Buslov (2022-01-20 08:38:05)
> >> 
> >> 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):
> >>     [<0000000097659d47>] metadata_dst_alloc+0x1f/0x470
> >>     [<000000007571c30f>] tun_dst_unclone+0xee/0x360 [vxlan]
> >>     [<00000000d2dcfd00>] vxlan_xmit_one+0x131d/0x2a00 [vxlan]

[...]

> >> 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.
> 
> I tried to remove the dst_hold(), but that caused underflows[0], so I
> guess the current reference counting is required at least for some
> use-cases.
> 
> [0]:
> 
> [  118.803011] dst_release: dst:000000001fc13e61 refcnt:-2                             

[...]

I finally had some time to look at this. Does the diff below fix your
issue?

diff --git a/include/net/dst_metadata.h b/include/net/dst_metadata.h
index 14efa0ded75d..90a7a4daea9c 100644
--- a/include/net/dst_metadata.h
+++ b/include/net/dst_metadata.h
@@ -110,8 +110,8 @@ static inline struct metadata_dst *tun_rx_dst(int md_size)
 static inline struct metadata_dst *tun_dst_unclone(struct sk_buff *skb)
 {
        struct metadata_dst *md_dst = skb_metadata_dst(skb);
-       int md_size;
        struct metadata_dst *new_md;
+       int md_size, ret;
 
        if (!md_dst || md_dst->type != METADATA_IP_TUNNEL)
                return ERR_PTR(-EINVAL);
@@ -123,8 +123,15 @@ static inline struct metadata_dst *tun_dst_unclone(struct sk_buff *skb)
 
        memcpy(&new_md->u.tun_info, &md_dst->u.tun_info,
               sizeof(struct ip_tunnel_info) + md_size);
+#ifdef CONFIG_DST_CACHE
+       ret = dst_cache_init(&new_md->u.tun_info.dst_cache, GFP_ATOMIC);
+       if (ret) {
+               metadata_dst_free(new_md);
+               return ERR_PTR(ret);
+       }
+#endif
+
        skb_dst_drop(skb);
-       dst_hold(&new_md->dst);
        skb_dst_set(skb, &new_md->dst);
        return new_md;
 }

Antoine