Subject: Re: IPv6: race condition in __ipv6_ifa_notify() and dst_free() ?

From: Stephen Hemminger <[email-masked]>
Date: Tue, 20 Apr 2010 14:16:55 -0700

On Tue, 20 Apr 2010 22:57:23 +0200
Eric Dumazet <[email-masked]> wrote:

> Le mardi 20 avril 2010 à 22:49 +0200, Jiri Bohac a écrit :
> > On Tue, Apr 20, 2010 at 07:57:27PM +0200, Eric Dumazet wrote:
> > > Le mardi 20 avril 2010 à 19:44 +0200, Jiri Bohac a écrit :
> > > > --- a/net/ipv6/addrconf.c	2010-04-17 00:12:32.000000000 +0200
> > > > +++ b/net/ipv6/addrconf.c	2010-04-20 19:07:35.000000000 +0200
> > > > @@ -3974,8 +3974,7 @@ static void __ipv6_ifa_notify(int event,
> > > >  			addrconf_leave_anycast(ifp);
> > > >  		addrconf_leave_solict(ifp->idev, &ifp->addr);
> > > >  		dst_hold(&ifp->rt->u.dst);
> > > > -		if (ip6_del_rt(ifp->rt))
> > > > -			dst_free(&ifp->rt->u.dst);
> > > > +		ip6_del_rt(ifp->rt);
> > > >  		break;
> > > >  	}
> > > >  }
> > > > 
> > > 
> > > 
> > > I dont understand the problem Jiri.
> > > 
> > > We just did dst_hold(&ifp->rt->u.dst), so if ip6_del_rt() fails we must
> > > dst_free(), or we leak a refcount.
> > 
> > Well, no ... dst_free() does not decrement the refcount.
> > The "opposite" of dst_hold() is dst_release(). And it does not
> > automatically call dst_free() when the refcount drops to 0.
> > dst_free() needs to be called explicitly and it apparently
> > expects the caller to ensure that two dst_free()s won't be called
> > simultaneously. But my bonding example shows this is not the
> > case.
> > 
> > 
> 
> Ah yes you're right
> 
> Maybe ask Stephen ?
> 
> commit 93fa159abe50d3c55c7f83622d3f5c09b6e06f4b
> Author: stephen hemminger <[email-masked]>
> Date:   Mon Apr 12 05:41:31 2010 +0000
> 
>     IPv6: keep route for tentative address
>     
>     Recent changes preserve IPv6 address when link goes down (good).
>     But would cause address to point to dead dst entry (bad).
>     The simplest fix is to just not delete route if address is
>     being held for later use.
>     
>     Signed-off-by: Stephen Hemminger <[email-masked]>
>     Signed-off-by: David S. Miller <[email-masked]>
> 
> diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
> index 1b00bfe..a9913d2 100644
> --- a/net/ipv6/addrconf.c
> +++ b/net/ipv6/addrconf.c
> @@ -4047,7 +4047,8 @@ static void __ipv6_ifa_notify(int event, struct
> inet6_ifaddr *ifp)
>                         addrconf_leave_anycast(ifp);
>                 addrconf_leave_solict(ifp->idev, &ifp->addr);
>                 dst_hold(&ifp->rt->u.dst);
> -               if (ip6_del_rt(ifp->rt))
> +
> +               if (ifp->dead && ip6_del_rt(ifp->rt))
>                         dst_free(&ifp->rt->u.dst);
>                 break;

Is this problem new to net-next where we hold onto addresses, or was
the issue there before?