Re: [PATCH net 1/3] l2tp: hold reference on tunnels in netlink dumps

[Guillaume Nault <g.nault@alphalink.fr>]

On Fri, Apr 13, 2018 at 10:57:03AM -0400, David Miller wrote:
> From: Guillaume Nault <g.nault@alphalink.fr>
> Date: Thu, 12 Apr 2018 20:50:33 +0200
> 
> > l2tp_tunnel_find_nth() is unsafe: no reference is held on the returned
> > tunnel, therefore it can be freed whenever the caller uses it.
> > This patch defines l2tp_tunnel_get_nth() which works similarly, but
> > also takes a reference on the returned tunnel. The caller then has to
> > drop it after it stops using the tunnel.
> > 
> > Convert netlink dumps to make them safe against concurrent tunnel
> > deletion.
> > 
> > Fixes: 309795f4bec2 ("l2tp: Add netlink control API for L2TP")
> > Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
> 
> During the entire invocation of l2tp_nl_cmd_tunnel_dump(), the RTNL
> mutex is held.
> 
> Therefore no tunnel configuration changes may occur and the tunnel
> object will persist and is safe to access.
> 
Yes, but only for updates done with the genl API. For L2TPv2, the
tunnel can be created by connecting a PPPOL2TP and a UDP socket.
Closing these sockets destroys the tunnel without any RTNL
synchronisation.

> The netlink dump should be safe as-is.
> 
> Were you actually able to trigger a crash or KASAN warning or is
> this purely from code inspection?
> 
Yes, I have a KASAN use-after-free for this case. I remember I saw a
few complains about stack traces in commit messages, so I've stopped
putting them there. I can paste (stripped) traces again. Just let me
know if you have any preference.

Guillaume