Subject: Re: [PATCH] l2tp: Avoid possible recursive deadlock in l2tp_tunnel_register()

From: Shigeru Yoshida <[email-masked]>
Date: Thu, 2 Feb 2023 00:43:49 +0900

Hi Guillaume,

On Mon, Jan 30, 2023 at 06:03:52PM +0100, Guillaume Nault wrote:
> On Tue, Jan 31, 2023 at 12:44:38AM +0900, Shigeru Yoshida wrote:
> > This patch fixes the issue by returning error when a pppol2tp socket
> > itself is passed.
> 
> Fixes: 0b2c59720e65 ("l2tp: close all race conditions in l2tp_tunnel_register()")
> 
> > Signed-off-by: Shigeru Yoshida <[email-masked]>
> > ---
> >  net/l2tp/l2tp_ppp.c | 7 +++++--
> >  1 file changed, 5 insertions(+), 2 deletions(-)
> > 
> > diff --git a/net/l2tp/l2tp_ppp.c b/net/l2tp/l2tp_ppp.c
> > index db2e584c625e..88d1a339500b 100644
> > --- a/net/l2tp/l2tp_ppp.c
> > +++ b/net/l2tp/l2tp_ppp.c
> > @@ -702,11 +702,14 @@ static int pppol2tp_connect(struct socket *sock, struct sockaddr *uservaddr,
> >  			struct l2tp_tunnel_cfg tcfg = {
> >  				.encap = L2TP_ENCAPTYPE_UDP,
> >  			};
> > +			int dummy = 0;
> 
> There's no need to initialise dummy here. This is just confusing.
> We could even do without any extra variable and reuse error in
> sockfd_lookup().
> 
> >  			/* Prevent l2tp_tunnel_register() from trying to set up
> > -			 * a kernel socket.
> > +			 * a kernel socket.  Also, prevent l2tp_tunnel_register()
> > +			 * from trying to use pppol2tp socket itself.
> >  			 */
> > -			if (info.fd < 0) {
> > +			if (info.fd < 0 ||
> > +			    sock == sockfd_lookup(info.fd, &dummy)) {
> >  				error = -EBADF;
> >  				goto end;
> >  			}
> 
> That should work, but the real problem is calling l2tp_tunnel_register()
> under lock_sock(). We should instead get/create the tunnel before
> locking the pppol2tp socket.

Thank you so much for your comment, and sorry for the late response.

Do you mean we can call l2tp_tunnel_register() without pppol2tp socket
lock?  I've read the source code of pppol2tp_connect(), but I'm not
sure why pppol2tp socket is locked at the beginning of this function.

If we can call l2tp_tunnel_register() without pppol2tp socket lock, I
think we can move lock_sock() after l2tp_tunnel_register().

Thanks,
Shigeru