netdev - Re: [PATCH net] l2tp: Don't sleep and disable BH under writer-side sk_callback

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Date:   Thu, 24 Nov 2022 11:27:35 +0100
From:   Guillaume Nault <gnault@...hat.com>
To:     Tom Parkin <tparkin@...alix.com>
Cc:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Jakub Sitnicki <jakub@...udflare.com>, netdev@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        syzbot+703d9e154b3b58277261@...kaller.appspotmail.com,
        syzbot+50680ced9e98a61f7698@...kaller.appspotmail.com,
        syzbot+de987172bb74a381879b@...kaller.appspotmail.com
Subject: Re: [PATCH net] l2tp: Don't sleep and disable BH under writer-side
 sk_callback_lock

On Thu, Nov 24, 2022 at 10:07:51AM +0000, Tom Parkin wrote:
> On  Wed, Nov 23, 2022 at 16:24:00 +0100, Guillaume Nault wrote:
> > On Tue, Nov 22, 2022 at 11:28:45PM +0900, Tetsuo Handa wrote:
> > > That's what I thought at https://lkml.kernel.org/r/c64284f4-2c2a-ecb9-a08e-9e49d49c720b@I-love.SAKURA.ne.jp .
> > > 
> > > But the problem is not that setup_udp_tunnel_sock() can sleep. The problem is that lockdep
> > > gets confused due to changing lockdep class after the socket is already published. We need
> > > to avoid calling lockdep_set_class_and_name() on a socket retrieved via sockfd_lookup().
> > 
> > This is a second problem. The problem of setting sk_user_data under
> > sk_callback_lock write protection (while still calling
> > udp_tunnel_encap_enable() from sleepable context) still remains.
> > 
> > For lockdep_set_class_and_name(), maybe we could store the necessary
> > socket information (addresses, ports and checksum configuration) in the
> > l2tp_tunnel structure, thus avoiding the need to read them from the
> > socket. This way, we could stop locking the user space socket in
> > l2tp_xmit_core() and drop the lockdep_set_class_and_name() call.
> > I think either you or Jakub proposed something like this in another
> > thread.
> 
> I note that l2tp_xmit_core calls ip_queue_xmit which expects a socket
> atomic context*.
> 
> It also accesses struct inet_sock corking data which may also need locks
> to safely access.
> 
> Possibly we could somehow work around that, but on the face of it we'd
> need to do a bit more work to avoid the socket lock in the tx path.

I was thinking of avoiding using the socket entirely, which indeed
means replacing ip_queue_xmit(). We should probably use the different
variants of udp_tunnel_xmit_skb() instead.

> * davem fixed locking in the l2tp xmit path in:
> 
> 6af88da14ee2 ("l2tp: Fix locking in l2tp_core.c")
> -- 
> Tom Parkin
> Katalix Systems Ltd
> https://katalix.com
> Catalysts for your Embedded Linux software development