netdev - Re: [PATCH v2] net: rds: acquire refcount on TCP sockets

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Date:   Tue, 3 May 2022 06:43:27 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     Paolo Abeni <pabeni@...hat.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Santosh Shilimkar <santosh.shilimkar@...cle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        syzbot <syzbot+694120e1002c117747ed@...kaller.appspotmail.com>,
        netdev <netdev@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        OFED mailing list <linux-rdma@...r.kernel.org>
Subject: Re: [PATCH v2] net: rds: acquire refcount on TCP sockets

On Tue, May 3, 2022 at 6:27 AM David Laight <David.Laight@...lab.com> wrote:
>
> From: Paolo Abeni
> > Sent: 03 May 2022 10:03
> >
> > Hello,
> >
> > On Mon, 2022-05-02 at 10:40 +0900, Tetsuo Handa wrote:
> > > syzbot is reporting use-after-free read in tcp_retransmit_timer() [1],
> > > for TCP socket used by RDS is accessing sock_net() without acquiring a
> > > refcount on net namespace. Since TCP's retransmission can happen after
> > > a process which created net namespace terminated, we need to explicitly
> > > acquire a refcount.
> > >
> > > Link: https://syzkaller.appspot.com/bug?extid=694120e1002c117747ed [1]
> > > Reported-by: syzbot <syzbot+694120e1002c117747ed@...kaller.appspotmail.com>
> > > Fixes: 26abe14379f8e2fa ("net: Modify sk_alloc to not reference count the netns of kernel sockets.")
> > > Fixes: 8a68173691f03661 ("net: sk_clone_lock() should only do get_net() if the parent is not a
> > kernel socket")
> > > Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
> > > Tested-by: syzbot <syzbot+694120e1002c117747ed@...kaller.appspotmail.com>
> > > ---
> > > Changes in v2:
> > >   Add Fixes: tag.
> > >   Move to inside lock_sock() section.
> > >
> > > I chose 26abe14379f8e2fa and 8a68173691f03661 which went to 4.2 for Fixes: tag,
> > > for refcount was implicitly taken when 70041088e3b97662 ("RDS: Add TCP transport
> > > to RDS") was added to 2.6.32.
> > >
> > >  net/rds/tcp.c | 8 ++++++++
> > >  1 file changed, 8 insertions(+)
> > >
> > > diff --git a/net/rds/tcp.c b/net/rds/tcp.c
> > > index 5327d130c4b5..2f638f8b7b1e 100644
> > > --- a/net/rds/tcp.c
> > > +++ b/net/rds/tcp.c
> > > @@ -495,6 +495,14 @@ void rds_tcp_tune(struct socket *sock)
> > >
> > >     tcp_sock_set_nodelay(sock->sk);
> > >     lock_sock(sk);
> > > +   /* TCP timer functions might access net namespace even after
> > > +    * a process which created this net namespace terminated.
> > > +    */
> > > +   if (!sk->sk_net_refcnt) {
> > > +           sk->sk_net_refcnt = 1;
> > > +           get_net_track(net, &sk->ns_tracker, GFP_KERNEL);
> > > +           sock_inuse_add(net, 1);
> > > +   }
> > >     if (rtn->sndbuf_size > 0) {
> > >             sk->sk_sndbuf = rtn->sndbuf_size;
> > >             sk->sk_userlocks |= SOCK_SNDBUF_LOCK;
> >
> > This looks equivalent to the fix presented here:
> >
> > https://lore.kernel.org/all/CANn89i+484ffqb93aQm1N-tjxxvb3WDKX0EbD7318RwRgsatjw@mail.gmail.com/
> >
> > but the latter looks a more generic solution. @Tetsuo could you please
> > test the above in your setup?
>
> Wouldn't a more generic solution be to add a flag to sock_create_kern()
> so that it acquires a reference to the namespace?
> This could be a bit on one of the existing parameters - like SOCK_NONBLOCK.
>
> I've a driver that uses __sock_create() in order to get that reference.
> I'm pretty sure the extra 'security' check will never fail.
>

This would be silly really.

Definition of a 'kernel socket' is that it does not hold a reference
to the namespace.
(otherwise a netns could not be destroyed by user space)

A kernel layer using kernel sockets needs to properly dismantle them
when a namespace is destroyed.

In the RDS case, the socket was a user socket, or RDS lacked proper
tracking of all the sockets
so that they can be dismantled properly.