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

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Message-ID: <8783dad64b0d41af9624f923cb4e4f03@AcuMS.aculab.com>
Date:   Tue, 3 May 2022 13:27:30 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Paolo Abeni' <pabeni@...hat.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Eric Dumazet <edumazet@...gle.com>
CC:     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

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.

	David

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