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

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Date:   Wed, 4 May 2022 06:25:49 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     patchwork-bot+netdevbpf@...nel.org,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Santosh Shilimkar <santosh.shilimkar@...cle.com>,
        David 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>,
        linux-rdma <linux-rdma@...r.kernel.org>
Subject: Re: [PATCH v2] net: rds: acquire refcount on TCP sockets

On Wed, May 4, 2022 at 6:09 AM Paolo Abeni <pabeni@...hat.com> wrote:
>
> On Tue, 2022-05-03 at 14:17 -0700, Eric Dumazet wrote:
> > On Tue, May 3, 2022 at 4:40 AM <patchwork-bot+netdevbpf@...nel.org> wrote:
> > >
> > > Hello:
> > >
> > > This patch was applied to netdev/net.git (master)
> > > by Paolo Abeni <pabeni@...hat.com>:
> > >
> > > On Mon, 2 May 2022 10:40:18 +0900 you 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>
> > > >
> > > > [...]
> > >
> > > Here is the summary with links:
> > >   - [v2] net: rds: acquire refcount on TCP sockets
> > >     https://git.kernel.org/netdev/net/c/3a58f13a881e
> > >
> > > You are awesome, thank you!
> > > --
> > > Deet-doot-dot, I am a bot.
> > > https://korg.docs.kernel.org/patchwork/pwbot.html
> > >
> > >
> >
> > I think we merged this patch too soon.
>
> My fault.
>
>
> > My question is : What prevents rds_tcp_conn_path_connect(), and thus
> > rds_tcp_tune() to be called
> > after the netns refcount already reached 0 ?
> >
> > I guess we can wait for next syzbot report, but I think that get_net()
> > should be replaced
> > by maybe_get_net()
> >
> Should we revert this patch before the next pull request, if a suitable
> incremental fix is not available by then?
>
> It looks like the window of opportunity for the race is roughly the
> same?
>

No need to revert the patch, we certainly are in a better situation,
as refcount_t helps here.

We can refine the logic in a followup.

Thanks.