netdev - Re: net/l2tp:BUG: KASAN: use-after-free in l2tp_ip6

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Date:   Wed, 16 Nov 2016 22:07:41 +0100
From:   Guillaume Nault <g.nault@...halink.fr>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Baozeng Ding <sploving1@...il.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: net/l2tp:BUG: KASAN: use-after-free in l2tp_ip6_close

On Wed, Nov 16, 2016 at 11:08:23AM -0800, Cong Wang wrote:
> On Wed, Nov 16, 2016 at 8:30 AM, Guillaume Nault <g.nault@...halink.fr> wrote:
> > diff --git a/net/l2tp/l2tp_ip.c b/net/l2tp/l2tp_ip.c
> > index fce25af..982f6c4 100644
> > --- a/net/l2tp/l2tp_ip.c
> > +++ b/net/l2tp/l2tp_ip.c
> > @@ -251,8 +251,6 @@ static int l2tp_ip_bind(struct sock *sk, struct sockaddr *uaddr, int addr_len)
> >         int ret;
> >         int chk_addr_ret;
> >
> > -       if (!sock_flag(sk, SOCK_ZAPPED))
> > -               return -EINVAL;
> >         if (addr_len < sizeof(struct sockaddr_l2tpip))
> >                 return -EINVAL;
> >         if (addr->l2tp_family != AF_INET)
> > @@ -267,6 +265,9 @@ static int l2tp_ip_bind(struct sock *sk, struct sockaddr *uaddr, int addr_len)
> >         read_unlock_bh(&l2tp_ip_lock);
> >
> >         lock_sock(sk);
> > +       if (!sock_flag(sk, SOCK_ZAPPED))
> > +               goto out;
> > +
> >         if (sk->sk_state != TCP_CLOSE || addr_len < sizeof(struct sockaddr_l2tpip))
> >                 goto out;
> >
> > diff --git a/net/l2tp/l2tp_ip6.c b/net/l2tp/l2tp_ip6.c
> > index ad3468c..9978d01 100644
> > --- a/net/l2tp/l2tp_ip6.c
> > +++ b/net/l2tp/l2tp_ip6.c
> > @@ -269,8 +269,6 @@ static int l2tp_ip6_bind(struct sock *sk, struct sockaddr *uaddr, int addr_len)
> >         int addr_type;
> >         int err;
> >
> > -       if (!sock_flag(sk, SOCK_ZAPPED))
> > -               return -EINVAL;
> >         if (addr->l2tp_family != AF_INET6)
> >                 return -EINVAL;
> >         if (addr_len < sizeof(*addr))
> > @@ -296,6 +294,9 @@ static int l2tp_ip6_bind(struct sock *sk, struct sockaddr *uaddr, int addr_len)
> >         lock_sock(sk);
> >
> >         err = -EINVAL;
> > +       if (!sock_flag(sk, SOCK_ZAPPED))
> > +               goto out_unlock;
> > +
> >         if (sk->sk_state != TCP_CLOSE)
> >                 goto out_unlock;
> 
> 
> Makes sense, it should prevent a concurrent caller adding the socket
> into bind table
> twice after passing __l2tp_ip_bind_lookup() check.

Yes, and the __l2tp_ip_bind_lookup() call is also racy. But, by
properly checking the SOCK_ZAPPED flag, we probably can remove this
call entirely.

For now, I only wanted to make sure the issue was well identified. I'll
submit a more complete patch for net (with protected SOCK_ZAPPED check
in l2tp_ip_connect() too).