Re: Re: [PATCH bpf-next v3 2/5] af_unix: add unix_stream_proto for sockmap

["Jiang Wang ." <jiang.wang@bytedance.com>]

On Wed, Aug 4, 2021 at 9:59 AM Jakub Sitnicki <jakub@cloudflare.com> wrote:
>
> On Mon, Aug 02, 2021 at 11:19 PM CEST, Jiang Wang wrote:
>
> [...]
>
> > diff --git a/net/core/sock_map.c b/net/core/sock_map.c
> > index ae5fa4338..42f50ea7a 100644
> > --- a/net/core/sock_map.c
> > +++ b/net/core/sock_map.c
> > @@ -517,9 +517,15 @@ static bool sk_is_tcp(const struct sock *sk)
> >              sk->sk_protocol == IPPROTO_TCP;
> >  }
> >
> > +static bool sk_is_unix_stream(const struct sock *sk)
> > +{
> > +     return sk->sk_type == SOCK_STREAM &&
> > +            sk->sk_protocol == PF_UNIX;
> > +}
> > +
> >  static bool sock_map_redirect_allowed(const struct sock *sk)
> >  {
> > -     if (sk_is_tcp(sk))
> > +     if (sk_is_tcp(sk) || sk_is_unix_stream(sk))
> >               return sk->sk_state != TCP_LISTEN;
> >       else
> >               return sk->sk_state == TCP_ESTABLISHED;
>
> Let me provide some context.
>
> The reason why we check != TCP_LISTEN for TCP sockets is that we want to
> allow redirect redirect to sockets that are about to transition from
> TCP_SYN_RECV to TCP_ESTABLISHED, in addition to sockets already in
> TCP_ESTABLISHED state.
>
> That's because BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB callback happens when
> socket is still in TCP_SYN_RECV state. With BPF sockops program, we can
> insert such socket into a sockmap. Hence, there is a short window of
> opportunity when we could redirect to a socket in TCP_SYN_RECV.
>
> UNIX sockets can be only in TCP_{CLOSE,LISTEN,ESTABLISHED} state,
> AFAIK. So it is sufficient to rely on the default == TCP_ESTABLISHED
> check.
>
Got it. Thanks for the explanation. I will change unix sockets to only
check == TCP_ESTABLISHED condition.

> > diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
> > index 0ae3fc4c8..9c1711c67 100644
> > --- a/net/unix/af_unix.c
> > +++ b/net/unix/af_unix.c
> > @@ -791,17 +791,35 @@ static void unix_close(struct sock *sk, long timeout)
> >        */
> >  }
> >
> > -struct proto unix_proto = {
> > -     .name                   = "UNIX",
> > +static void unix_unhash(struct sock *sk)
> > +{
> > +     /* Nothing to do here, unix socket does not need a ->unhash().
> > +      * This is merely for sockmap.
> > +      */
> > +}
> > +
> > +struct proto unix_dgram_proto = {
> > +     .name                   = "UNIX-DGRAM",
> > +     .owner                  = THIS_MODULE,
> > +     .obj_size               = sizeof(struct unix_sock),
> > +     .close                  = unix_close,
> > +#ifdef CONFIG_BPF_SYSCALL
> > +     .psock_update_sk_prot   = unix_dgram_bpf_update_proto,
> > +#endif
> > +};
> > +
> > +struct proto unix_stream_proto = {
> > +     .name                   = "UNIX-STREAM",
> >       .owner                  = THIS_MODULE,
> >       .obj_size               = sizeof(struct unix_sock),
> >       .close                  = unix_close,
> > +     .unhash                 = unix_unhash,
> >  #ifdef CONFIG_BPF_SYSCALL
> > -     .psock_update_sk_prot   = unix_bpf_update_proto,
> > +     .psock_update_sk_prot   = unix_stream_bpf_update_proto,
> >  #endif
> >  };
> >
> > -static struct sock *unix_create1(struct net *net, struct socket *sock, int kern)
> > +static struct sock *unix_create1(struct net *net, struct socket *sock, int kern, int type)
> >  {
> >       struct sock *sk = NULL;
> >       struct unix_sock *u;
> > @@ -810,7 +828,11 @@ static struct sock *unix_create1(struct net *net, struct socket *sock, int kern)
> >       if (atomic_long_read(&unix_nr_socks) > 2 * get_max_files())
> >               goto out;
> >
> > -     sk = sk_alloc(net, PF_UNIX, GFP_KERNEL, &unix_proto, kern);
> > +     if (type == SOCK_STREAM)
> > +             sk = sk_alloc(net, PF_UNIX, GFP_KERNEL, &unix_stream_proto, kern);
> > +     else /*dgram and  seqpacket */
> > +             sk = sk_alloc(net, PF_UNIX, GFP_KERNEL, &unix_dgram_proto, kern);
> > +
> >       if (!sk)
> >               goto out;
> >
> > @@ -872,7 +894,7 @@ static int unix_create(struct net *net, struct socket *sock, int protocol,
> >               return -ESOCKTNOSUPPORT;
> >       }
> >
> > -     return unix_create1(net, sock, kern) ? 0 : -ENOMEM;
> > +     return unix_create1(net, sock, kern, sock->type) ? 0 : -ENOMEM;
> >  }
> >
> >  static int unix_release(struct socket *sock)
> > @@ -1286,7 +1308,7 @@ static int unix_stream_connect(struct socket *sock, struct sockaddr *uaddr,
> >       err = -ENOMEM;
> >
> >       /* create new sock for complete connection */
> > -     newsk = unix_create1(sock_net(sk), NULL, 0);
> > +     newsk = unix_create1(sock_net(sk), NULL, 0, sock->type);
> >       if (newsk == NULL)
> >               goto out;
> >
> > @@ -2214,7 +2236,7 @@ static int unix_dgram_recvmsg(struct socket *sock, struct msghdr *msg, size_t si
> >       struct sock *sk = sock->sk;
> >
> >  #ifdef CONFIG_BPF_SYSCALL
> > -     if (sk->sk_prot != &unix_proto)
> > +     if (sk->sk_prot != &unix_dgram_proto)
> >               return sk->sk_prot->recvmsg(sk, msg, size, flags & MSG_DONTWAIT,
> >                                           flags & ~MSG_DONTWAIT, NULL);
> >  #endif
>
>
> KASAN might be unhappy about access to sk->sk_prot not annotated with
> READ_ONCE. In unix_bpf we have WRITE_ONCE(sk->sk_prot, ...) [1]
>
Got it.  Will check and add READ_ONCE if necessary.

> [...]
>
> [1] https://github.com/google/ktsan/wiki/READ_ONCE-and-WRITE_ONCE#why-kernel-code-should-use-read_once-and-write_once-for-shared-memory-accesses