Re: [PATCH v2 1/3] unix: fix use-after-free in unix_dgram_poll()

[Mathias Krause <minipli@googlemail.com>]

On 2 October 2015 at 22:43, Jason Baron <jbaron@akamai.com> wrote:
> The unix_dgram_poll() routine calls sock_poll_wait() not only for the wait
> queue associated with the socket s that we are poll'ing against, but also calls
> sock_poll_wait() for a remote peer socket p, if it is connected. Thus,
> if we call poll()/select()/epoll() for the socket s, there are then
> a couple of code paths in which the remote peer socket p and its associated
> peer_wait queue can be freed before poll()/select()/epoll() have a chance
> to remove themselves from the remote peer socket.
>
> The way that remote peer socket can be freed are:
>
> 1. If s calls connect() to a connect to a new socket other than p, it will
> drop its reference on p, and thus a close() on p will free it.
>
> 2. If we call close on p(), then a subsequent sendmsg() from s, will drop
> the final reference to p, allowing it to be freed.
>
> Address this issue, by reverting unix_dgram_poll() to only register with
> the wait queue associated with s and register a callback with the remote peer
> socket on connect() that will wake up the wait queue associated with s. If
> scenarios 1 or 2 occur above we then simply remove the callback from the
> remote peer. This then presents the expected semantics to poll()/select()/
> epoll().
>
> I've implemented this for sock-type, SOCK_RAW, SOCK_DGRAM, and SOCK_SEQPACKET
> but not for SOCK_STREAM, since SOCK_STREAM does not use unix_dgram_poll().
>
> Signed-off-by: Jason Baron <jbaron@akamai.com>
> ---
>  include/net/af_unix.h |  1 +
>  net/unix/af_unix.c    | 32 +++++++++++++++++++++++++++++++-
>  2 files changed, 32 insertions(+), 1 deletion(-)
>
> diff --git a/include/net/af_unix.h b/include/net/af_unix.h
> index 4a167b3..9698aff 100644
> --- a/include/net/af_unix.h
> +++ b/include/net/af_unix.h
> @@ -62,6 +62,7 @@ struct unix_sock {
>  #define UNIX_GC_CANDIDATE      0
>  #define UNIX_GC_MAYBE_CYCLE    1
>         struct socket_wq        peer_wq;
> +       wait_queue_t            wait;
>  };
>  #define unix_sk(__sk) ((struct unix_sock *)__sk)
>
> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
> index 03ee4d3..f789423 100644
> --- a/net/unix/af_unix.c
> +++ b/net/unix/af_unix.c
> @@ -420,6 +420,9 @@ static void unix_release_sock(struct sock *sk, int embrion)
>         skpair = unix_peer(sk);
>
>         if (skpair != NULL) {
> +               if (sk->sk_type != SOCK_STREAM)
> +                       remove_wait_queue(&unix_sk(skpair)->peer_wait,
> +                                         &u->wait);
>                 if (sk->sk_type == SOCK_STREAM || sk->sk_type == SOCK_SEQPACKET) {
>                         unix_state_lock(skpair);
>                         /* No more writes */
> @@ -636,6 +639,16 @@ static struct proto unix_proto = {
>   */
>  static struct lock_class_key af_unix_sk_receive_queue_lock_key;
>
> +static int peer_wake(wait_queue_t *wait, unsigned mode, int sync, void *key)
> +{
> +       struct unix_sock *u;
> +
> +       u = container_of(wait, struct unix_sock, wait);
> +       wake_up_interruptible_sync_poll(sk_sleep(&u->sk), key);
> +
> +       return 0;
> +}
> +
>  static struct sock *unix_create1(struct net *net, struct socket *sock, int kern)
>  {
>         struct sock *sk = NULL;
> @@ -664,6 +677,7 @@ static struct sock *unix_create1(struct net *net, struct socket *sock, int kern)
>         INIT_LIST_HEAD(&u->link);
>         mutex_init(&u->readlock); /* single task reading lock */
>         init_waitqueue_head(&u->peer_wait);
> +       init_waitqueue_func_entry(&u->wait, peer_wake);
>         unix_insert_socket(unix_sockets_unbound(sk), sk);
>  out:
>         if (sk == NULL)
> @@ -1030,7 +1044,11 @@ restart:
>          */
>         if (unix_peer(sk)) {
>                 struct sock *old_peer = unix_peer(sk);
> +
> +               remove_wait_queue(&unix_sk(old_peer)->peer_wait,
> +                                 &unix_sk(sk)->wait);
>                 unix_peer(sk) = other;
> +               add_wait_queue(&unix_sk(other)->peer_wait, &unix_sk(sk)->wait);
>                 unix_state_double_unlock(sk, other);
>
>                 if (other != old_peer)
> @@ -1038,8 +1056,12 @@ restart:
>                 sock_put(old_peer);
>         } else {
>                 unix_peer(sk) = other;
> +               add_wait_queue(&unix_sk(other)->peer_wait, &unix_sk(sk)->wait);
>                 unix_state_double_unlock(sk, other);
>         }
> +       /* New remote may have created write space for us */
> +       wake_up_interruptible_sync_poll(sk_sleep(sk),
> +                                       POLLOUT | POLLWRNORM | POLLWRBAND);
>         return 0;
>
>  out_unlock:
> @@ -1194,6 +1216,8 @@ restart:
>
>         sock_hold(sk);
>         unix_peer(newsk)        = sk;
> +       if (sk->sk_type == SOCK_SEQPACKET)
> +               add_wait_queue(&unix_sk(sk)->peer_wait, &unix_sk(newsk)->wait);
>         newsk->sk_state         = TCP_ESTABLISHED;
>         newsk->sk_type          = sk->sk_type;
>         init_peercred(newsk);
> @@ -1220,6 +1244,8 @@ restart:
>
>         smp_mb__after_atomic(); /* sock_hold() does an atomic_inc() */
>         unix_peer(sk)   = newsk;
> +       if (sk->sk_type == SOCK_SEQPACKET)
> +               add_wait_queue(&unix_sk(newsk)->peer_wait, &unix_sk(sk)->wait);
>
>         unix_state_unlock(sk);
>
> @@ -1254,6 +1280,10 @@ static int unix_socketpair(struct socket *socka, struct socket *sockb)
>         sock_hold(skb);
>         unix_peer(ska) = skb;
>         unix_peer(skb) = ska;
> +       if (ska->sk_type != SOCK_STREAM) {
> +               add_wait_queue(&unix_sk(ska)->peer_wait, &unix_sk(skb)->wait);
> +               add_wait_queue(&unix_sk(skb)->peer_wait, &unix_sk(ska)->wait);
> +       }
>         init_peercred(ska);
>         init_peercred(skb);
>
> @@ -1565,6 +1595,7 @@ restart:
>                 unix_state_lock(sk);
>                 if (unix_peer(sk) == other) {
>                         unix_peer(sk) = NULL;
> +                       remove_wait_queue(&unix_sk(other)->peer_wait, &u->wait);
>                         unix_state_unlock(sk);
>
>                         unix_dgram_disconnected(sk, other);
> @@ -2441,7 +2472,6 @@ static unsigned int unix_dgram_poll(struct file *file, struct socket *sock,
>         other = unix_peer_get(sk);
>         if (other) {
>                 if (unix_peer(other) != sk) {
> -                       sock_poll_wait(file, &unix_sk(other)->peer_wait, wait);
>                         if (unix_recvq_full(other))
>                                 writable = 0;
>                 }
> --
> 1.8.2.rc2
>

My reproducer runs on this patch for more than 3 days now without
triggering anything anymore.

Tested-by: Mathias Krause <minipli@googlemail.com>

Thanks Jason!

Regards,
Mathias