On Tue, 3 Sep 2019 at 17:22, Daniel Borkmann [off-list ref] wrote:

On 8/26/19 8:10 AM, Björn Töpel wrote:

quoted

From: Björn Töpel <redacted>

The state variable was read, and written outside the control mutex
(struct xdp_sock, mutex), without proper barriers and {READ,
WRITE}_ONCE correctness.

In this commit this issue is addressed, and the state member is now
used a point of synchronization whether the socket is setup correctly
or not.

This also fixes a race, found by syzcaller, in xsk_poll() where umem
could be accessed when stale.

Suggested-by: Hillf Danton <redacted>
Reported-by: syzbot+c82697e3043781e08802@syzkaller.appspotmail.com
Fixes: 77cd0d7b3f25 ("xsk: add support for need_wakeup flag in AF_XDP rings")
Signed-off-by: Björn Töpel <redacted>

Sorry for the delay.

quoted

---
  net/xdp/xsk.c | 57 ++++++++++++++++++++++++++++++++++++---------------
  1 file changed, 40 insertions(+), 17 deletions(-)

diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index f3351013c2a5..8fafa3ce3ae6 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c

@@ -162,10 +162,23 @@ static int __xsk_rcv_zc(struct xdp_sock *xs, struct xdp_buff *xdp, u32 len)
      return err;
  }

+static bool xsk_is_bound(struct xdp_sock *xs)
+{
+     if (READ_ONCE(xs->state) == XSK_BOUND) {
+             /* Matches smp_wmb() in bind(). */
+             smp_rmb();
+             return true;
+     }
+     return false;
+}
+
  int xsk_rcv(struct xdp_sock *xs, struct xdp_buff *xdp)
  {
      u32 len;

+     if (!xsk_is_bound(xs))
+             return -EINVAL;
+
      if (xs->dev != xdp->rxq->dev || xs->queue_id != xdp->rxq->queue_index)
              return -EINVAL;

@@ -362,7 +375,7 @@ static int xsk_sendmsg(struct socket *sock, struct msghdr *m, size_t total_len)
      struct sock *sk = sock->sk;
      struct xdp_sock *xs = xdp_sk(sk);

-     if (unlikely(!xs->dev))
+     if (unlikely(!xsk_is_bound(xs)))
              return -ENXIO;
      if (unlikely(!(xs->dev->flags & IFF_UP)))
              return -ENETDOWN;

@@ -378,10 +391,15 @@ static unsigned int xsk_poll(struct file *file, struct socket *sock,
                           struct poll_table_struct *wait)
  {
      unsigned int mask = datagram_poll(file, sock, wait);
-     struct sock *sk = sock->sk;
-     struct xdp_sock *xs = xdp_sk(sk);
-     struct net_device *dev = xs->dev;
-     struct xdp_umem *umem = xs->umem;
+     struct xdp_sock *xs = xdp_sk(sock->sk);
+     struct net_device *dev;
+     struct xdp_umem *umem;
+
+     if (unlikely(!xsk_is_bound(xs)))
+             return mask;
+
+     dev = xs->dev;
+     umem = xs->umem;

      if (umem->need_wakeup)
              dev->netdev_ops->ndo_xsk_wakeup(dev, xs->queue_id,

@@ -417,10 +435,9 @@ static void xsk_unbind_dev(struct xdp_sock *xs)
  {
      struct net_device *dev = xs->dev;

-     if (!dev || xs->state != XSK_BOUND)
+     if (xs->state != XSK_BOUND)
              return;
-
-     xs->state = XSK_UNBOUND;
+     WRITE_ONCE(xs->state, XSK_UNBOUND);

      /* Wait for driver to stop using the xdp socket. */
      xdp_del_sk_umem(xs->umem, xs);

@@ -495,7 +512,9 @@ static int xsk_release(struct socket *sock)
      local_bh_enable();

      xsk_delete_from_maps(xs);
+     mutex_lock(&xs->mutex);
      xsk_unbind_dev(xs);
+     mutex_unlock(&xs->mutex);

      xskq_destroy(xs->rx);
      xskq_destroy(xs->tx);

@@ -589,19 +608,18 @@ static int xsk_bind(struct socket *sock, struct sockaddr *addr, int addr_len)
              }

              umem_xs = xdp_sk(sock->sk);
-             if (!umem_xs->umem) {
-                     /* No umem to inherit. */
+             if (!xsk_is_bound(umem_xs)) {
                      err = -EBADF;
                      sockfd_put(sock);
                      goto out_unlock;
-             } else if (umem_xs->dev != dev || umem_xs->queue_id != qid) {
+             }
+             if (umem_xs->dev != dev || umem_xs->queue_id != qid) {
                      err = -EINVAL;
                      sockfd_put(sock);
                      goto out_unlock;
              }
-
              xdp_get_umem(umem_xs->umem);
-             xs->umem = umem_xs->umem;
+             WRITE_ONCE(xs->umem, umem_xs->umem);
              sockfd_put(sock);
      } else if (!xs->umem || !xdp_umem_validate_queues(xs->umem)) {
              err = -EINVAL;

@@ -626,10 +644,15 @@ static int xsk_bind(struct socket *sock, struct sockaddr *addr, int addr_len)
      xdp_add_sk_umem(xs->umem, xs);

  out_unlock:
-     if (err)
+     if (err) {
              dev_put(dev);
-     else
-             xs->state = XSK_BOUND;
+     } else {
+             /* Matches smp_rmb() in bind() for shared umem
+              * sockets, and xsk_is_bound().
+              */
+             smp_wmb();

You write with what this barrier matches/pairs, but would be useful for readers
to have an explanation against what it protects. I presume to have things like
xs->umem public as you seem to guard it behind xsk_is_bound() in xsk_poll() and
other cases? Would be great to have a detailed analysis of all this e.g. in the
commit message so one wouldn't need to guess; right now it feels this is doing
many things at once and w/o further explanation of why READ_ONCE() or others are
omitted sometimes. Would be great to get a lot more clarity into this, perhaps
splitting it up a bit might also help.

I'll address that. Thanks for the review!


Björn

quoted

+             WRITE_ONCE(xs->state, XSK_BOUND);
+     }

Thanks,
Daniel