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Date: Tue, 3 Sep 2019 17:26:53 +0200
From: Björn Töpel <bjorn.topel@...il.com>
To: Daniel Borkmann <daniel@...earbox.net>
Cc: Alexei Starovoitov <ast@...nel.org>,
Netdev <netdev@...r.kernel.org>,
Björn Töpel <bjorn.topel@...el.com>,
"Karlsson, Magnus" <magnus.karlsson@...el.com>,
Magnus Karlsson <magnus.karlsson@...il.com>,
bpf <bpf@...r.kernel.org>,
Jonathan Lemon <jonathan.lemon@...il.com>,
syzbot+c82697e3043781e08802@...kaller.appspotmail.com,
Hillf Danton <hdanton@...a.com>,
Ilya Maximets <i.maximets@...sung.com>
Subject: Re: [PATCH bpf-next v2 2/4] xsk: add proper barriers and {READ,
WRITE}_ONCE-correctness for state
On Tue, 3 Sep 2019 at 17:22, Daniel Borkmann <daniel@...earbox.net> wrote:
>
> On 8/26/19 8:10 AM, Björn Töpel wrote:
> > From: Björn Töpel <bjorn.topel@...el.com>
> >
> > 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 <hdanton@...a.com>
> > Reported-by: syzbot+c82697e3043781e08802@...kaller.appspotmail.com
> > Fixes: 77cd0d7b3f25 ("xsk: add support for need_wakeup flag in AF_XDP rings")
> > Signed-off-by: Björn Töpel <bjorn.topel@...el.com>
>
> Sorry for the delay.
>
> > ---
> > 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
> > + WRITE_ONCE(xs->state, XSK_BOUND);
> > + }
>
> Thanks,
> Daniel
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