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Date: Tue, 3 Sep 2019 17:22:11 +0200
From: Daniel Borkmann <daniel@...earbox.net>
To: Björn Töpel <bjorn.topel@...il.com>,
ast@...nel.org, netdev@...r.kernel.org
Cc: Björn Töpel <bjorn.topel@...el.com>,
magnus.karlsson@...el.com, magnus.karlsson@...il.com,
bpf@...r.kernel.org, jonathan.lemon@...il.com,
syzbot+c82697e3043781e08802@...kaller.appspotmail.com,
hdanton@...a.com, i.maximets@...sung.com
Subject: Re: [PATCH bpf-next v2 2/4] xsk: add proper barriers and {READ,
WRITE}_ONCE-correctness for state
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.
> + WRITE_ONCE(xs->state, XSK_BOUND);
> + }
Thanks,
Daniel
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