Re: [PATCH bpf-next] xsk: honor SO_BINDTODEVICE on bind

[Jason Wang]

On Tue, Jul 4, 2023 at 1:53 AM Ilya Maximets <i.maximets@xxxxxxx> wrote:
>
> Initial creation of an AF_XDP socket requires CAP_NET_RAW capability.
> A privileged process might create the socket and pass it to a
> non-privileged process for later use.  However, that process will be
> able to bind the socket to any network interface.  Even though it will
> not be able to receive any traffic without modification of the BPF map,
> the situation is not ideal.
>
> Sockets already have a mechanism that can be used to restrict what
> interface they can be attached to.  That is SO_BINDTODEVICE.
>
> To change the SO_BINDTODEVICE binding the process will need CAP_NET_RAW.
>
> Make xsk_bind() honor the SO_BINDTODEVICE in order to allow safer
> workflow when non-privileged process is using AF_XDP.
>
> The intended workflow is following:
>
>   1. First process creates a bare socket with socket(AF_XDP, ...).
>   2. First process loads the XSK program to the interface.
>   3. First process adds the socket fd to a BPF map.
>   4. First process ties socket fd to a particular interface using
>      SO_BINDTODEVICE.
>   5. First process sends socket fd to a second process.
>   6. Second process allocates UMEM.
>   7. Second process binds socket to the interface with bind(...).
>   8. Second process sends/receives the traffic.
>
> All the steps above are possible today if the first process is
> privileged and the second one has sufficient RLIMIT_MEMLOCK and no
> capabilities.  However, the second process will be able to bind the
> socket to any interface it wants on step 7 and send traffic from it.
> With the proposed change, the second process will be able to bind
> the socket only to a specific interface chosen by the first process
> at step 4.
>
> Acked-by: Magnus Karlsson <magnus.karlsson@xxxxxxxxx>
> Signed-off-by: Ilya Maximets <i.maximets@xxxxxxx>

Acked-by: Jason Wang <jasowang@xxxxxxxxxx>

Is this a stable material or not?

Thanks

> ---
>
> RFC --> PATCH:
>   * Better explained intended workflow in a commit message.
>   * Added ACK from Magnus.
>
>  Documentation/networking/af_xdp.rst | 9 +++++++++
>  net/xdp/xsk.c                       | 6 ++++++
>  2 files changed, 15 insertions(+)
>
> diff --git a/Documentation/networking/af_xdp.rst b/Documentation/networking/af_xdp.rst
> index 247c6c4127e9..1cc35de336a4 100644
> --- a/Documentation/networking/af_xdp.rst
> +++ b/Documentation/networking/af_xdp.rst
> @@ -433,6 +433,15 @@ start N bytes into the buffer leaving the first N bytes for the
>  application to use. The final option is the flags field, but it will
>  be dealt with in separate sections for each UMEM flag.
>
> +SO_BINDTODEVICE setsockopt
> +--------------------------
> +
> +This is a generic SOL_SOCKET option that can be used to tie AF_XDP
> +socket to a particular network interface.  It is useful when a socket
> +is created by a privileged process and passed to a non-privileged one.
> +Once the option is set, kernel will refuse attempts to bind that socket
> +to a different interface.  Updating the value requires CAP_NET_RAW.
> +
>  XDP_STATISTICS getsockopt
>  -------------------------
>
> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index 5a8c0dd250af..386ff641db0f 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -886,6 +886,7 @@ static int xsk_bind(struct socket *sock, struct sockaddr *addr, int addr_len)
>         struct sock *sk = sock->sk;
>         struct xdp_sock *xs = xdp_sk(sk);
>         struct net_device *dev;
> +       int bound_dev_if;
>         u32 flags, qid;
>         int err = 0;
>
> @@ -899,6 +900,11 @@ static int xsk_bind(struct socket *sock, struct sockaddr *addr, int addr_len)
>                       XDP_USE_NEED_WAKEUP))
>                 return -EINVAL;
>
> +       bound_dev_if = READ_ONCE(sk->sk_bound_dev_if);
> +
> +       if (bound_dev_if && bound_dev_if != sxdp->sxdp_ifindex)
> +               return -EINVAL;
> +
>         rtnl_lock();
>         mutex_lock(&xs->mutex);
>         if (xs->state != XSK_READY) {
> --
> 2.40.1
>