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

[Magnus Karlsson]

On Mon, 3 Jul 2023 at 13:16, Ilya Maximets <i.maximets@xxxxxxx> wrote:
>
> On 7/3/23 12:24, Magnus Karlsson wrote:
> > On Mon, 3 Jul 2023 at 12:13, Ilya Maximets <i.maximets@xxxxxxx> wrote:
> >>
> >> On 7/3/23 12:06, Ilya Maximets wrote:
> >>> On 7/3/23 11:48, Magnus Karlsson wrote:
> >>>> On Fri, 30 Jun 2023 at 16:58, 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 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.
> >>>>
> >>>> Rebinding an AF_XDP socket is not allowed today. Any such attempt will
> >>>> return an error from bind. So if I understand the purpose of
> >>>> SO_BINDTODEVICE correctly, you could say that this option is always
> >>>> set for an AF_XDP socket and it is not possible to toggle it. The only
> >>>> way to "rebind" an AF_XDP socket is to close it and open a new one.
> >>>> This was a conscious design decision from day one as it would be very
> >>>> hard to support this, especially in zero-copy mode.
> >>>
> >>> Hi, Magnus.
> >>>
> >>> The purpose of this patch is not to allow re-binding.  The use case 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 sends socket fd to a second process.
> >>> 5. Second process allocates UMEM.
> >>> 6. Second process binds socket to the interface.
> >>
> >> 7. Second process sends/receives the traffic. :)
> >>
> >>>
> >>> The idea is that the first process will call SO_BINDTODEVICE before
> >>> sending socket fd to a second process, so the second process is limited
> >>> in to which interface it can bind the socket.
> >>>
> >>> Does that make sense?
> >
> > Thanks for explaining this to me. Yes, that makes sense and seems
> > useful. Could you please send a v2 and include the flow (1-7) above in
> > your commit message? Would be good to add one step with the setsockopt
> > SO_BINDTODEVICE before step #4 just to be clear. With those changes
> > please feel free to include my ack:
> >
> >  Acked-by: Magnus Karlsson <magnus.karlsson@xxxxxxxxx>
>
> Thanks!  I'll update the commit message with the steps above to make it
> more clear.
>
> I was planning to send a non-RFC version of this patch once the tree is
> open (in a week).  Or are the rules for bpf-next different?

Bpf-next is always open I believe.

> >
> > Thank you!
> >
> >>> This workflow allows the second process to have no capabilities
> >>> as long as it has sufficient RLIMIT_MEMLOCK.
> >>
> >> Note that steps 1-7 are working just fine today.  i.e. the umem
> >> registration, bind, ring mapping and traffic send/receive do not
> >> require any extra capabilities.
> >>
> >> We may restrict the bind() call to require CAP_NET_RAW and then
> >> allow it for sockets that had SO_BINDTODEVICE as an alternative.
> >> But restriction will break the current uAPI.
> >>
> >>>
> >>> Best regards, Ilya Maximets.
> >>>
> >>>>
> >>>>> Signed-off-by: Ilya Maximets <i.maximets@xxxxxxx>
> >>>>> ---
> >>>>>
> >>>>> Posting as an RFC for now to probably get some feedback.
> >>>>> Will re-post once the tree is open.
> >>>>>
> >>>>>  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
> >>>>>
> >>>>>
> >>>
> >>
>