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

[Ilya Maximets]

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.

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?

This workflow allows the second process to have no capabilities
as long as it has sufficient RLIMIT_MEMLOCK.

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
>>
>>