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

[Ilya Maximets]

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?

> 
> 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
>>>>>
>>>>>
>>>
>>