[PATCH v9 12/12] landlock: Document Landlock's network support

[Konstantin Meskhidze (A)]

3/6/2023 7:09 PM, Mickaël Salaün пишет:
> 
> On 06/03/2023 14:43, Konstantin Meskhidze (A) wrote:
>> 
>> 
>> 2/21/2023 7:16 PM, Mickaël Salaün пишет:
>>>
>>> On 30/01/2023 11:03, Konstantin Meskhidze (A) wrote:
>>>>
>>>>
>>>> 1/27/2023 9:22 PM, Mickaël Salaün пишет:
>>>>>
>>>>> On 23/01/2023 10:38, Konstantin Meskhidze (A) wrote:
>>>>>>
>>>>>>
>>>>>> 1/22/2023 2:07 AM, Günther Noack пишет:
>>>>>
>>>>> [...]
>>>>>
>>>>>>>> @@ -143,10 +157,24 @@ for the ruleset creation, by filtering access rights according to the Landlock
>>>>>>>>     ABI version.  In this example, this is not required because all of the requested
>>>>>>>>     ``allowed_access`` rights are already available in ABI 1.
>>>>>>>>     
>>>>>>>> -We now have a ruleset with one rule allowing read access to ``/usr`` while
>>>>>>>> -denying all other handled accesses for the filesystem.  The next step is to
>>>>>>>> -restrict the current thread from gaining more privileges (e.g. thanks to a SUID
>>>>>>>> -binary).
>>>>>>>> +For network access-control, we can add a set of rules that allow to use a port
>>>>>>>> +number for a specific action. All ports values must be defined in network byte
>>>>>>>> +order.
>>>>>>>
>>>>>>> What is the point of asking user space to convert this to network byte
>>>>>>> order? It seems to me that the kernel would be able to convert it to
>>>>>>> network byte order very easily internally and in a single place -- why
>>>>>>> ask all of the users to deal with that complexity? Am I overlooking
>>>>>>> something?
>>>>>>
>>>>>>      I had a discussion about this issue with Mickaёl.
>>>>>>      Please check these threads:
>>>>>>      1.
>>>>>> https://lore.kernel.org/netdev/49391484-7401-e7c7-d909-3bd6bd024731@digikod.net/
>>>>>>      2.
>>>>>> https://lore.kernel.org/netdev/1ed20e34-c252-b849-ab92-78c82901c979@huawei.com/
>>>>>
>>>>> I'm definitely not sure if this is the right solution, or if there is
>>>>> one. The rationale is to make it close to the current (POSIX) API. We
>>>>> didn't get many opinion about that but I'd really like to have a
>>>>> discussion about port endianness for this Landlock API.
>>>>
>>>>      As for me, the kernel should take care about port converting. This
>>>> work should be done under the hood.
>>>>
>>>>      Any thoughts?
>>>>
>>>>>
>>>>> I looked at some code (e.g. see [1]) and it seems that using htons()
>>>>> might make application patching more complex after all. What do you
>>>>> think? Is there some network (syscall) API that don't use this convention?
>>>>>
>>>>> [1] https://github.com/landlock-lsm/tuto-lighttpd
>>>>>
>>>>>>>
>>>>>>>> +
>>>>>>>> +.. code-block:: c
>>>>>>>> +
>>>>>>>> +    struct landlock_net_service_attr net_service = {
>>>>>>>> +        .allowed_access = LANDLOCK_ACCESS_NET_BIND_TCP,
>>>>>>>> +        .port = htons(8080),
>>>>>>>> +    };
>>>>>>>
>>>>>>> This is a more high-level comment:
>>>>>>>
>>>>>>> The notion of a 16-bit "port" seems to be specific to TCP and UDP --
>>>>>>> how do you envision this struct to evolve if other protocols need to
>>>>>>> be supported in the future?
>>>>>>
>>>>>>       When TCP restrictions land into Linux, we need to think about UDP
>>>>>> support. Then other protocols will be on the road. Anyway you are right
>>>>>> this struct will be evolving in long term, but I don't have a particular
>>>>>> envision now. Thanks for the question - we need to think about it.
>>>>>>>
>>>>>>> Should this struct and the associated constants have "TCP" in its
>>>>>>> name, and other protocols use a separate struct in the future?
>>>>>
>>>>> Other protocols such as AF_VSOCK uses a 32-bit port. We could use a
>>>>> 32-bits port field or ever a 64-bit one. The later could make more sense
>>>>> because each field would eventually be aligned on 64-bit. Picking a
>>>>> 16-bit value was to help developers (and compilers/linters) with the
>>>>> "correct" type (for TCP).
>>>
>>> Thinking more about this, let's use a __u64 port (and remove the
>>> explicit packing). The landlock_append_net_rule() function should use a
>>> __u16 port argument, but the add_rule_net_service() function should
>>> check that there is no overflow with the port attribute (not higher than
>>> U16_MAX) before passing it to landlock_append_net_rule(). We should
>>> prioritize flexibility for the kernel UAPI over stricter types. User
>>> space libraries can improve this kind of types with a more complex API.
>>>
>>> Big endian can make sense for a pure network API because the port value
>>> (and the IP address) is passed to other machines through the network,
>>> as-is. However, with Landlock, the port value is only used by the
>>> kernel. Moreover, in practice, port values are mostly converted when
>>> filling the sockaddr*_in structs. It would then make it more risky to
>>> ask developers another explicit htons() conversion for Landlock
>>> syscalls. Let's stick to the host endianess and let the kernel do the
>>> conversion.
>>>
>>> Please include these rationales in code comments. We also need to update
>>> the tests for endianess, but still check big and little endian
>>> consistency as it is currently done in these tests. A new test should be
>>> added to check port boundaries with:
>>> - port = 0
>>> - port = U16_MAX
>>       port = U16_MAX value passes.
> 
> correct
> 
>> 
>>> - port = U16_MAX + 1 (which should get an EINVAL)
>>       port = U16_MAX + 1 after casting is 0, EINVAL is returned.
> 
> In the tests, we want the casting to be be done by the kernel. The test
> should then pass 0x10000 to the struct and the kernel should return
> EINVAL because it is greater than U16_MAX, not because it is zero.
> 
>> 
>>> - port = U16_MAX + 2 (to check u16 casting != 0)
>>       port = U16_MAX + 2 after casting is 1, is it passes?
> 
> In this case, 0x10001 should be rejected by the kernel (and return
> EINVAL) because it is greater than U16_MAX.
> 
>> 
>>> - port = U32_MAX + 1
>>> - port = U32_MAX + 2
>> 
>>       Don't you think that all port values >= U16_MAX + 1, EINVAL should
>>       be returned?
> 
> All port values > U16_MAX should indeed return EINVAL, and tests should
> check kernel casting (i.e. the kernel must check the 64-bit value before
> casting it to a 16-bit value and only check the casted zero). I didn't
> mean that these cases should pass, only that they should be tested, but
> I think you got it. ;)

   Yep. I got the point. Thanks.
> 
>>>
>>>
>>>>>
>>>>> If we think about protocols other than TCP and UDP (e.g. AF_VSOCK), it
>>>>> could make sense to have a dedicated attr struct specifying other
>>>>> properties (e.g. CID). Anyway, the API is flexible but it would be nice
>>>>> to not mess with it too much. What do you think?
>>>>>
>>>>>
>>>>>>>
>>>>>>>> +
>>>>>>>> +    err = landlock_add_rule(ruleset_fd, LANDLOCK_RULE_NET_SERVICE,
>>>>>>>> +                            &net_service, 0);
>>>>>>>> +
>>>>>>>> +The next step is to restrict the current thread from gaining more privileges
>>>>>>>> +(e.g. thanks to a SUID binary). We now have a ruleset with the first rule allowing
>>>>>>>              ^^^^^^
>>>>>>>              "through" a SUID binary? "thanks to" sounds like it's desired
>>>>>>>              to do that, while we're actually trying to prevent it here?
>>>>>>
>>>>>>       This is Mickaёl's part. Let's ask his opinion here.
>>>>>>
>>>>>>       Mickaёl, any thoughts?
>>>>>
>>>>> Yep, "through" looks better.
>>>>> .
>>> .
> .