[RFC PATCH 0/1] xattr: Allow user.* xattr on symlink/special files if caller has CAP_SYS_RESOURCE

[Casey Schaufler]

On 6/29/2021 9:35 AM, Dr. David Alan Gilbert wrote:
> * Casey Schaufler (casey at schaufler-ca.com) wrote:
>> On 6/29/2021 8:20 AM, Vivek Goyal wrote:
>>> On Tue, Jun 29, 2021 at 07:38:15AM -0700, Casey Schaufler wrote:
>>>
>>> [..]
>>>>>>>> User xattrs are less protected than security xattrs. You are exposing the
>>>>>>>> security xattrs on the guest to the possible whims of a malicious, unprivileged
>>>>>>>> actor on the host. All it needs is the right UID.
>>>>>>> Yep, we realise that; but when you're mainly interested in making sure
>>>>>>> the guest can't attack the host, that's less worrying.
>>>>>> That's uncomfortable.
>>>>> Why exactly?
>>>> If a mechanism is designed with a known vulnerability you
>>>> fail your validation/evaluation efforts.
>>> We are working with the constraint that shared directory should not be
>>> accessible to unpriviliged users on host. And with that constraint, what
>>> you are referring to is not a vulnerability.
>> Sure, that's quite reasonable for your use case. It doesn't mean
>> that the vulnerability doesn't exist, it means you've mitigated it. 
>>
>>
>>>> Your mechanism is
>>>> less general because other potential use cases may not be
>>>> as cavalier about the vulnerability.
>>> Prefixing xattrs with "user.virtiofsd" is just one of the options.
>>> virtiofsd has the capability to prefix "trusted.virtiofsd" as well.
>>> We have not chosen that because we don't want to give it CAP_SYS_ADMIN.
>>>
>>> So other use cases which don't like prefixing "user.virtiofsd", can
>>> give CAP_SYS_ADMIN and work with it.
>>>
>>>> I think that you can
>>>> approach this differently, get a solution that does everything
>>>> you want, and avoid the known problem.
>>> What's the solution? Are you referring to using "trusted.*" instead? But
>>> that has its own problem of giving CAP_SYS_ADMIN to virtiofsd.
>> I'm coming to the conclusion that xattr namespaces, analogous
>> to user namespaces, are the correct solution. They generalize
>> for multiple filesystem and LSM use cases. The use of namespaces
>> is well understood, especially in the container community. It
>> looks to me as if it would address your use case swimmingly.
> Yeh; although the details of getting the semantics right is tricky;
> in particular, the stuff which clears capabilitiies/setuid/etc on writes
> - should it clear xattrs that represent capabilities?  If the host
>   performs a write, should it clear mapped xattrs capabilities?  If the
> namespace performs a write should it clear just the mapped ones or the
> host ones as well?  Our virtiofsd code performs acrobatics to make
> sure they get cleared on write that are painful.

Dealing with tricky semantics is the difference between a feature
and a hack. Doing so in a way that other people can take advantage
of the feature is the hallmark of a feature well done.

>
> Dave
>
>>> Thanks
>>> Vivek
>>>