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Reply-To: dwalsh@redhat.com
Subject: Re: [RFC PATCH 0/1] xattr: Allow user.* xattr on symlink/special
 files if caller has CAP_SYS_RESOURCE
To:     Vivek Goyal <vgoyal@redhat.com>,
        Casey Schaufler <casey@schaufler-ca.com>
Cc:     "Dr. David Alan Gilbert" <dgilbert@redhat.com>,
        "Schaufler, Casey" <casey.schaufler@intel.com>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "viro@zeniv.linux.org.uk" <viro@zeniv.linux.org.uk>,
        "virtio-fs@redhat.com" <virtio-fs@redhat.com>,
        "berrange@redhat.com" <berrange@redhat.com>,
        linux-security-module <linux-security-module@vger.kernel.org>,
        "selinux@vger.kernel.org" <selinux@vger.kernel.org>
References: <BN0PR11MB57275823CE05DED7BC755460FD069@BN0PR11MB5727.namprd11.prod.outlook.com>
 <20210628131708.GA1803896@redhat.com>
 <1b446468-dcf8-9e21-58d3-c032686eeee5@redhat.com>
 <5d8f033c-eba2-7a8b-f19a-1005bbb615ea@schaufler-ca.com>
 <YNn4p+Zn444Sc4V+@work-vm>
 <a13f2861-7786-09f4-99a8-f0a5216d0fb1@schaufler-ca.com>
 <YNrhQ9XfcHTtM6QA@work-vm>
 <e6f9ed0d-c101-01df-3dff-85c1b38f9714@schaufler-ca.com>
 <20210629152007.GC5231@redhat.com>
 <78663f5c-d2fd-747a-48e3-0c5fd8b40332@schaufler-ca.com>
 <20210629173530.GD5231@redhat.com>
From:   Daniel Walsh <dwalsh@redhat.com>
Organization: Red Hat
Message-ID: <f4992b3a-a939-5bc4-a5da-0ce8913bd569@redhat.com>
Date:   Tue, 29 Jun 2021 16:28:24 -0400
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On 6/29/21 13:35, Vivek Goyal wrote:
> On Tue, Jun 29, 2021 at 09:13:48AM -0700, Casey Schaufler 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.
> Even if xattrs were namespaced, I am not sure it solves the issue
> of unpriviliged UID being able to modify security xattrs of file.
> If it happens to be correct UID, it should be able to spin up a
> user namespace and modify namespaced xattrs?
>
> Anyway, once namespaced xattrs are available, I will gladly make use
> of it. But that probably should not be a blocker for this patch.
>
> Vivek
>
All this conversation is great, and I look forward to a better solution, 
but if we go back to the patch, it was to fix an issue where the kernel 
is requiring CAP_SYS_ADMIN for writing user Xattrs on link files and 
other special files.

The documented reason for this is to prevent the users from using XATTRS 
to avoid quota.

The CAP_SYS_RESOURCE capability is denfined to allow processes with this 
capability to ignore quota.

This PR allows processes with CAP_SYS_RESOURCE to create user Xattrs.

To me this makes sense.

Is there any argument against this?