linux-kernel - Re: [PATCH v2] fs/ceph/quota: ignore quota with CAP_SYS

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Message-ID: <8dfd2741-a2a4-4fbd-9800-5d19de2c4377@redhat.com>
Date: Thu, 12 Sep 2024 10:00:49 +0800
From: Xiubo Li <xiubli@...hat.com>
To: Patrick Donnelly <pdonnell@...hat.com>,
 Max Kellermann <max.kellermann@...os.com>
Cc: idryomov@...il.com, ceph-devel@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] fs/ceph/quota: ignore quota with CAP_SYS_RESOURCE


On 9/12/24 04:14, Patrick Donnelly wrote:
> On Wed, Sep 11, 2024 at 3:21 PM Max Kellermann <max.kellermann@...os.com> wrote:
>> On Wed, Sep 11, 2024 at 8:04 PM Patrick Donnelly <pdonnell@...hat.com> wrote:
>>> CephFS has many components that are cooperatively maintained by the
>>> MDS **and** the clients; i.e. the clients are trusted to follow the
>>> protocols and restrictions in the file system. For example,
>>> capabilities grant a client read/write permissions on an inode but a
>>> client could easily just open any file and write to it at will. There
>>> is no barrier preventing that misbehavior.
>> To me, that sounds like you confirm my assumption on how Ceph works -
>> both file permissions and quotas. As a superuser (CAP_DAC_OVERRIDE), I
>> can write arbitrary files, and just as well CAP_SYS_RESOURCE should
>> allow me to exceed quotas - that's how both capabilities are
>> documented.
>>
>>> Having root on a client does not extend to arbitrary superuser
>>> permissions on the distributed file system. Down that path lies chaos
>>> and inconsistency.
>> Fine for me - I'll keep my patch in our kernel fork (because we need
>> the feature), together with the other Ceph patches that were rejected.
> If you want to upstream this, the appropriate change would go in
> ceph.git as a new cephx capability (not cephfs capability) for the
> "mds" auth cap that would allow a client with root (or
> CAP_SYS_RESOURCE) to bypass quotas. I would support merging such a
> patch (and the corresponding userspace / kernel client changes).
>
Yeah, Patrick is correct. This really will by pass the protocols and 
restrictions in cephfs and introduces inconsistency. By adding a new 
cephx caps we can broadcast this to all the users or clients.

Thanks