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

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Message-ID: <CA+2bHPbMwsg8NkvW=FCSwCs9p2B0wBkrfW6AbPj+SOWNHAD45w@mail.gmail.com>
Date: Wed, 11 Sep 2024 16:14:14 -0400
From: Patrick Donnelly <pdonnell@...hat.com>
To: Max Kellermann <max.kellermann@...os.com>
Cc: xiubli@...hat.com, 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 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).

-- 
Patrick Donnelly, Ph.D.
He / Him / His
Red Hat Partner Engineer
IBM, Inc.
GPG: 19F28A586F808C2402351B93C3301A3E258DD79D