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

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Message-ID: <CAKPOu+9KauLTWWkF+JcY4RXft+pyhCGnC0giyd82K35oJ2FraA@mail.gmail.com>
Date: Wed, 11 Sep 2024 21:21:22 +0200
From: Max Kellermann <max.kellermann@...os.com>
To: Patrick Donnelly <pdonnell@...hat.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 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.

Max