linux-ext4 - Re: [PATCH v5 00/42] idmapped mounts

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Date:   Fri, 15 Jan 2021 16:24:23 +0000
From:   Christoph Hellwig <hch@...radead.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     "Darrick J. Wong" <djwong@...nel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        linux-fsdevel@...r.kernel.org,
        John Johansen <john.johansen@...onical.com>,
        James Morris <jmorris@...ei.org>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Arnd Bergmann <arnd@...db.de>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
        Geoffrey Thomas <geofft@...reload.com>,
        Mrunal Patel <mpatel@...hat.com>,
        Josh Triplett <josh@...htriplett.org>,
        Andy Lutomirski <luto@...nel.org>,
        Theodore Tso <tytso@....edu>, Alban Crequy <alban@...volk.io>,
        Tycho Andersen <tycho@...ho.ws>,
        David Howells <dhowells@...hat.com>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Seth Forshee <seth.forshee@...onical.com>,
        St?phane Graber <stgraber@...ntu.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Aleksa Sarai <cyphar@...har.com>,
        Lennart Poettering <lennart@...ttering.net>,
        "Eric W. Biederman" <ebiederm@...ssion.com>, smbarber@...omium.org,
        Phil Estes <estesp@...il.com>, Serge Hallyn <serge@...lyn.com>,
        Kees Cook <keescook@...omium.org>,
        Todd Kjos <tkjos@...gle.com>, Paul Moore <paul@...l-moore.com>,
        Jonathan Corbet <corbet@....net>,
        containers@...ts.linux-foundation.org,
        linux-security-module@...r.kernel.org, linux-api@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-integrity@...r.kernel.org, selinux@...r.kernel.org
Subject: Re: [PATCH v5 00/42] idmapped mounts

On Fri, Jan 15, 2021 at 07:43:34AM +1100, Dave Chinner wrote:
> > That sounds neat.  AFAICT, the VFS passes the filesystem a mount userns
> > structure, which is then carried down the call stack to whatever
> > functions actually care about mapping kernel [ug]ids to their ondisk
> > versions?
> > 
> > Does quota still work after this patchset is applied?  There isn't any
> > mention of that in the cover letter and I don't see a code patch, so
> > does that mean everything just works?  I'm particularly curious about
> > whether there can exist processes with CAP_SYS_ADMIN and an idmapped
> > mount?  Syscalls like bulkstat and quotactl present file [ug]ids to
> > programs, but afaict there won't be any translating going on?
> 
> bulkstat is not allowed inside user namespaces. It's an init
> namespace only thing because it provides unchecked/unbounded access
> to all inodes in the filesystem, not just those contained within a
> specific mount container.
> 
> Hence I don't think bulkstat output (and other initns+root only
> filesystem introspection APIs) should be subject to or concerned
> about idmapping.

That is what the capabilities are designed for and we already check
for them.