linux-ext4 - Re: [PATCH v6 23/40] exec: handle idmapped mounts

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Message-ID: <20210127055012.GA32153@mail.hallyn.com>
Date:   Tue, 26 Jan 2021 23:50:12 -0600
From:   "Serge E. Hallyn" <serge@...lyn.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@....de>, 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>,
        Stéphane Graber <stgraber@...ntu.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Aleksa Sarai <cyphar@...har.com>,
        Lennart Poettering <lennart@...ttering.net>,
        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 v6 23/40] exec: handle idmapped mounts

On Mon, Jan 25, 2021 at 05:44:04PM +0100, Christian Brauner wrote:
> On Mon, Jan 25, 2021 at 10:39:01AM -0600, Eric W. Biederman wrote:
> > Christian Brauner <christian.brauner@...ntu.com> writes:
> > 
> > > When executing a setuid binary the kernel will verify in bprm_fill_uid()
> > > that the inode has a mapping in the caller's user namespace before
> > > setting the callers uid and gid. Let bprm_fill_uid() handle idmapped
> > > mounts. If the inode is accessed through an idmapped mount it is mapped
> > > according to the mount's user namespace. Afterwards the checks are
> > > identical to non-idmapped mounts. If the initial user namespace is
> > > passed nothing changes so non-idmapped mounts will see identical
> > > behavior as before.
> > 
> > This does not handle the v3 capabilites xattr with embeds a uid.
> > So at least at that level you are missing some critical conversions.
> 
> Thanks for looking. Vfs v3 caps are handled earlier in the series. I'm
> not sure what you're referring to here. There are tests in xfstests that
> verify vfs3 capability behavior.
> 
> Christian

So fwiw I just tested it manually as well.  Scenario:

uid 1000 user ubuntu
uid 1001 user u2
sudo ./mount-idmapped --map-mount b:1000:1001:1 /home/ubuntu/ /mnt
su - u2
  cp /usr/bin/id /mnt/
  unshare -Urm
    setcap cap_setuid=pe /mnt/id
At this point, from init_user_ns,
ubuntu@...aps:~/mount-idmapped$ /home/u2/rcap /mnt/id
v3, rootid is 1001
ubuntu@...aps:~/mount-idmapped$ /home/u2/rcap //home/ubuntu/id
v3, rootid is 1000

-serge