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Date:   Tue, 26 Jan 2021 23:50:12 -0600
From:   "Serge E. Hallyn" <serge@hallyn.com>
To:     Christian Brauner <christian.brauner@ubuntu.com>
Cc:     "Eric W. Biederman" <ebiederm@xmission.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@lst.de>, linux-fsdevel@vger.kernel.org,
        John Johansen <john.johansen@canonical.com>,
        James Morris <jmorris@namei.org>,
        Mimi Zohar <zohar@linux.ibm.com>,
        Dmitry Kasatkin <dmitry.kasatkin@gmail.com>,
        Stephen Smalley <stephen.smalley.work@gmail.com>,
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        Arnd Bergmann <arnd@arndb.de>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>,
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        Mrunal Patel <mpatel@redhat.com>,
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Subject: Re: [PATCH v6 23/40] exec: handle idmapped mounts
Message-ID: <20210127055012.GA32153@mail.hallyn.com>
References: <20210121131959.646623-1-christian.brauner@ubuntu.com>
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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@ubuntu.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@fscaps:~/mount-idmapped$ /home/u2/rcap /mnt/id
v3, rootid is 1001
ubuntu@fscaps:~/mount-idmapped$ /home/u2/rcap //home/ubuntu/id
v3, rootid is 1000

-serge