From: ebiederm@xmission.com (Eric W. Biederman)
To: Matthias Schniedermeyer <ms@citd.de>
Cc: "Serge E. Hallyn" <serge@hallyn.com>, Miklos Szeredi <miklos@szeredi.hu>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Linux-Fsdevel <linux-fsdevel@vger.kernel.org>,
        Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andy Lutomirski <luto@amacapital.net>, Rob Landley <rob@landley.net>,
        Linus Torvalds <torvalds@linux-foundation.org>
References: <CAJfpegsxgnSRUW-E5HM3uT5QfGyUtn_v=i4Ppkkkutp34287AA@mail.gmail.com>
	<87a9kkax0j.fsf@xmission.com>
	<CAJfpeguv+7giYNpAuXE9Ja_9BEwB0-fZBVgRSeVqpzSXgQYZ6Q@mail.gmail.com>
	<8761v7h2pt.fsf@tw-ebiederman.twitter.com>
	<CAJfpegtT7y-1HhbEVAMKkdQugTG_w7G_epGtQHGvQLpcZB5FVA@mail.gmail.com>
	<87li281wx6.fsf_-_@xmission.com> <87a9ioo37a.fsf_-_@xmission.com>
	<20131007043919.GB10284@mail.hallyn.com> <87vc191sf2.fsf@xmission.com>
	<20131008104202.GA20639@citd.de>
Date: Tue, 08 Oct 2013 06:13:14 -0700
In-Reply-To: <20131008104202.GA20639@citd.de> (Matthias Schniedermeyer's
	message of "Tue, 8 Oct 2013 12:42:02 +0200")
Message-ID: <87ob6zzz11.fsf@tw-ebiederman.twitter.com>
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Subject: Re: [RFC][PATCH 4/3] vfs: Allow rmdir to remove mounts in all but the current mount namespace
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Matthias Schniedermeyer <ms@citd.de> writes:

> On 06.10.2013 23:55, Eric W. Biederman wrote:
>> "Serge E. Hallyn" <serge@hallyn.com> writes:
>> 
>> So if we can feel safe just depending on the parent directory
>> permissions (which are not hidden by a mount) protecting our mount
>> points, I feel much better about this patchset.
>
> As far as i can tell, the permissions of the host-directory of a 
> mount-point are hidden, at least for user-space.
> (Ignoring (bind-)mounting the parent-mount somewhere else)
>
> As root:
> $ mkdir /tmp/test
> $ ls -ld /tmp/test
> drwxr-xr-x 2 root root 40 Oct  8 12:33 /tmp/test
>
> $ mount tmpfs -t tmpfs /tmp/test
> $ ls -ld /tmp/test
> drwxrwxrwt 2 root root 40 Oct  8 12:33 /tmp/test
>
> $ chown nobody.users /tmp/test
> $ ls -ld /tmp/test
> drwxrwxrwt 2 nobody users 40 Oct  8 12:33 /tmp/test
>
> $ umount /tmp/test
> $ ls -ld /tmp/test
> drwxr-xr-x 2 root root 40 Oct  8 12:33 /tmp/test
>
>
> So if the kernel would check the host-directory-permissions for allowing 
> umounting by rmdir it follows that a "plain user" doesn't have any 
> possibility to know beforehand if rmdir/umount would be possible.

Except the directory that is relevant to unlink/rmdir in your example is /tmp
not /tmp/test.

But thanks for the eyeball.

Eric
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