linux-kernel - Re: [RFC][PATCH 4/3] vfs: Allow rmdir to remove mounts in all but the current mount namespace

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Date:	Tue, 08 Oct 2013 06:13:14 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Matthias Schniedermeyer <ms@...d.de>
Cc:	"Serge E. Hallyn" <serge@...lyn.com>,
	Miklos Szeredi <miklos@...redi.hu>,
	Al Viro <viro@...iv.linux.org.uk>,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Rob Landley <rob@...dley.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC][PATCH 4/3] vfs: Allow rmdir to remove mounts in all but the current mount namespace

Matthias Schniedermeyer <ms@...d.de> writes:

> On 06.10.2013 23:55, Eric W. Biederman wrote:
>> "Serge E. Hallyn" <serge@...lyn.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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