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Subject: Re: pivot_root(".", ".") and the fchdir() dance
To:     "Eric W. Biederman" <ebiederm@xmission.com>
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From:   "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
Message-ID: <cc21557f-1568-68c3-e322-47ceb52fdf53@gmail.com>
Date:   Mon, 23 Sep 2019 13:10:56 +0200
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Hello Eric,

On 9/15/19 8:17 PM, Eric W. Biederman wrote:
> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
> 
>> Hello Eric,
>>
>> On 9/11/19 1:06 AM, Eric W. Biederman wrote:
>>> "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
>>>
>>>> Hello Christian,
>>>>
>>>>>> All: I plan to add the following text to the manual page:
>>>>>>
>>>>>>        new_root and put_old may be the same  directory.   In  particular,
>>>>>>        the following sequence allows a pivot-root operation without need‐
>>>>>>        ing to create and remove a temporary directory:
>>>>>>
>>>>>>            chdir(new_root);
>>>>>>            pivot_root(".", ".");
>>>>>>            umount2(".", MNT_DETACH);
>>>>>
>>>>> Hm, should we mention that MS_PRIVATE or MS_SLAVE is usually needed
>>>>> before the umount2()? Especially for the container case... I think we
>>>>> discussed this briefly yesterday in person.
>>>> Thanks for noticing. That detail (more precisely: not MS_SHARED) is
>>>> already covered in the numerous other changes that I have pending
>>>> for this page:
>>>>
>>>>        The following restrictions apply:
>>>>        ...
>>>>        -  The propagation type of new_root and its parent mount must  not
>>>>           be MS_SHARED; similarly, if put_old is an existing mount point,
>>>>           its propagation type must not be MS_SHARED.
>>>
>>> Ugh.  That is close but not quite correct.
>>>
>>> A better explanation:
>>>
>>>     The pivot_root system call will never propagate any changes it makes.
>>>     The pivot_root system call ensures this is safe by verifying that
>>>     none of put_old, the parent of new_root, and parent of the root directory
>>>     have a propagation type of MS_SHARED.
>>
>> Thanks for that. However, another question. You text has two changes.
>> First, I understand why you reword the discussion to indicate the
>> _purpose_ of the rules. However, you also, AFAICS, list a different set of
>> of directories that can't be MS_SHARED:
>>
>> I said: new_root, the parent of new_root, and put_old
>> You said: the parent of new_root, and put_old, and parent of the
>> root directory.
> 
> 
>> Was I wrong on this detail also?
> 
> That is how I read the code.  The code says:
> 
> 	if (IS_MNT_SHARED(old_mnt) ||
> 		IS_MNT_SHARED(new_mnt->mnt_parent) ||
> 		IS_MNT_SHARED(root_mnt->mnt_parent))
> 		goto out4;
> 
> We both agree on put_old and the parent of new_mnt.
> 
> When I look at the code root_mnt comes from the root directory, not new_mnt.

Hmm -- I had checked the code when I wrote my text, but somehow
I misread things. Going back to recheck the code, you are obviously
correct. Thanks for catching that.

> Furthermore those checks fundamentally makes sense as the root directory
> and new_root that are moving.  The directory put_old simply has
> something moving onto it.
> 
>>> The concern from our conversation at the container mini-summit was that
>>> there is a pathology if in your initial mount namespace all of the
>>> mounts are marked MS_SHARED like systemd does (and is almost necessary
>>> if you are going to use mount propagation), that if new_root itself
>>> is MS_SHARED then unmounting the old_root could propagate.
>>>
>>> So I believe the desired sequence is:
>>>
>>>>>>            chdir(new_root);
>>> +++            mount("", ".", MS_SLAVE | MS_REC, NULL);
>>>>>>            pivot_root(".", ".");
>>>>>>            umount2(".", MNT_DETACH);
>>>
>>> The change to new new_root could be either MS_SLAVE or MS_PRIVATE.  So
>>> long as it is not MS_SHARED the mount won't propagate back to the
>>> parent mount namespace.
>>
>> Thanks. I made that change.
> 
> For what it is worth.  The sequence above without the change in mount
> attributes will fail if it is necessary to change the mount attributes
> as "." is both put_old as well as new_root.
> 
> When I initially suggested the change I saw "." was new_root and forgot
> "." was also put_old.  So I thought there was a silent danger without
> that sequence.

So, now I am a little confused by the comments you added here. Do you
now mean that the 

mount("", ".", MS_SLAVE | MS_REC, NULL);

call is not actually necessary?

Thanks,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/