Re: MNT_DETACH and mount namespace issue

[Richard Weinberger]

On Sat, Aug 2, 2014 at 12:09 AM, Eric W. Biederman
<ebiederm@xxxxxxxxxxxx> wrote:
> Richard Weinberger <richard@xxxxxx> writes:
>
>> Am 01.08.2014 17:44, schrieb Ram Pai:
>>> On Fri, Aug 01, 2014 at 12:17:13AM +0200, Richard Weinberger wrote:
>>>> Am 30.07.2014 22:46, schrieb Richard Weinberger:
>>>>> Am 30.07.2014 15:59, schrieb Richard Weinberger:
>>>>>> If we use the plain list_empty() we might not see the
>>>>>> hlist_del_init_rcu() and therefore miss one member of the
>>>>>> list.
>>>>>>
>>>>>> It fixes the following issue:
>>>>>> $ unshare -m /usr/bin/sleep 10000 &
>>>>>> $ mkdir -p foo/proc
>>>>>> $ mount -t proc none foo/proc
>>>>>> $ mount -t binfmt_misc none foo/proc/sys/fs/binfmt_misc
>>>>>> $ umount -l foo/proc
>>>>>> $ rmdir foo/proc
>>>>>> rmdir: failed to remove âfoo/procâ: Device or resource busy
>>>>>
>>>>> Although my fix was wrong, the issue is real, it seems to exist for a very long
>>>>> time. Just was able to reproduce it on 2.6.32.
>>>>> Please note that you need a shared root subtree to trigger the issue.
>>>>> i.e. mount --shared /
>>>>> Maybe this is why nobody noticed it so far as only systemd distros
>>>>> have the root subtree shared by default.
>>>>>
>>>>> I hit the issue on openSUSE 13.1 where an application creates a chroot environment
>>>>> and then lazy umounts /proc.
>>>>> It happened on very few machines. An analysis showed that only boxes with an OpenVPN tunnel
>>>>> were affected. This did not make any sense until I discovered that the OpenVPN systemd
>>>>> service file has set "PrivateTmp=true". This setting creates
>>>>> a mount namespace for the said service...
>>>>>
>>>>> In __propagate_umount() the following piece of code is interesting:
>>>>>
>>>>>  /*
>>>>>  * umount the child only if the child has no
>>>>>  * other children
>>>>>  */
>>>>> if (child && list_empty(&child->mnt_mounts)) {
>>>>>         hlist_del_init_rcu(&child->mnt_hash);
>>>>>         hlist_add_before_rcu(&child->mnt_hash, &mnt->mnt_hash);
>>>>> }
>>>>>
>>>>> child->mnt_mounts is non-empty for the "proc" although the "binfmt_misc"
>>>>> subtree was removed.
>>>>> I'm not sure whether this is only one more symptom or the main culprit.
>>>>
>>>> CC'ing Ram Pai.
>>>>
>>>> Ram, you are the author of the said code. Can you please explain why we need that
>>>> list_empty() check?
>>>> To my (limited) understanding of VFS, the following change should be fine to fix the issue:
>>>
>>> We had made a rule then, that busy vfsmounts cannot be lazily unmounted
>>> **implicitly**. Propagated unmounts are implicit unmounts, and if such
>>> implicit vfsmounts have child-mounts than obviously they are busy, and
>>> hence they cannot be lazy-unmounted implicitly.
>>>
>>> the list_empty() is checking for no child-mounts on the vfsmount before
>>> letting it unmount.
>>>
>>> We did not want a bunch of mounts disappear without the users knowledge.
>>> Hence we made the above rule.
>>>
>>> Al Viro, will have more insights into this.
>>
>> Hmm, with the root subtree shared by default this policy will be problematic and
>> lead to problems.
>> As I observe on openSUSE 13.1.
>>
>> Al, what do you think?
>
> I have a pending patchset that causes the rmdir to cause all of the
> mounts to go away.  It has passed review and has not been merged only
> because of stack overflow concerns (which I have not had time to fully
> address).
>
> Sigh.  It badly breaks unix semantics for rmdir unlink with no mounts in
> the local namespace to fail, and it introduces as denial of service
> attack from unprivielged users.

Thanks for the pointer!
I fear your patch series is nothing we can easily feed into -stable.
Is this really the only acceptable solution?

The thing is, users get already bitten by that.
i.e. run openSUSE's KIWI tool on a machine where a systemd service
with PrivateTmp=yes is installed
and you'll end up with a stale mount point.
KIWI creates a chroot, populates /proc, lazily unmounts it later and
then fails to remove the temporary chroot directory
because of EBUSY.

-- 
Thanks,
//richard
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