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    SubjectRe: [CRIU] Introspecting userns relationships to other namespaces?
    On Fri, Jul 08, 2016 at 10:05:18PM -0500, Eric W. Biederman wrote:
    > James Bottomley <James.Bottomley@HansenPartnership.com> writes:
    >
    > > On Fri, 2016-07-08 at 18:52 -0500, Eric W. Biederman wrote:
    > >> James Bottomley <James.Bottomley@Hansenpartnership.com> writes:
    > >>
    > >> > On July 8, 2016 1:38:19 PM PDT, Andrew Vagin <avagin@virtuozzo.com>
    > >> > wrote:
    > >>
    > >> > > What do you think about the idea to mount nsfs and be able to
    > >> > > look up any alive namespace by inum:
    > >> >
    > >> > I think I like it. It will give us a way to enter any extant
    > >> > namespace. It will work for Eric's fs namespaces as well. Perhaps
    > >> > a /process/ns/<inum> Directory?
    > >
    > > As you understood, I meant /proc/ns/<inum> (damn mobile phone
    > > completions).
    > >
    > >> *Shivers*
    > >>
    > >> That makes it very easy to bypass any existing controls that exist
    > >> for getting at namespaces. It is true that everything of that kind
    > >> is directory based but still.
    > >>
    > >> Plus I think it would serve as information leak to information
    > >> outside of the container.
    > >>
    > >> An operation to get a user namespace file descriptor from some kernel
    > >> object sounds reasonably sane.
    > >>
    > >> A great big list of things sounds about as scary as it can get. This
    > >> is not the time to be making it easier to escape from containers.
    > >
    > > To be honest, I think this argument is rubbish. If we're afraid of
    > > giving out a list of all the namespaces, it means we're afraid there's
    > > some security bug and we're trying to obscure it by making the list
    > > hard to get. All we've done is allayed fears about the bug but the
    > > hackers still know the portals to get through.
    > >
    > > If such a bug exists, it will be possible to exploit it by simply
    > > reconstructing the information from the individual process directories,
    > > so obscurity doesn't protect us and all it does is give us a false
    > > sense of security. If such a bug doesn't exist, then all the security
    > > mechanisms currently in place (like no re-entry to prior namespace)
    > > should protect us and we can give out the list.
    > >
    > > Let's deal with the world as we'd like it to be (no obscure namespace
    > > bugs) and accept the consequences and the responsibility for fixing
    > > them if we turn out to be slightly incorrect. We'll end up in a far
    > > better place than security by obscurity would land us.
    >
    > No. That is not the fear. The permission checks on /proc/self/ns/xxx
    > are different than if the namespace is bind mounted somewhere.
    >
    > That was done deliberately and with a reasonable amount of forethought.
    > You are asking to throw those permission checks out. The answer is no.
    >
    > Furthermore there is a much clearer reason not to go with a list of all
    > namespaces. A list of all namespaces breaks CRIU. As you have described
    > it the list will change depending upon which machine you restore a
    > checkpoint on. I honestly don't know what kind of havoc that will cause
    > but it is certainly something we won't be able to checkpoint no matter
    > how hard we try.

    It's right. I hadn't thought about this.

    >
    > A global list of namespaces especially of the kind that you can open
    > and get a handle to the namespace is just not appropriate.
    >
    > I know inode numbers comes darn close to names but they aren't really
    > names and if it comes to it we can figure out how to preserve an
    > applications view of it all across a checkpoint/restart. So far it
    > hasn't proven necessary to preserve any inode numbers across
    > checkpoint/restart but again it is theoretically possible if it becomes
    > necessary.
    >
    > Throwing away checkpoint/restart support for the sake of
    > checkpoint/restart is a no-go.
    >
    > Containers fundamentally imply you don't have global visibility,
    > and that is a good thing.

    All these thoughts about security make me thinking that kcmp is what we
    should use here. It's maybe something like this:

    kcmp(pid1, pid2, KCMP_NS_USERNS, fd1, fd2)

    - to check if userns of the fd1 namepsace is equal to the fd2 userns

    kcmp(pid1, pid2, KCMP_NS_PARENT, fd1, fd2)

    - to check if a parent namespace of the fd1 pidns is equal to fd pidns.

    fd1 and fd2 is file descriptors to namespace files.

    So if we want to build a hierarchy, we need to collect all namespaces
    and then enumerate them to check dependencies with help of kcmp.

    >
    > Eric

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