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    SubjectRe: [CRIU] Introspecting userns relationships to other namespaces?
    On Sat, Jul 09, 2016 at 01:29:20PM -0500, Eric W. Biederman wrote:
    > ebiederm@xmission.com (Eric W. Biederman) writes:
    >
    > > Andrew Vagin <avagin@virtuozzo.com> writes:
    > >
    > >> 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.
    > >
    > > That is certainly one way to go.
    > >
    > > There is a funny case where we would want to compare a user namespace
    > > file descriptor to a parent user namespace file descriptor.
    > >
    > >
    > > Grumble, Grumble. I think this may actually a case for creating ioctls
    > > for these two cases. Now that random nsfs file descriptors are bind
    > > mountable the original reason for using proc files is not as pressing.
    > >
    > > One ioctl for the user namespace that owns a file descriptor.
    > > One ioctl for the parent namespace of a namespace file descriptor.
    > >
    > > We also need some way to get a command file descriptor for a file system
    > > super block. Al Viro has a pet project for cleaning up the mount API
    > > and this might be the idea excuse to start looking at that.
    > >
    > > (In principle we might be able to run commands through the namespace
    > > file descriptor and using an ioctl feels dirty. But an ioctl that
    > > only uses the fd and request argument does not suffer from the same
    > > problems that ioctls that have to pass additional arguments suffer
    > > from.)
    >
    > Of course it should be an error perhaps -EINVAL to get a user
    > namespace owner or parent namespace that is outside of a processes
    > current user namespace or pid namespace. That way thing stay bounded
    > within the current namespaces the process is in. Which prevents any
    > leak possibilities, and keeps CRIU working.

    I prepared patches with ioctl-s to understand how it looks like.

    Here is a whole series:
    https://github.com/avagin/linux-task-diag/commits/namespaces

    Here is a patch to get an owning user namespace:
    https://github.com/avagin/linux-task-diag/commit/7fad8ff3fc4110bebf0920cec2388390b3bd2238
    https://github.com/avagin/linux-task-diag/commit/2663bc803d324785e328261f3c07a0fef37d2088

    Here is an example how it looks from user-space:
    https://github.com/avagin/linux-task-diag/blob/namespaces/tools/testing/selftests/nsfs/owner.c#L49

    I like the idea with ioctl-s. James, Michael, Trevor, what is your
    opinion about this?

    >
    > Eric

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