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    SubjectRe: [CRIU] Introspecting userns relationships to other namespaces?
    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.

    Eric

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