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    SubjectRe: [PATCH -mm 5/7] add user namespace
    Quoting Eric W. Biederman (ebiederm@xmission.com):
    > "Serge E. Hallyn" <serue@us.ibm.com> writes:
    >
    > > Quoting Eric W. Biederman (ebiederm@xmission.com):
    > >> Dave Hansen <haveblue@us.ibm.com> writes:
    > >>
    > >> > On Thu, 2006-07-13 at 12:14 -0600, Eric W. Biederman wrote:
    > >> >> Maybe. I really think the sane semantics are in a different uid namespace.
    > >> >> So you can't assumes uids are the same. Otherwise you can't handle open
    > >> >> file descriptors or files passed through unix domain sockets.
    > >> >
    > >> > Eric, could you explain this a little bit more? I'm not sure I
    > >> > understand the details of why this is a problem?
    > >>
    > >> Very simply.
    > >>
    > >> In the presence of a user namespace.
    > >> All comparisons of a user equality need to be of the tuple (user namespace,
    > > user id).
    > >> Any comparison that does not do that is an optimization.
    > >>
    > >> Because you can have access to files created in another user namespace it
    > >> is very unlikely that optimization will apply very frequently. The easy
    > > scenario
    > >> to get access to a file descriptor from another context is to consider unix
    > >> domain sockets.
    > >
    > > What does that have to do with uids? If you receive an fd, uids don't
    > > matter in any case. The only permission checks which happen are LSM
    > > hooks, which should be uid-agnostic.
    >
    > You are guest uid 0. You get a directory file descriptor from another namespace.
    > You call fchdir.
    >
    > If you permission checks are not (user namespace, uid) what can't you do?

    File descripters can only be passed over a unix socket, right?

    So this seems to fall into the same "userspace should set things up
    sanely" argument you've brought up before.

    Don't get me wrong though - the idea of using in-kernel keys as
    cross-namespace uid's is definately interesting.

    -serge
    -
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