Messages in this thread | | | Date | Fri, 14 Jul 2006 09:02:37 -0500 | From | "Serge E. Hallyn" <> | Subject | Re: [PATCH -mm 5/7] add user namespace |
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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.
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