Messages in this thread | | | From | (Eric W. Biederman) | Subject | Re: [PATCH -mm 5/7] add user namespace | Date | Thu, 13 Jul 2006 21:52:40 -0600 |
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"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?
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