[patch] unprivileged mounts update

[Serge E. Hallyn]

Quoting Eric W. Biederman (ebiederm at xmission.com):
> "Serge E. Hallyn" <serge at hallyn.com> writes:
> 
> > Quoting Eric W. Biederman (ebiederm at xmission.com):
> >> 
> >> Are there other permission checks that mount is doing that we
> >> care about.
> >
> > Not mount itself, but in looking up /share/fa/root/home/fa,
> > user fa doesn't have the rights to read /share, and by setting
> > fsuid to fa and dropping CAP_DAC_READ_SEARCH the mount action fails.
> 
> Got it. 
> 
> I'm not certain this is actually a problem it may be a feature.
> But it does fly in the face of the general principle of just
> getting out of roots way so things can get done.
> 
> I think we can solve your basic problem by simply doing like:
> chdir(/share); mount(.);  To simply avoid the permission problem.
> 
> The practical question is how much do we care.
> 
> > But the solution you outlined in your previous post would work around
> > this perfectly.
> 
> If we are not using usual permissions which user do we use current->uid?
> Or do we pass that user someplace?

Right, I figure if the normal action is to always do
mnt->user = current->fsuid, then for the special case we
pass a uid in someplace.  Of course...  do we not have a
place to do that?  Would it be a no-no to use 'data' for
a non-fs-specific arg?

> >> > If it were really the equivalent then I could keep my capabilities :)
> >> > after changing it.
> >> 
> >> We drop all capabilities after we change the euid.
> >
> > Not if we've done prctl(PR_SET_KEEPCAPS, 1)
> 
> Ah cap_clear doesn't do the obvious thing.
> 
> Eric