Re: libcap vs libcap-ng mess

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Paolo Bonzini (pbonzini@redhat.com) wrote:
> On 29/11/19 19:01, Dr. David Alan Gilbert wrote:
> >> It's not entirely trivial because fsdev-proxy-helper wants to keep the
> >> effective set and clear the permitted set; in libcap-ng you can only
>                      ^^^^^
> 
> (Wrong, this is "modify" the permitted set.  The permitted set is
> already cleared by setresuid/setresgid).
> 
> >> apply both sets at once, and you cannot choose only one of them in
> >> capng_clear/capng_get_caps_process.  But it's doable, I'll take a look.
> > I'm having some difficulties making the same conversion for virtiofsd;
> > all it wants to do is drop (and later recover) CAP_FSETID
> > from it's effective set;  so I'm calling capng_get_caps_process
> > (it used to be cap_get_proc).  While libcap survives just using the
> > capget syscall, libcap-ng wants to read /proc/<TID>/status - and
> > that's a problem because we're in a sandbox without /proc mounted
> > at that point.
> 
> The state of libcap-ng persists after capng_apply.  So you can just call
> capng_update({CAP_ADD,CAP_DROP}) followed by capng_apply.

But the internal state needs initialising doesn't it? So that when you
capng_update it tweaks a set that was originally read from somewhere?
(and that's per-thread?)

> Does virtiofsd have to do uid/gid dances like virtfs-proxy-helper?

It looks like it; I can see setresuid calls to save and restore
euid/egid.

Dave

> Thanks,
> 
> Paolo
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK