Re: The Future of Linux Capabilities ...

[Herbert Poetzl <herbert@13thfloor.at>]

On Mon, Dec 27, 2004 at 03:22:32PM -0800, Ulrich Drepper wrote:
> On Mon, 27 Dec 2004 02:40:41 +0100, Herbert Poetzl <herbert@13thfloor.at> wrote:
> >   II)   add 32 (or more) sub-capabilities which depend
> >         on the parent capability to be usable, and add
> >         appropriate syscalls for them.
> > 
> >         example: CAP_IPC_LOCK gets two subcapabilities
> >         (e.g. SCAP_SHM_LOCK and SCAP_MEM_LOCK) which
> 
> I won't try to say anything about III, but I is not really suitable,
> it breaks code currently using capabilities.  Or at least makes them
> less secure.  

let me assure you that III) does neither break the existing code
nor reduce security, for the following reasons:

  a) the per process capability is a requirement for
     _all_ subcapabilities (when the cap is in the cap_mask)

  b) the capability system isn't changed for caps not
     in the cap_mask

  c) reducing a cap by removing a subcapability can only
     increase security not lower it

> With sub-capabilities the interface diverges from the
> POSIX capabilities interfaces, but at least one can keep backward
> compatibilities.

to some extend, yes ...

> An alternative would be to keep the existing capabilities, and add new
> ones for all the cases which need splitting.  If the old value is
> set/reset, all the split-out values are "magically" affected as well. 

I consider the 'magically' part another solution I didn't
list in my previous mail, but it is a kind of variation
from II) where we do not necessarily need subcaps for _all_
aspects of a capability (as a matter of fact it's one less)

> This would help keeping the interfaces in line with POSIX and no
> additions to the userlevel libcap would be needed.  Yes, new cap[gs]et
> syscalls would be needed, but this fact is hidden from the user.

I guess it might be doable, although the 'magically' part
would require to keep masks for all caps which got split
to select the corresponding sub-capabilities ...

thanks,
Herbert

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