ucred when euid/egid

[Clifton Royston]

On Sun, Nov 29, 2009 at 01:19:02PM +0300, Anthony Pankov wrote:
> 
> Thank you for reply.
> 
> So, seteuid/gid isn't enough to gain group access as for real uid.
> But how i can achieve this? What functions should i call from
> 'theprog' to gain access for the groups euid user belongs to?
> 
> May be i solve the problem in wrong way?
> 
> The full problem is:
> 
> There is a file owned by group filegroup:
>  rw-rw----   someone:filegroup    thefile
> 
> There is a programs data owned by group proggroup:
> 
>  rw-rw----   someone2:proggroup    progdata
> 
> I need a program (theprog) that can access 'thefile' and
> 'progdata' simultaneously. Program can be executed by anyone.
 
This is a clearer statement of the problem, in terms of what you're
trying to accomplish.  

If you can make the program data owned by a special program user, and
require the users of the program to make their files group-accessible
by this special filegroup, then you can do it fairly simply, like this:

Make each users' "thefile" be owned by group filegroup, for example:
  rw-rw----   someone:filegroup    ~someone/thefile
  rw-rw----   someone2:filegroup   ~someone2/thefile
  rw-rw----   someone3:filegroup   ~someone3/thefile
  ...

Make the program's data file owned by *user* proguser:
  rw-rw----   proguser:proggroup    progdata

Now you can make the program setuid proguser/setgid filegroup:
  r-sr-sr-x   proguser:filegroup    theprog
 
This lets it be executed by any user and access its own data (via the
suid) and the files the users have put into filegroup (via the sgid).


Note that the users should not themselves be members of filegroup 
unless it's OK for them to read/write each others' data.  You may need
either to provide an sgid utility which can be used to create or chown
that file to filegroup, or require them to be put in a shared directory
with filegroup gid and the directory sticky bit set.

Alteratively you could drop the sgid and simply require the file be group
readable/writable by the user's own group.  In that case you have
  r-sr-xr-x   proguser:bin          theprog
and
  rw-rw----   someone:somegroup    ~someone/thefile


> My idea was to seteuid theprog to user who is memeber of one group
> (filegroup) and setegid theprog to another group (proggroup). In that
> way i was going to give theprog rights to work with both files.
>
> P.S. I don't want to use file ACLs.

The standard Unix permissions aren't really extensible in that way. 

You can do it as I've outlined above; that's getting close to the
limits of what you can readily do with the standard permissions.  If it
gets more complicated, you will need to either do ACLs or something
still more creative.  

sudo, for instance, does allow you to set a vector of groups to match
the user you're executing as.  It may be possible to leverage the sudo
command into doing something more elaborate if you need to, with a
suitably crafted sudoers config file; you could also look into the code
that sudo uses to set the group vector, but that will require you to
write a suid root utility which adds a lot of security risks.

Hope this helps,
  -- Clifton

-- 
    Clifton Royston  --  cliftonr at iandicomputing.com / cliftonr at lava.net
       President  - I and I Computing * http://www.iandicomputing.com/
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