ACLs, umask and shared directories

Fri Mar 12 08:03:34 UTC 2010

Hi Gary,

Parts of the filesystem are written by all users - /tmp and /var/tmp.
Users don't often write files there deliberately, but many programs run
by the user do.

With a umask of 002, one user can modify another user's file in these
locations. (The sticky bit only protects against file deletion.)

Also note that a user can change permissions on their home directory -
the users in question are students, and a few do this accidentally every
semester. With a umask of 002, and every user in the same group, your
home directory is effectively world-writable!

The cronjobs and so forth would work - but functional ACLs would be so
much simpler :-)

Thanks
Rob.

On 12/03/2010, at 12:52 AM, Gary Gatten wrote:

> "Project Groups" are the key. A secondary group owns the dir, only users working on that project are in that group - so 002 works.  I get umask is system wide, but it should be ok if directory ownership is "correct" everywhere in the system and/or users know they should only put certain files in certain places.
> 
> If for some reason that won't work (if you could explain why for my benefit) what about: A cron job that runs every x mins and sets perms as you wish. Ideally a daemon process would "monitor" the directory in question looking for opens/writes/etc, and then set perms; event driven.  Lasty, maybe train the users to set perms, or have a script they can run that will do it.  OR...., have them create files in a temp dir and a cron job mv's them to the permanent dir and sets perms the same time?
> 
> 
> 
> ----- Original Message -----
> From: Rob <listone at deathbeforedecaf.net>
> To: Gary Gatten
> Cc: freebsd-questions at freebsd.org <freebsd-questions at freebsd.org>
> Sent: Thu Mar 11 05:04:28 2010
> Subject: Re: ACLs, umask and shared directories
> 
> Hi Gary,
> 
> Directory group inheritance is the default in FreeBSD - see open(2):
> 
>         When a new file is created it is given the group of the
>         directory which contains it.
> 
> In SysV, this behaviour is controlled by the setgid bit.
> 
> So the file has the correct group, but it's not writeable by other users
> unless it has g+w permissions. The way to guarantee this is to set
> everyone's umask to 002 - but then they can write each other's files
> anywhere else in the filesystem, because they're all in the same primary
> group.
> 
> I just can't see a tidy solution.
> 
> Thanks
> Rob.
> 
> On 09/03/2010, at 10:34 AM, Gary Gatten wrote:
> 
> >
> > chmod g+s "ParentDirectory"
> >
> > Files created in the dir now have the group of the dir.
> >
> > Not sure if this will help or not, as it appears the new files do not
> > inherit the perms of the group, the umask still over-rides so....
> >
> > What about a secondary group + SGID + umask 002?  The users that need to
> > edit each others files in this directory are in a secondary group
> > (ShareMe).  This same group owns the parent directory and the SGID bit
> > is set.  This should allow you to set the umask to 002 correct?  Maybe?
> >
> > So:
> >
> > www1 primary group = domain_users;
> >
> > www1$ pwd
> > /WorkgroupXShare
> >
> > drwxrws---  4 root ShareMe 0 Mar  8 03:11 .
> > www1$ touch file1
> > drwxrws---  4 www1 ShareMe 0 Mar  8 03:11 file1
> >
> > umask of 002 should give files 664 (I'd change umask to 004, group
> > "ShareMe" should get rw perms, right?
> >
> > I think this will work?
> >
> > G
> >
> >
> > -----Original Message-----
> > From: Gary Gatten
> > Sent: Monday, March 08, 2010 4:49 PM
> > To: 'listone at deathbeforedecaf.net'
> > Subject: RE: ACLs, umask and shared directories
> >
> > This may also work:
> >
> > SGID (set group ID) on a directory: in this special case every file
> > created in the directory will have the same group owner as the directory
> > itself (while normal behavior would be that new files are owned by the
> > users who create them). This way, users don't need to worry about file
> > ownership when sharing directories:
> >
> > G
> >
> >
> > -----Original Message-----
> > From: Gary Gatten
> > Sent: Monday, March 08, 2010 4:13 PM
> > To: Gary Gatten; 'listone at deathbeforedecaf.net'
> > Subject: RE: ACLs, umask and shared directories
> >
> > What about sticky bit on the parent directory - in combination with
> > appropriate owner and group perms.  I used sticky in my ftpd solution,
> > HOWEVER, this was on SCO Unix and sticky may have different behavior on
> > FBSD.  Worth a look though!
> >
> > G
> >
> >
> > -----Original Message-----
> > From: Gary Gatten
> > Sent: Monday, March 08, 2010 8:25 AM
> > To: 'listone at deathbeforedecaf.net'
> > Subject: Re: ACLs, umask and shared directories
> >
> > I ran into a similar issue long ago with an ftp folder and "shared"
> > files.  If I recall umask solved my issue for me but understand it
> > doesn't solve yours.
> >
> > If nothing else, could you write a shell script that "monitors" the
> > directory/directories for writes and then sets the perms as needed?
> >
> > ----- Original Message -----
> > From: owner-freebsd-questions at freebsd.org
> > <owner-freebsd-questions at freebsd.org>
> > To: freebsd-questions at freebsd.org <freebsd-questions at freebsd.org>
> > Sent: Mon Mar 08 06:41:03 2010
> > Subject: ACLs, umask and shared directories
> >
> > Hi Folks,
> >
> > I need to give a group of users write access to a shared directory. The
> > problem is, when one user creates a file,
> >
> >  www1$ touch file1
> >  www1$ ll
> >  total 8
> >  drwxrwxr-x  2 root  domain_users  512 Mar  8 03:11 .
> >  drwxr-xr-x  4 root  wheel         512 Mar  8 03:10 ..
> >  -rw-r--r--  1 www1  domain_users    0 Mar  8 03:11 file1
> >
> > other users can't edit it.
> >
> > Solution 1
> > ----------
> >
> > Change everyone's umask to 002. Unfortunately, these users are defined
> > in Active Directory and they're all in the same primary group - 002 is
> > not secure in this scenario.
> >
> > Solution 2
> > ----------
> >
> > Set a default ACL on the parent directory,
> >
> >  www1$ getfacl -d .
> >  # file: .
> >  # owner: root
> >  # group: domain_users
> >  user::rwx
> >  group::rwx
> >  mask::rwx
> >  other::r-x
> >
> > but it doesn't have the desired effect,
> >
> >  www1$ touch file1
> >  www1$ getfacl file1
> >  # file: file1
> >  # owner: www1
> >  # group: domain_users
> >  user::rw-
> >  group::rwx           # effective: r--
> >  mask::r--
> >  other::r--
> >
> > as the umask seems to override it - this was confirmed by Robert
> > Watson[1] in 2005.
> >
> > So does anyone have a better idea?
> >
> > Thanks
> > Rob.
> >
> > [1]
> > http://lists.freebsd.org/pipermail/freebsd-fs/2005-October/001382.html
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