negative group permissions?

[Anton Shterenlikht]

On Fri, Feb 24, 2012 at 09:34:02AM +0000, Matthew Seaman wrote:
> On 24/02/2012 09:08, Anton Shterenlikht wrote:
> > Recently I started seeing this line
> > in daily security output:
> > 
> >   Checking negative group permissions:
> >   70834 -rw-r----x  1 root  daemon  4 Feb 21 12:54:02 2012 /var/spool/output/lpd/.seq
> > 
> > I've a parallel printer attached to
> > a 9.9-CURRENT #2 r230787M box.
> > 
> > What does it mean?
> 
> This means that non-root users in group daemon have only read
> permissions on that file.  Users that aren't root and that aren't in
> group daemon have execute permission only.
> 
> It does look a bit odd, and I believe that file would just contain a job
> number (IIRC -- haven't dealt much with lpd or lprng much recently)
> so executing it doesn't really achieve anything.
> 
> This is the standard idiom to allow access for 'everyone, except members
> of a particular group.'

yes, I get this.


> One way you can get weird permissions is if you happen to use decimal
> for permissions bitmaps rather than octal.  A umask of '77' is not the
> same thing at all as a umask of '077'.  (It's effectively 0115, which
> doesn't make much sense to me.)  Most shells nowadays will assume you
> mean octal whether you include the leading zero or not: the same is not
> true if you use umask(2) to set the mask programatically.  Ditto for
> other places you can set permissions like open(2) with O_CREAT or mkdir(2).

# umask
0022
# pwd
/var/spool/output/lpd
# ls -al
total 8
drwxr-xr-x  2 root  daemon  512 Feb 24 12:43 .
drwxr-xr-x  3 root  daemon  512 Mar  9  2010 ..
-rw-rw-r--  1 root  daemon   41 Feb 21 12:54 lock
-rw-rw-r--  1 root  daemon   25 Feb 21 12:54 status
#

Then I print something:

% pwd | lpr

Then this .seq file appears with weird permissions:

# ls -al
total 10
drwxr-xr-x  2 root  daemon  512 Feb 24 12:46 .
drwxr-xr-x  3 root  daemon  512 Mar  9  2010 ..
-rw-r----x  1 root  daemon    4 Feb 24 12:45 .seq
-rw-rw-r--  1 root  daemon   41 Feb 24 12:45 lock
-rw-rw-r--  1 root  daemon   25 Feb 24 12:45 status
# 

# cat .seq 
001
#

So presumably lpd(8) created this file, but I'm still
unsure why permissions are so strange. But interests
me more, is why I didn't see it until about 1-2 months
ago? Has something chaged in -current, e.g. in open(2)
like you suggest? Or has I messed up with my setup?
Or maybe it was always like this, but the security
check didn't pick it up?

> 
> > Should I be worried?
> 
> No more than a normal level of paranoia is indicated here.

Thanks

-- 
Anton Shterenlikht
Room 2.6, Queen's Building
Mech Eng Dept
Bristol University
University Walk, Bristol BS8 1TR, UK
Tel: +44 (0)117 331 5944
Fax: +44 (0)117 929 4423