negative group permissions?
Anton Shterenlikht
mexas at bristol.ac.uk
Fri Feb 24 14:49:06 UTC 2012
On Fri, Feb 24, 2012 at 03:25:52PM +0100, Trond Endrest?l wrote:
> On Fri, 24 Feb 2012 14:04-0000, Anton Shterenlikht wrote:
>
> > On Fri, Feb 24, 2012 at 02:41:44PM +0100, Trond Endrest?l wrote:
> > > On Fri, 24 Feb 2012 12:54-0000, Anton Shterenlikht wrote:
> > >
> > > > 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.
> > >
> > > Looking at usr.sbin/lpr/lpr/lpr.c at around line 847 (RELENG_9):
> > >
> > > (void) snprintf(buf, sizeof(buf), "%s/.seq", pp->spool_dir);
> > > seteuid(euid);
> > > if ((fd = open(buf, O_RDWR|O_CREAT, 0661)) < 0) {
> > > printf("%s: cannot create %s\n", progname, buf);
> > > exit(1);
> > > }
> > > if (flock(fd, LOCK_EX)) {
> > > printf("%s: cannot lock %s\n", progname, buf);
> > > exit(1);
> > > }
> > >
> > > It remains a mystery why these files are created with mode 0661. Mode
> >
> > Isn't .seq above has mode 641?
> >
> > % chmod 641 z
> > % ls -al z
> > -rw-r----x 1 mexas wheel 0 Feb 24 13:59 z
> > %
>
> It sure is, in all cases quoted above.
>
> All handling of the .seq files seems to be contained within the
> mktemps() function of usr.sbin/lpr/lpr/lpr.c.
>
> The call to open(2) with the mode set to 0661 has been there since CVS
> revision 1.1 of usr.sbin/lpr/lpr/lpr.c, see
> http://www.freebsd.org/cgi/cvsweb.cgi/src/usr.sbin/lpr/lpr/lpr.c?annotate=1.45.2.1.2.1
>
> No calls to chmod(2) of the .seq files anywhere else, as far as I can
> tell.
>
> I usually keep tight permissions on the spool directories, mode 0770.
It seems I need 755, otherwise dialer and smmsp
will not have access:
# ls -al /var/spool/
total 28
drwxr-xr-x 8 root wheel 512 Nov 21 2009 .
drwxr-xr-x 25 root wheel 512 Jan 31 02:03 ..
drwxrwx--- 2 smmsp smmsp 512 Feb 24 03:39 clientmqueue
drwxrwxr-x 2 uucp dialer 512 Jan 31 02:04 lock
drwxr-xr-x 2 root daemon 512 Nov 21 2009 lpd
drwxr-xr-x 2 root daemon 14336 Feb 24 03:40 mqueue
drwx------ 2 root daemon 512 Nov 21 2009 opielocks
drwxr-xr-x 3 root daemon 512 Mar 9 2010 output
#
>
> It's still a mystery. Thus it's time to bring in people with more
> knowledge on lpr and friends.
sure
--
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
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