Re: problem with capabilities inheritance and auditing in python

["Serge E. Hallyn" <serue@us.ibm.com>]

Quoting Stephen Smalley (sds@tycho.nsa.gov):
> On Tue, 2009-02-10 at 14:59 -0600, Xavier Toth wrote:
> > On Tue, Feb 10, 2009 at 2:33 PM, Stephen Smalley <sds@tycho.nsa.gov> wrote:
> > > On Tue, 2009-02-10 at 14:20 -0600, Xavier Toth wrote:
> > >> On Tue, Feb 10, 2009 at 12:34 PM, Serge E. Hallyn <serue@us.ibm.com> wrote:
> > >> > Quoting Xavier Toth (txtoth@gmail.com):
> > >> >> I was not putting capabilities on the script but rather on a compiled
> > >> >> wrapper which execs a python script in which I need to do auditing.
> > >> >> Will this not work?
> > >> >
> > >> > No, because of the way capabilities are re-calculated on exec().
> > >> >
> > >> >        pI' = pI
> > >> >        pP' = (X&fP) | (pI & fI)
> > >> >        pE' = fE ? pP' : 0
> > >> >
> > >> > So since the interpreter has fI=fP=fE=0 and is not setuid root (which
> > >> > would fill in fP and/or fE to emulate privileged root), pP' and pE' will
> > >> > be empty after exec().
> > >> >
> > >> > Now you could use a wrapper as follows:  Have the wrapper fill pI,
> > >> > and then fill fI on the python interpreter.  Any user who has an
> > >> > empty pI (which generally is all users) will execute python scripts
> > >> > with no privilege, but when the wrapper execs the script, pP' will
> > >> > be filled with (pI&fI) = full.
> > >> >
> > >> > -serge
> > >> >
> > >>
> > >> Thanks for the clarification.
> > >> For anyone one that is interested I've included some test code. The
> > >> wrapper is a modified version of a wrapper Stephen sent me a link to.
> > >> Basic steps to test are:
> > >> 1) edit the wrapper to set the path to the audit_test.py script
> > >> 2) compiler the wrapper
> > >>     gcc -o audit-wrapper audit-wrapper.c -lcap
> > >> 3) set the capabilities on the wrapper and python
> > >>     setcap cap_audit_write,cap_setfcap=epi audit-wrapper
> > >
> > > Why cap_setfcap (set file capability)?
> > 
> > The wrapper adds the 'i' back to cap_audit_write as it goes away when
> > audit-wrapper runs. I was printing the capabilities in the wrapper for
> > debug purposes when I noticed that it capabilities were "=
> > cap_audit_write,cap_setfcap+ep". I think without the i cap_audit_write
> > can't be inherited by the child process.
> 
> cap_setfcap controls setting of file capabilities via setxattr(2).
> cap_setpcap may have an effect on setting of process capabilities, but I
> don't think it is required if you are just setting inheritable to
> something in your permitted set.

Right, to be able to put cap_audit_write in pI, you either need to
have cap_audit_write in pP, or to have cap_setpcap in pP.

-serge

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