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@xxxxxxxxxx> wrote:
> > Quoting Xavier Toth (txtoth@xxxxxxxxx):
> >> 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
> setcap cap_audit_write=ei /usr/bin/python
> 4) run audit-wrapper
> 5) check audit log for audit records.
>
> I also ran audit_test.py without the wrapper to verify that no audit
> would occur.
>
> Ted
>
> ------------------------------------------------------------------------------
> audit-wrapper.c
> ------------------------------------------------------------------------
>
<snip>
> cap_t cur = cap_from_text("cap_audit_write+i");
> int ret = cap_set_proc(cur);
> if (ret) {
> perror("cap_set_proc");
> return 1;
> }
> cap_free(cur);
Looks like you could make your wrapper generic for any capabilities by
having it copy the permitted (or effective) set to the inheritable set.
Something like this:
cap_t cur = cap_get_proc();
cap_value_t caps[1];
cap_flag_value_t value;
int i, rc;
for (i = 0; i < CAP_LAST_CAP; i++) {
rc = cap_get_flag(cur, i, CAP_PERMITTED, &value);
if (rc < 0) {
perror("cap_get_flag");
return 1;
}
caps[0] = i;
rc = cap_set_flag(cur, CAP_INHERITABLE, 1, caps, value);
if (rc < 0) {
perror("cap_set_flag");
return 1;
}
}
rc = cap_set_proc(cur);
if (rc < 0) {
perror("cap_set_proc");
return 1;
}
cap_free(cur);
Serge - is there any easier way?
--
Stephen Smalley
National Security Agency
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