[PATCH] capabilities: do not audit log BPRM_FCAPS on set*id

[Richard Guy Briggs]

On 2017-03-02 20:07, Serge E. Hallyn wrote:
> On Thu, Mar 02, 2017 at 08:10:29PM -0500, Richard Guy Briggs wrote:
> > The audit subsystem is adding a BPRM_FCAPS record when auditing setuid
> > application execution (SYSCALL execve). This is not expected as it was
> > supposed to be limited to when the file system actually had capabilities
> > in an extended attribute.  It lists all capabilities making the event
> > really ugly to parse what is happening.  The PATH record correctly
> > records the setuid bit and owner.  Suppress the BPRM_FCAPS record on
> > set*id.
> > 
> > See: https://github.com/linux-audit/audit-kernel/issues/16
> 
> Hey Richard,

Hi Serge,

> one possibly audit-worth case which (if I read correctly) this will
> skip is where a setuid-root binary has filecaps which *limit* its privs.
> Does that matter?

I hadn't thought of that case, but I did consider in the setuid case
comparing before and after without setuid forcing the drop of all
capabilities via "ambient".  Mind you, this bug has been around before
Luto's patch that adds the ambient capabilities set.

Paul?

> > Signed-off-by: Richard Guy Briggs <rgb at redhat.com>
> > ---
> >  security/commoncap.c |    5 +++--
> >  1 files changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/security/commoncap.c b/security/commoncap.c
> > index 14540bd..8f6bedf 100644
> > --- a/security/commoncap.c
> > +++ b/security/commoncap.c
> > @@ -594,16 +594,17 @@ skip:
> >  	/*
> >  	 * Audit candidate if current->cap_effective is set
> >  	 *
> > -	 * We do not bother to audit if 3 things are true:
> > +	 * We do not bother to audit if 4 things are true:
> >  	 *   1) cap_effective has all caps
> >  	 *   2) we are root
> >  	 *   3) root is supposed to have all caps (SECURE_NOROOT)
> > +	 *   4) we are running a set*id binary
> >  	 * Since this is just a normal root execing a process.
> >  	 *
> >  	 * Number 1 above might fail if you don't have a full bset, but I think
> >  	 * that is interesting information to audit.
> >  	 */
> > -	if (!cap_issubset(new->cap_effective, new->cap_ambient)) {
> > +	if (!is_setid && !cap_issubset(new->cap_effective, new->cap_ambient)) {
> >  		if (!cap_issubset(CAP_FULL_SET, new->cap_effective) ||
> >  		    !uid_eq(new->euid, root_uid) || !uid_eq(new->uid, root_uid) ||
> >  		    issecure(SECURE_NOROOT)) {
> > -- 
> > 1.7.1

- RGB

--
Richard Guy Briggs <rgb at redhat.com>
Kernel Security Engineering, Base Operating Systems, Red Hat
Remote, Ottawa, Canada
Voice: +1.647.777.2635, Internal: (81) 32635