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

[Richard Guy Briggs]

On 2017-03-02 21:50, Richard Guy Briggs wrote:
> 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.

Can you suggest a scenario where that might happen?

Can you come up with an idea for a test case?  At first I figured I
could simply go from root and su to an unprivileged user, but that
doesn't trigger it and then naively thought I could strace both
directions to find out the difference and su or sudo to root really
doesn't like being straced.

> 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

- 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