Re: [PATCH v6 3/3] fanotify,audit: Allow audit to use the full permission event response

[Steve Grubb]

On Friday, January 27, 2023 3:00:37 PM EST Paul Moore wrote:
> On Wed, Jan 25, 2023 at 5:06 PM Richard Guy Briggs <rgb@xxxxxxxxxx> wrote:
> > On 2023-01-20 13:52, Paul Moore wrote:
> > > On Wed, Jan 18, 2023 at 1:34 PM Steve Grubb <sgrubb@xxxxxxxxxx> wrote:
> > > > Hello Richard,
> > > > 
> > > > I built a new kernel and tested this with old and new user space. It
> > > > is
> > > > working as advertised. The only thing I'm wondering about is why we
> > > > have 3F as the default value when no additional info was sent? Would
> > > > it be better to just make it 0?
> > > 
> > > ...
> > > 
> > > > On Tuesday, January 17, 2023 4:14:07 PM EST Richard Guy Briggs wrote:
> > > > > diff --git a/kernel/auditsc.c b/kernel/auditsc.c
> > > > > index d1fb821de104..3133c4175c15 100644
> > > > > --- a/kernel/auditsc.c
> > > > > +++ b/kernel/auditsc.c
> > > > > @@ -2877,10 +2878,19 @@ void __audit_log_kern_module(char *name)
> > > > > 
> > > > >       context->type = AUDIT_KERN_MODULE;
> > > > >  
> > > > >  }
> > > > > 
> > > > > -void __audit_fanotify(u32 response)
> > > > > +void __audit_fanotify(u32 response, struct
> > > > > fanotify_response_info_audit_rule *friar) {
> > > > > -     audit_log(audit_context(), GFP_KERNEL,
> > > > > -             AUDIT_FANOTIFY, "resp=%u", response);
> > > > > +     /* {subj,obj}_trust values are {0,1,2}: no,yes,unknown */
> > > > > +     if (friar->hdr.type == FAN_RESPONSE_INFO_NONE) {
> > > > > +             audit_log(audit_context(), GFP_KERNEL,
> > > > > AUDIT_FANOTIFY,
> > > > > +                       "resp=%u fan_type=%u fan_info=3F
> > > > > subj_trust=2
> > > > 
> > > > obj_trust=2",
> > > > 
> > > > > +                       response, FAN_RESPONSE_INFO_NONE);
> > > > > +             return;
> > > > > +     }
> > > 
> > > (I'm working under the assumption that the "fan_info=3F" in the record
> > > above is what Steve was referring to in his comment.)
> > > 
> > > I vaguely recall Richard commenting on this in the past, although
> > > maybe not ... my thought is that the "3F" is simply the hex encoded
> > > "?" character in ASCII ('man 7 ascii' is your friend).  I suppose the
> > > question is what to do in the FAN_RESPONSE_INFO_NONE case.
> > > 
> > > Historically when we had a missing field we would follow the "field=?"
> > > pattern, but I don't recall doing that for a field which was
> > > potentially hex encoded, is there an existing case where we use "?"
> > > for a field that is hex encoded?  If so, we can swap out the "3F" for
> > > a more obvious "?".
> > 
> > I was presuming encoding the zero: "30"
> 
> I'm sorry, but you've lost me here.
> 
> > > However, another option might be to simply output the current
> > > AUDIT_FANOTIFY record format in the FAN_RESPONSE_INFO_NONE case, e.g.
> > > only "resp=%u".  This is a little against the usual guidance of
> > > "fields should not disappear from a record", but considering that
> > > userspace will always need to support the original resp-only format
> > > for compatibility reasons this may be an option.
> > 
> > I don't have a strong opinion.
> 
> I'm not sure I care too much either.  I will admit that the "3F" seems
> to be bordering on the "bit too clever" side of things, but it's easy
> to argue it is in keeping with the general idea of using "?" to denote
> absent/unknown fields.

The translation will be from %X to %u. In that case, someone might think 63 
has some meaning. It would be better to leave it as 0 so there's less to 
explain.

-Steve

> As Steve was the one who raised the question in this latest round, and
> he knows his userspace tools the best, it seems wise to get his input
> on this.