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

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Date:   Fri, 20 Jan 2023 13:52:37 -0500
From:   Paul Moore <paul@...l-moore.com>
To:     Steve Grubb <sgrubb@...hat.com>
Cc:     Linux-Audit Mailing List <linux-audit@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
        Richard Guy Briggs <rgb@...hat.com>,
        Eric Paris <eparis@...isplace.org>, Jan Kara <jack@...e.cz>,
        Amir Goldstein <amir73il@...il.com>
Subject: Re: [PATCH v6 3/3] fanotify,audit: Allow audit to use the full
 permission event response

On Wed, Jan 18, 2023 at 1:34 PM Steve Grubb <sgrubb@...hat.com> 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 "?".

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.

--
paul-moore.com