Re: [PATCH v4 3/4] fanotify,audit: Allow audit to use the full permission event response

[Richard Guy Briggs <rgb@redhat.com>]

On 2022-09-09 10:22, Steve Grubb wrote:
> On Friday, September 9, 2022 7:09:44 AM EDT Jan Kara wrote:
> > Hello Steve!
> > 
> > On Fri 09-09-22 00:03:53, Steve Grubb wrote:
> > > On Thursday, September 8, 2022 10:41:44 PM EDT Richard Guy Briggs wrote:
> > > > > I'm trying to abide by what was suggested by the fs-devel folks. I
> > > > > can
> > > > > live with it. But if you want to make something non-generic for all
> > > > > users of fanotify, call the new field "trusted". This would decern
> > > > > when
> > > > > a decision was made because the file was untrusted or access denied
> > > > > for
> > > > > another reason.
> > > > 
> > > > So, "u32 trusted;" ?  How would you like that formatted?
> > > > "fan_trust={0|1}"
> > > 
> > > So how does this play out if there is another user? Do they want a num=
> > > and trust=  if not, then the AUDIT_FANOTIFY record will have multiple
> > > formats which is not good. I'd rather suggest something generic that can
> > > be interpreted based on who's attached to fanotify. IOW we have a
> > > fan_type=0 and then followed by info0= info1=  the interpretation of
> > > those solely depend on fan_type. If the fan_type does not need both,
> > > then any interpretation skips what it doesn't need. If fan_type=1, then
> > > it follows what arg0= and arg1= is for that format. But make this pivot
> > > on fan_type and not actual names.
> > So I think there is some misunderstanding so let me maybe spell out in
> > detail how I see things so that we can get on the same page:
> > 
> > It was a requirement from me (and probably Amir) that there is a generic
> > way to attach additional info to a response to fanotify permission event.
> > This is achieved by defining:
> > 
> > struct fanotify_response_info_header {
> >        __u8 type;
> >        __u8 pad;
> >        __u16 len;
> > };
> > 
> > which is a generic header and kernel can based on 'len' field decide how
> > large the response structure is (to safely copy it from userspace) and
> > based on 'type' field it can decide who should be the recipient of this
> > extra information (or generally what to do with it). So any additional
> > info needs to start with this header.
> > 
> > Then there is:
> > 
> > struct fanotify_response_info_audit_rule {
> >        struct fanotify_response_info_header hdr;
> >        __u32 audit_rule;
> > };
> > 
> > which properly starts with the header and hdr.type is expected to be
> > FAN_RESPONSE_INFO_AUDIT_RULE. What happens after the header with type
> > FAN_RESPONSE_INFO_AUDIT_RULE until length hdr.len is fully within *audit*
> > subsystem's responsibility. Fanotify code will just pass this as an opaque
> > blob to the audit subsystem.
> > 
> > So if you know audit subsystem will also need some other field together
> > with 'audit_rule' now is a good time to add it and it doesn't have to be
> > useful for anybody else besides audit. If someone else will need other
> > information passed along with the response, he will append structure with
> > another header with different 'type' field. In principle, there can be
> > multiple structures appended to fanotify response like
> > 
> > <hdr> <data> <hdr> <data> ...
> > 
> > and fanotify subsystem will just pass them to different receivers based
> > on the type in 'hdr' field.
> > 
> > Also if audit needs to pass even more information along with the respose,
> > we can define a new 'type' for it. But the 'type' space is not infinite so
> > I'd prefer this does not happen too often...
> > 
> > I hope this clears out things a bit.
> 
> Yes. Thank you.
> 
> Richard,  add subj_trust and obj_trust. These can be 0|1|2 for no, yes, 
> unknown.

type?  bitfield?  My gut would say that "0" should be "unset"/"unknown",
but that is counterintuitive to the values represented.

Or "trust" with sub-fields "subj" and "obj"?

> -Steve

- RGB

--
Richard Guy Briggs <rgb@redhat.com>
Sr. S/W Engineer, Kernel Security, Base Operating Systems
Remote, Ottawa, Red Hat Canada
IRC: rgb, SunRaycer
Voice: +1.647.777.2635, Internal: (81) 32635