Re: [PATCH v2 1/2] selinux: add tracepoint on denials

[Steven Rostedt <rostedt@goodmis.org>]

On Fri, 14 Aug 2020 19:22:13 +0200
peter enderborg <peter.enderborg@sony.com> wrote:

> On 8/14/20 7:08 PM, Stephen Smalley wrote:
> > On Fri, Aug 14, 2020 at 1:07 PM peter enderborg
> > <peter.enderborg@sony.com> wrote:  
> >> On 8/14/20 6:51 PM, Stephen Smalley wrote:  
> >>> On Fri, Aug 14, 2020 at 9:05 AM Thiébaud Weksteen <tweek@google.com> wrote:  
> >>>> On Thu, Aug 13, 2020 at 5:41 PM Stephen Smalley
> >>>> <stephen.smalley.work@gmail.com> wrote:  
> >>>>> An explanation here of how one might go about decoding audited and
> >>>>> tclass would be helpful to users (even better would be a script to do it
> >>>>> for them).  Again, I know how to do that but not everyone using
> >>>>> perf/ftrace will.  
> >>>> What about something along those lines:
> >>>>
> >>>> The tclass value can be mapped to a class by searching
> >>>> security/selinux/flask.h. The audited value is a bit field of the
> >>>> permissions described in security/selinux/av_permissions.h for the
> >>>> corresponding class.  
> >>> Sure, I guess that works.  Would be nice if we just included the class
> >>> and permission name(s) in the event itself but I guess you viewed that
> >>> as too heavyweight?  
> >> The class name is added in part 2. Im not sure how a proper format for permission
> >> would look like in trace terms. It is a list, right?  
> > Yes.  See avc_audit_pre_callback() for example code to log the permission names.  
> 
> I wrote about that on some of the previous sets. The problem is that trace format is quite fixed. So it is lists are not
> that easy to handle if you want to filter in them. You can have a trace event for each of them. You can also add
> additional trace event "selinux_audied_permission" for each permission. With that you can filter out tclass or permissions.
> 
> But the basic thing we would like at the moment is a event that we can debug in user space.

We have a trace_seq p helper, that lets you create strings in
TP_printk(). I should document this more. Thus you can do:

extern const char *audit_perm_to_name(struct trace_seq *p, u16 class, u32 audited);
#define __perm_to_name(p, class, audited) audit_perm_to_name(p, class, audited)

	TP_printk("tclass=%u audited=%x (%s)",
		__entry->tclass,
		__entry->audited,
		__perm_to_name(__entry->tclass, __entry->audited))


const char *audit_perm_to_name(struct trace_seq *p, u16 tclass, u32 av)
{
	const char *ret = trace_seq_buffer_ptr(p);
	int i, perm;

	( some check for tclass integrity here)

	perms = secclass_map[tclass-1].perms;

	i = 0;
	perm = 1;
	while (i < (sizeof(av) * 8)) {
		if ((perm & av) && perms[i]) {
			trace_seq_printf(p, " %s", perms[i]);
			av &= ~perm;
		}
		i++;
		perm <<= 1;
	}

	return ret;
}

Note, this wont work for perf and trace-cmd as it wouldn't know how to
parse it, but if the tclass perms are stable, you could create a plugin
to libtraceevent that can do the above as well.

-- Steve