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

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Message-ID: <4427545e-d4ea-b64e-91d9-3ccd2a483df1@sony.com>
Date:   Sat, 15 Aug 2020 09:17:07 +0200
From:   peter enderborg <peter.enderborg@...y.com>
To:     Steven Rostedt <rostedt@...dmis.org>,
        Thiébaud Weksteen <tweek@...gle.com>
CC:     Stephen Smalley <stephen.smalley.work@...il.com>,
        Paul Moore <paul@...l-moore.com>,
        Nick Kralevich <nnk@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Eric Paris <eparis@...isplace.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Rob Herring <robh@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        SElinux list <selinux@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] selinux: add tracepoint on denials

On 8/14/20 7:46 PM, Steven Rostedt wrote:
> On Fri, 14 Aug 2020 19:22:13 +0200
> peter enderborg <peter.enderborg@...y.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@...y.com> wrote:  
>>>> On 8/14/20 6:51 PM, Stephen Smalley wrote:  
>>>>> On Fri, Aug 14, 2020 at 9:05 AM Thiébaud Weksteen <tweek@...gle.com> wrote:  
>>>>>> On Thu, Aug 13, 2020 at 5:41 PM Stephen Smalley
>>>>>> <stephen.smalley.work@...il.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

That works fine. I will do this as third patch in our patch-set.  But I think we also should export the permission-map
somewhere. I don’t think there is any good place for it in tracefs. So selinuxfs or debugfs might do? And I think it is
more useful to print what is denied than what is audited but that does not match the trace event name.