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Date: Thu, 13 Aug 2020 17:35:53 +0200
From: peter enderborg <peter.enderborg@...y.com>
To: Casey Schaufler <casey@...aufler-ca.com>,
Thiébaud Weksteen <tweek@...gle.com>,
Paul Moore <paul@...l-moore.com>
CC: Nick Kralevich <nnk@...gle.com>,
Stephen Smalley <stephen.smalley.work@...il.com>,
Eric Paris <eparis@...isplace.org>,
Steven Rostedt <rostedt@...dmis.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@...r.kernel.org>, <selinux@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] selinux: add basic filtering for audit trace
events
On 8/13/20 5:05 PM, Casey Schaufler wrote:
> On 8/13/2020 7:48 AM, Thiébaud Weksteen wrote:
>> From: Peter Enderborg <peter.enderborg@...y.com>
>>
>> This patch adds further attributes to the event. These attributes are
>> helpful to understand the context of the message and can be used
>> to filter the events.
>>
>> There are three common items. Source context, target context and tclass.
>> There are also items from the outcome of operation performed.
>>
>> An event is similar to:
>> <...>-1309 [002] .... 6346.691689: selinux_audited:
>> requested=0x4000000 denied=0x4000000 audited=0x4000000
>> result=-13 ssid=315 tsid=61
> It may not be my place to ask, but *please please please* don't
> externalize secids. I understand that it's easier to type "42"
> than "system_r:cupsd_t:s0-s0:c0.c1023", and that it's easier for
> your tools to parse and store the number. Once you start training
> people that system_r:cupsd_t:s0-s0:c0.c1023 is secid 42 you'll
> never be able to change it. The secid will start showing up in
> scripts. Bad Things Will Happen.
Ok, it seems to mostly against having this performance options.
Yes, it is a kernel internal data. So is most of the kernel tracing.
I see it is a primary tool for kernel debugging but than can also be
used for user-space debugging tools. Hiding data for debuggers
does not make any sense too me.
>> scontext=system_u:system_r:cupsd_t:s0-s0:c0.c1023
>> tcontext=system_u:object_r:bin_t:s0 tclass=file
>>
>> With systems where many denials are occurring, it is useful to apply a
>> filter. The filtering is a set of logic that is inserted with
>> the filter file. Example:
>> echo "tclass==\"file\" && ssid!=42" > events/avc/selinux_audited/filter
>>
>> This adds that we only get tclass=file but not for ssid 42.
>>
>> The trace can also have extra properties. Adding the user stack
>> can be done with
>> echo 1 > options/userstacktrace
>>
>> Now the output will be
>> runcon-1365 [003] .... 6960.955530: selinux_audited:
>> requested=0x4000000 denied=0x4000000 audited=0x4000000
>> result=-13 ssid=315 tsid=61
>> scontext=system_u:system_r:cupsd_t:s0-s0:c0.c1023
>> tcontext=system_u:object_r:bin_t:s0 tclass=file
>> runcon-1365 [003] .... 6960.955560: <user stack trace>
>> => <00007f325b4ce45b>
>> => <00005607093efa57>
>>
>> Note that the ssid is the internal numeric representation of scontext
>> and tsid is numeric for tcontext. They are useful for filtering.
>>
>> Signed-off-by: Peter Enderborg <peter.enderborg@...y.com>
>> Reviewed-by: Thiébaud Weksteen <tweek@...gle.com>
>> ---
>> v2 changes:
>> - update changelog to include usage examples
>>
>> include/trace/events/avc.h | 41 ++++++++++++++++++++++++++++----------
>> security/selinux/avc.c | 22 +++++++++++---------
>> 2 files changed, 44 insertions(+), 19 deletions(-)
>>
>> diff --git a/include/trace/events/avc.h b/include/trace/events/avc.h
>> index 07c058a9bbcd..ac5ef2e1c2c5 100644
>> --- a/include/trace/events/avc.h
>> +++ b/include/trace/events/avc.h
>> @@ -1,6 +1,7 @@
>> /* SPDX-License-Identifier: GPL-2.0 */
>> /*
>> - * Author: Thiébaud Weksteen <tweek@...gle.com>
>> + * Authors: Thiébaud Weksteen <tweek@...gle.com>
>> + * Peter Enderborg <Peter.Enderborg@...y.com>
>> */
>> #undef TRACE_SYSTEM
>> #define TRACE_SYSTEM avc
>> @@ -12,23 +13,43 @@
>>
>> TRACE_EVENT(selinux_audited,
>>
>> - TP_PROTO(struct selinux_audit_data *sad),
>> + TP_PROTO(struct selinux_audit_data *sad,
>> + char *scontext,
>> + char *tcontext,
>> + const char *tclass
>> + ),
>>
>> - TP_ARGS(sad),
>> + TP_ARGS(sad, scontext, tcontext, tclass),
>>
>> TP_STRUCT__entry(
>> - __field(unsigned int, tclass)
>> - __field(unsigned int, audited)
>> + __field(u32, requested)
>> + __field(u32, denied)
>> + __field(u32, audited)
>> + __field(int, result)
>> + __string(scontext, scontext)
>> + __string(tcontext, tcontext)
>> + __string(tclass, tclass)
>> + __field(u32, ssid)
>> + __field(u32, tsid)
>> ),
>>
>> TP_fast_assign(
>> - __entry->tclass = sad->tclass;
>> - __entry->audited = sad->audited;
>> + __entry->requested = sad->requested;
>> + __entry->denied = sad->denied;
>> + __entry->audited = sad->audited;
>> + __entry->result = sad->result;
>> + __entry->ssid = sad->ssid;
>> + __entry->tsid = sad->tsid;
>> + __assign_str(tcontext, tcontext);
>> + __assign_str(scontext, scontext);
>> + __assign_str(tclass, tclass);
>> ),
>>
>> - TP_printk("tclass=%u audited=%x",
>> - __entry->tclass,
>> - __entry->audited)
>> + TP_printk("requested=0x%x denied=0x%x audited=0x%x result=%d ssid=%u tsid=%u scontext=%s tcontext=%s tclass=%s",
>> + __entry->requested, __entry->denied, __entry->audited, __entry->result,
>> + __entry->ssid, __entry->tsid, __get_str(scontext), __get_str(tcontext),
>> + __get_str(tclass)
>> + )
>> );
>>
>> #endif
>> diff --git a/security/selinux/avc.c b/security/selinux/avc.c
>> index b0a0af778b70..7de5cc5169af 100644
>> --- a/security/selinux/avc.c
>> +++ b/security/selinux/avc.c
>> @@ -705,35 +705,39 @@ static void avc_audit_post_callback(struct audit_buffer *ab, void *a)
>> {
>> struct common_audit_data *ad = a;
>> struct selinux_audit_data *sad = ad->selinux_audit_data;
>> - char *scontext;
>> + char *scontext = NULL;
>> + char *tcontext = NULL;
>> + const char *tclass = NULL;
>> u32 scontext_len;
>> + u32 tcontext_len;
>> int rc;
>>
>> - trace_selinux_audited(sad);
>> -
>> rc = security_sid_to_context(sad->state, sad->ssid, &scontext,
>> &scontext_len);
>> if (rc)
>> audit_log_format(ab, " ssid=%d", sad->ssid);
>> else {
>> audit_log_format(ab, " scontext=%s", scontext);
>> - kfree(scontext);
>> }
>>
>> - rc = security_sid_to_context(sad->state, sad->tsid, &scontext,
>> - &scontext_len);
>> + rc = security_sid_to_context(sad->state, sad->tsid, &tcontext,
>> + &tcontext_len);
>> if (rc)
>> audit_log_format(ab, " tsid=%d", sad->tsid);
>> else {
>> - audit_log_format(ab, " tcontext=%s", scontext);
>> - kfree(scontext);
>> + audit_log_format(ab, " tcontext=%s", tcontext);
>> }
>>
>> - audit_log_format(ab, " tclass=%s", secclass_map[sad->tclass-1].name);
>> + tclass = secclass_map[sad->tclass-1].name;
>> + audit_log_format(ab, " tclass=%s", tclass);
>>
>> if (sad->denied)
>> audit_log_format(ab, " permissive=%u", sad->result ? 0 : 1);
>>
>> + trace_selinux_audited(sad, scontext, tcontext, tclass);
>> + kfree(tcontext);
>> + kfree(scontext);
>> +
>> /* in case of invalid context report also the actual context string */
>> rc = security_sid_to_context_inval(sad->state, sad->ssid, &scontext,
>> &scontext_len);
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