Re: [PATCH v3 2/3] selinux: add basic filtering for audit trace events

[Stephen Smalley <stephen.smalley.work@gmail.com>]

On 8/17/20 1:07 PM, Thiébaud Weksteen wrote:

> From: Peter Enderborg <peter.enderborg@sony.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
>         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\" " > events/avc/selinux_audited/filter
>
> This adds that we only get tclass=file.
>
> 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
>       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>
>
> Signed-off-by: Peter Enderborg <peter.enderborg@sony.com>
> Reviewed-by: Thiébaud Weksteen <tweek@google.com>
> ---
>   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);
>   	}
I guess technically you should drop the { } above since it is reduced to 
a single statement body.
>   
> -	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);
>   	}
Ditto.