audit: format user messages to size of MAX_AUDIT_MESSAGE_LENGTH

Messages of type AUDIT_USER_TTY were being formatted to 1024 octets,
truncating messages approaching MAX_AUDIT_MESSAGE_LENGTH (8970 octets).

Set the formatting to 8560 characters, given maximum estimates for prefix and
suffix budgets.

See the problem discussion:
https://www.redhat.com/archives/linux-audit/2009-January/msg00030.html

And the new size rationale:
https://www.redhat.com/archives/linux-audit/2013-September/msg00016.html

Test ~8k messages with:
auditctl -m "$(for i in $(seq -w 001 820);do echo -n "${i}0______";done)"

Reported-by: LC Bruzenak <lenny@magitekltd.com>
Reported-by: Justin Stephenson <jstephen@redhat.com>
Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Eric Paris <eparis@redhat.com>

include/uapi/linux/audit.h

@@ -358,6 +358,12 @@ enum {
 #define AUDIT_PERM_READ		4
 #define AUDIT_PERM_ATTR		8
 
+/* MAX_AUDIT_MESSAGE_LENGTH is set in audit:lib/libaudit.h as:
+ * 8970 // PATH_MAX*2+CONTEXT_SIZE*2+11+256+1
+ * max header+body+tailer: 44 + 29 + 32 + 262 + 7 + pad
+ */
+#define AUDIT_MESSAGE_TEXT_MAX	8560
+
 struct audit_status {
 	__u32		mask;		/* Bit mask for valid entries */
 	__u32		enabled;	/* 1 = enabled, 0 = disabled */

kernel/audit.c

@@ -715,7 +715,8 @@ static int audit_receive_msg(struct sk_buff *skb, struct nlmsghdr *nlh)
 			}
 			audit_log_common_recv_msg(&ab, msg_type);
 			if (msg_type != AUDIT_USER_TTY)
-				audit_log_format(ab, " msg='%.1024s'",
+				audit_log_format(ab, " msg='%.*s'",
+						 AUDIT_MESSAGE_TEXT_MAX,
 						 (char *)data);
 			else {
 				int size;