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>

2 files changed, 8 insertions, 1 deletions

diff --git a/include/uapi/linux/audit.h b/include/uapi/linux/audit.h
index 75cef3fd97ad..5dfcd85037e2 100644
--- a/include/uapi/linux/audit.h
+++ b/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 */
diff --git a/kernel/audit.c b/kernel/audit.c
index 91e53d04b6a9..dd63d2f978d5 100644
--- a/kernel/audit.c
+++ b/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;