Fix DoS condition on recursive logging

This patch fixes a denial-of-service condition when exec is denied on a process
with a lot of parents. It's caused by the parent_task_walk() being a recursive
function.

Recursive function in the linux kernel? What was I thinking? I wasn't,
actually, as my main goal with the project initially was proof-of-concept. Now
that I'm actually using the thing in ad-hoc production environments, it's time
I search the code for problems. Well, I found one!

This patch also introduces a new sysctl entry: log_max

README

@@ -52,6 +52,7 @@ check_file	- check file owner/mode in addition to directory. default on
 kill		- kill the offending process and its parent when it gets denied
 		  execution from TPE, unless it's root. default off
 log		- whether to log denied execs to the ring buffer. default on
+log_max		- maximun parent processes in a single log entry. default 50
 log_floodburst	- number of log entries before logging is disabled. default 5
 log_floodtime	- seconds until re-enabling logging after floodburst. default 5
 paranoid	- denies execs for root of files not owned by root. default off

core.c

@@ -62,16 +62,25 @@ void parent_task_walk(struct task_struct *task) {
 
 	struct task_struct *parent;
 	char filename[MAX_FILE_LEN];
+	int c = 0;
 
+	walk:
+	c++;
 	if (task && task->mm) {
 
+		if (tpe_log_max && c > tpe_log_max) {
+			printk("tpe log_max %d reached", tpe_log_max);
+			return;
+		}
+
 		parent = get_task_parent(task);
 
 		printk("%s (uid:%d)", exe_from_mm(task->mm, filename, MAX_FILE_LEN), get_task_uid(task));
 
 		if (parent && task->pid != 1) {
 			printk(", ");
-			parent_task_walk(parent);
+			task = parent;
+			goto walk;
 		}
 	}
 

module.h

@@ -76,6 +76,7 @@ extern int tpe_check_file;
 extern int tpe_paranoid;
 extern int tpe_kill;
 extern int tpe_log;
+extern int tpe_log_max;
 extern int tpe_log_floodtime;
 extern int tpe_log_floodburst;
 extern int tpe_dmesg;

sysctl.c

@@ -10,6 +10,7 @@ int tpe_check_file = 1;
 int tpe_paranoid = 0;
 int tpe_kill = 0;
 int tpe_log = 1;
+int tpe_log_max = 50;
 int tpe_log_floodtime = LOG_FLOODTIME;
 int tpe_log_floodburst = LOG_FLOODBURST;
 int tpe_dmesg = 0;
@@ -117,6 +118,14 @@ static ctl_table tpe_table[] = {
 		.mode		= 0644,
 		.proc_handler	= &proc_dointvec,
 	},
+	{
+		.ctl_name	= CTL_UNNUMBERED,
+		.procname	= "log_max",
+		.data		= &tpe_log_max,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= &proc_dointvec,
+	},
 	{
 		.ctl_name	= CTL_UNNUMBERED,
 		.procname	= "log_floodtime",