AUDIT: Spawn kernel thread to list filter rules.

If we have enough rules to fill the netlink buffer space, it'll 
deadlock because auditctl isn't ever actually going to read from the 
socket until we return, and we aren't going to return until it 
reads... so we spawn a kernel thread to spew out the list and then
exit.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>

include/linux/audit.h

@@ -281,6 +281,7 @@ extern void		    audit_send_reply(int pid, int seq, int type,
 					     int done, int multi,
 					     void *payload, int size);
 extern void		    audit_log_lost(const char *message);
+extern struct semaphore audit_netlink_sem;
 #else
 #define audit_log(c,t,f,...) do { ; } while (0)
 #define audit_log_start(c,t) ({ NULL; })

kernel/audit.c

@@ -110,7 +110,7 @@ static DECLARE_WAIT_QUEUE_HEAD(kauditd_wait);
 /* The netlink socket is only to be read by 1 CPU, which lets us assume
  * that list additions and deletions never happen simultaneously in
  * auditsc.c */
-static DECLARE_MUTEX(audit_netlink_sem);
+DECLARE_MUTEX(audit_netlink_sem);
 
 /* AUDIT_BUFSIZ is the size of the temporary buffer used for formatting
  * audit records.  Since printk uses a 1024 byte buffer, this buffer

kernel/auditsc.c

@@ -39,6 +39,7 @@
 #include <linux/audit.h>
 #include <linux/personality.h>
 #include <linux/time.h>
+#include <linux/kthread.h>
 #include <asm/unistd.h>
 
 /* 0 = no checking
@@ -281,24 +282,60 @@ static int audit_copy_rule(struct audit_rule *d, struct audit_rule *s)
 	return 0;
 }
 
+static int audit_list_rules(void *_dest)
+{
+	int pid, seq;
+	int *dest = _dest;
+	struct audit_entry *entry;
+	int i;
+
+	pid = dest[0];
+	seq = dest[1];
+	kfree(dest);
+
+	down(&audit_netlink_sem);
+
+	/* The *_rcu iterators not needed here because we are
+	   always called with audit_netlink_sem held. */
+	for (i=0; i<AUDIT_NR_FILTERS; i++) {
+		list_for_each_entry(entry, &audit_filter_list[i], list)
+			audit_send_reply(pid, seq, AUDIT_LIST, 0, 1,
+					 &entry->rule, sizeof(entry->rule));
+	}
+	audit_send_reply(pid, seq, AUDIT_LIST, 1, 1, NULL, 0);
+
+	up(&audit_netlink_sem);
+	return 0;
+}
+
 int audit_receive_filter(int type, int pid, int uid, int seq, void *data,
 							uid_t loginuid)
 {
 	struct audit_entry *entry;
+	struct task_struct *tsk;
+	int *dest;
 	int		   err = 0;
-	int i;
 	unsigned listnr;
 
 	switch (type) {
 	case AUDIT_LIST:
-		/* The *_rcu iterators not needed here because we are
-		   always called with audit_netlink_sem held. */
-		for (i=0; i<AUDIT_NR_FILTERS; i++) {
-			list_for_each_entry(entry, &audit_filter_list[i], list)
-				audit_send_reply(pid, seq, AUDIT_LIST, 0, 1,
-						 &entry->rule, sizeof(entry->rule));
+		/* We can't just spew out the rules here because we might fill
+		 * the available socket buffer space and deadlock waiting for
+		 * auditctl to read from it... which isn't ever going to
+		 * happen if we're actually running in the context of auditctl
+		 * trying to _send_ the stuff */
+
+		dest = kmalloc(2 * sizeof(int), GFP_KERNEL);
+		if (!dest)
+			return -ENOMEM;
+		dest[0] = pid;
+		dest[1] = seq;
+
+		tsk = kthread_run(audit_list_rules, dest, "audit_list_rules");
+		if (IS_ERR(tsk)) {
+			kfree(dest);
+			err = PTR_ERR(tsk);
 		}
-		audit_send_reply(pid, seq, AUDIT_LIST, 1, 1, NULL, 0);
 		break;
 	case AUDIT_ADD:
 		if (!(entry = kmalloc(sizeof(*entry), GFP_KERNEL)))