Audit: add TTY input auditing

Add TTY input auditing, used to audit system administrator's actions.  This is
required by various security standards such as DCID 6/3 and PCI to provide
non-repudiation of administrator's actions and to allow a review of past
actions if the administrator seems to overstep their duties or if the system
becomes misconfigured for unknown reasons.  These requirements do not make it
necessary to audit TTY output as well.

Compared to an user-space keylogger, this approach records TTY input using the
audit subsystem, correlated with other audit events, and it is completely
transparent to the user-space application (e.g.  the console ioctls still
work).

TTY input auditing works on a higher level than auditing all system calls
within the session, which would produce an overwhelming amount of mostly
useless audit events.

Add an "audit_tty" attribute, inherited across fork ().  Data read from TTYs
by process with the attribute is sent to the audit subsystem by the kernel.
The audit netlink interface is extended to allow modifying the audit_tty
attribute, and to allow sending explanatory audit events from user-space (for
example, a shell might send an event containing the final command, after the
interactive command-line editing and history expansion is performed, which
might be difficult to decipher from the TTY input alone).

Because the "audit_tty" attribute is inherited across fork (), it would be set
e.g.  for sshd restarted within an audited session.  To prevent this, the
audit_tty attribute is cleared when a process with no open TTY file
descriptors (e.g.  after daemon startup) opens a TTY.

See https://www.redhat.com/archives/linux-audit/2007-June/msg00000.html for a
more detailed rationale document for an older version of this patch.

[akpm@linux-foundation.org: build fix]
Signed-off-by: Miloslav Trmac <mitr@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Paul Fulghum <paulkf@microgate.com>
Cc: Casey Schaufler <casey@schaufler-ca.com>
Cc: Steve Grubb <sgrubb@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 88 insertions, 8 deletions

diff --git a/kernel/audit.c b/kernel/audit.c
index d13276d..5ce8851 100644
--- a/kernel/audit.c
+++ b/kernel/audit.c
@@ -58,6 +58,7 @@
 #include <linux/selinux.h>
 #include <linux/inotify.h>
 #include <linux/freezer.h>
+#include <linux/tty.h>
 
 #include "audit.h"
 
@@ -423,6 +424,31 @@ static int kauditd_thread(void *dummy)
 	return 0;
 }
 
+static int audit_prepare_user_tty(pid_t pid, uid_t loginuid)
+{
+	struct task_struct *tsk;
+	int err;
+
+	read_lock(&tasklist_lock);
+	tsk = find_task_by_pid(pid);
+	err = -ESRCH;
+	if (!tsk)
+		goto out;
+	err = 0;
+
+	spin_lock_irq(&tsk->sighand->siglock);
+	if (!tsk->signal->audit_tty)
+		err = -EPERM;
+	spin_unlock_irq(&tsk->sighand->siglock);
+	if (err)
+		goto out;
+
+	tty_audit_push_task(tsk, loginuid);
+out:
+	read_unlock(&tasklist_lock);
+	return err;
+}
+
 int audit_send_list(void *_dest)
 {
 	struct audit_netlink_list *dest = _dest;
@@ -511,6 +537,8 @@ static int audit_netlink_ok(struct sk_buff *skb, u16 msg_type)
 	case AUDIT_DEL:
 	case AUDIT_DEL_RULE:
 	case AUDIT_SIGNAL_INFO:
+	case AUDIT_TTY_GET:
+	case AUDIT_TTY_SET:
 		if (security_netlink_recv(skb, CAP_AUDIT_CONTROL))
 			err = -EPERM;
 		break;
@@ -622,6 +650,11 @@ static int audit_receive_msg(struct sk_buff *skb, struct nlmsghdr *nlh)
 		err = audit_filter_user(&NETLINK_CB(skb), msg_type);
 		if (err == 1) {
 			err = 0;
+			if (msg_type == AUDIT_USER_TTY) {
+				err = audit_prepare_user_tty(pid, loginuid);
+				if (err)
+					break;
+			}
 			ab = audit_log_start(NULL, GFP_KERNEL, msg_type);
 			if (ab) {
 				audit_log_format(ab,
@@ -638,8 +671,17 @@ static int audit_receive_msg(struct sk_buff *skb, struct nlmsghdr *nlh)
 							" subj=%s", ctx);
 					kfree(ctx);
 				}
-				audit_log_format(ab, " msg='%.1024s'",
-					 (char *)data);
+				if (msg_type != AUDIT_USER_TTY)
+					audit_log_format(ab, " msg='%.1024s'",
+							 (char *)data);
+				else {
+					int size;
+
+					audit_log_format(ab, " msg=");
+					size = nlmsg_len(nlh);
+					audit_log_n_untrustedstring(ab, size,
+								    data);
+				}
 				audit_set_pid(ab, pid);
 				audit_log_end(ab);
 			}
@@ -730,6 +772,45 @@ static int audit_receive_msg(struct sk_buff *skb, struct nlmsghdr *nlh)
 				0, 0, sig_data, sizeof(*sig_data) + len);
 		kfree(sig_data);
 		break;
+	case AUDIT_TTY_GET: {
+		struct audit_tty_status s;
+		struct task_struct *tsk;
+
+		read_lock(&tasklist_lock);
+		tsk = find_task_by_pid(pid);
+		if (!tsk)
+			err = -ESRCH;
+		else {
+			spin_lock_irq(&tsk->sighand->siglock);
+			s.enabled = tsk->signal->audit_tty != 0;
+			spin_unlock_irq(&tsk->sighand->siglock);
+		}
+		read_unlock(&tasklist_lock);
+		audit_send_reply(NETLINK_CB(skb).pid, seq, AUDIT_TTY_GET, 0, 0,
+				 &s, sizeof(s));
+		break;
+	}
+	case AUDIT_TTY_SET: {
+		struct audit_tty_status *s;
+		struct task_struct *tsk;
+
+		if (nlh->nlmsg_len < sizeof(struct audit_tty_status))
+			return -EINVAL;
+		s = data;
+		if (s->enabled != 0 && s->enabled != 1)
+			return -EINVAL;
+		read_lock(&tasklist_lock);
+		tsk = find_task_by_pid(pid);
+		if (!tsk)
+			err = -ESRCH;
+		else {
+			spin_lock_irq(&tsk->sighand->siglock);
+			tsk->signal->audit_tty = s->enabled != 0;
+			spin_unlock_irq(&tsk->sighand->siglock);
+		}
+		read_unlock(&tasklist_lock);
+		break;
+	}
 	default:
 		err = -EINVAL;
 		break;
@@ -1185,7 +1266,7 @@ static void audit_log_n_string(struct audit_buffer *ab, size_t slen,
 }
 
 /**
- * audit_log_n_unstrustedstring - log a string that may contain random characters
+ * audit_log_n_untrustedstring - log a string that may contain random characters
  * @ab: audit_buffer
  * @len: lenth of string (not including trailing null)
  * @string: string to be logged
@@ -1201,25 +1282,24 @@ static void audit_log_n_string(struct audit_buffer *ab, size_t slen,
 const char *audit_log_n_untrustedstring(struct audit_buffer *ab, size_t len,
 					const char *string)
 {
-	const unsigned char *p = string;
+	const unsigned char *p;
 
-	while (*p) {
+	for (p = string; p < (const unsigned char *)string + len && *p; p++) {
 		if (*p == '"' || *p < 0x21 || *p > 0x7f) {
 			audit_log_hex(ab, string, len);
 			return string + len + 1;
 		}
-		p++;
 	}
 	audit_log_n_string(ab, len, string);
 	return p + 1;
 }
 
 /**
- * audit_log_unstrustedstring - log a string that may contain random characters
+ * audit_log_untrustedstring - log a string that may contain random characters
  * @ab: audit_buffer
  * @string: string to be logged
  *
- * Same as audit_log_n_unstrustedstring(), except that strlen is used to
+ * Same as audit_log_n_untrustedstring(), except that strlen is used to
  * determine string length.
  */
 const char *audit_log_untrustedstring(struct audit_buffer *ab, const char *string)