seccomp: Action to log before allowing

Add a new action, SECCOMP_RET_LOG, that logs a syscall before allowing
the syscall. At the implementation level, this action is identical to
the existing SECCOMP_RET_ALLOW action. However, it can be very useful when
initially developing a seccomp filter for an application. The developer
can set the default action to be SECCOMP_RET_LOG, maybe mark any
obviously needed syscalls with SECCOMP_RET_ALLOW, and then put the
application through its paces. A list of syscalls that triggered the
default action (SECCOMP_RET_LOG) can be easily gleaned from the logs and
that list can be used to build the syscall whitelist. Finally, the
developer can change the default action to the desired value.

This provides a more friendly experience than seeing the application get
killed, then updating the filter and rebuilding the app, seeing the
application get killed due to a different syscall, then updating the
filter and rebuilding the app, etc.

The functionality is similar to what's supported by the various LSMs.
SELinux has permissive mode, AppArmor has complain mode, SMACK has
bring-up mode, etc.

SECCOMP_RET_LOG is given a lower value than SECCOMP_RET_ALLOW as allow
while logging is slightly more restrictive than quietly allowing.

Unfortunately, the tests added for SECCOMP_RET_LOG are not capable of
inspecting the audit log to verify that the syscall was logged.

With this patch, the logic for deciding if an action will be logged is:

if action == RET_ALLOW:
  do not log
else if action == RET_KILL && RET_KILL in actions_logged:
  log
else if action == RET_LOG && RET_LOG in actions_logged:
  log
else if filter-requests-logging && action in actions_logged:
  log
else if audit_enabled && process-is-being-audited:
  log
else:
  do not log

Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

1 files changed, 18 insertions, 5 deletions

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index ed9fde4..59cde2e 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -533,10 +533,12 @@ static void seccomp_send_sigsys(int syscall, int reason)
 #define SECCOMP_LOG_TRAP		(1 << 2)
 #define SECCOMP_LOG_ERRNO		(1 << 3)
 #define SECCOMP_LOG_TRACE		(1 << 4)
-#define SECCOMP_LOG_ALLOW		(1 << 5)
+#define SECCOMP_LOG_LOG			(1 << 5)
+#define SECCOMP_LOG_ALLOW		(1 << 6)
 
 static u32 seccomp_actions_logged = SECCOMP_LOG_KILL  | SECCOMP_LOG_TRAP  |
-				    SECCOMP_LOG_ERRNO | SECCOMP_LOG_TRACE;
+				    SECCOMP_LOG_ERRNO | SECCOMP_LOG_TRACE |
+				    SECCOMP_LOG_LOG;
 
 static inline void seccomp_log(unsigned long syscall, long signr, u32 action,
 			       bool requested)
@@ -555,15 +557,18 @@ static inline void seccomp_log(unsigned long syscall, long signr, u32 action,
 	case SECCOMP_RET_TRACE:
 		log = requested && seccomp_actions_logged & SECCOMP_LOG_TRACE;
 		break;
+	case SECCOMP_RET_LOG:
+		log = seccomp_actions_logged & SECCOMP_LOG_LOG;
+		break;
 	case SECCOMP_RET_KILL:
 	default:
 		log = seccomp_actions_logged & SECCOMP_LOG_KILL;
 	}
 
 	/*
-	 * Force an audit message to be emitted when the action is RET_KILL or
-	 * the FILTER_FLAG_LOG bit was set and the action is allowed to be
-	 * logged by the admin.
+	 * Force an audit message to be emitted when the action is RET_KILL,
+	 * RET_LOG, or the FILTER_FLAG_LOG bit was set and the action is
+	 * allowed to be logged by the admin.
 	 */
 	if (log)
 		return __audit_seccomp(syscall, signr, action);
@@ -699,6 +704,10 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
 
 		return 0;
 
+	case SECCOMP_RET_LOG:
+		seccomp_log(this_syscall, 0, action, true);
+		return 0;
+
 	case SECCOMP_RET_ALLOW:
 		/*
 		 * Note that the "match" filter will always be NULL for
@@ -873,6 +882,7 @@ static long seccomp_get_action_avail(const char __user *uaction)
 	case SECCOMP_RET_TRAP:
 	case SECCOMP_RET_ERRNO:
 	case SECCOMP_RET_TRACE:
+	case SECCOMP_RET_LOG:
 	case SECCOMP_RET_ALLOW:
 		break;
 	default:
@@ -1023,12 +1033,14 @@ out:
 #define SECCOMP_RET_TRAP_NAME		"trap"
 #define SECCOMP_RET_ERRNO_NAME		"errno"
 #define SECCOMP_RET_TRACE_NAME		"trace"
+#define SECCOMP_RET_LOG_NAME		"log"
 #define SECCOMP_RET_ALLOW_NAME		"allow"
 
 static const char seccomp_actions_avail[] = SECCOMP_RET_KILL_NAME	" "
 					    SECCOMP_RET_TRAP_NAME	" "
 					    SECCOMP_RET_ERRNO_NAME	" "
 					    SECCOMP_RET_TRACE_NAME	" "
+					    SECCOMP_RET_LOG_NAME	" "
 					    SECCOMP_RET_ALLOW_NAME;
 
 struct seccomp_log_name {
@@ -1041,6 +1053,7 @@ static const struct seccomp_log_name seccomp_log_names[] = {
 	{ SECCOMP_LOG_TRAP, SECCOMP_RET_TRAP_NAME },
 	{ SECCOMP_LOG_ERRNO, SECCOMP_RET_ERRNO_NAME },
 	{ SECCOMP_LOG_TRACE, SECCOMP_RET_TRACE_NAME },
+	{ SECCOMP_LOG_LOG, SECCOMP_RET_LOG_NAME },
 	{ SECCOMP_LOG_ALLOW, SECCOMP_RET_ALLOW_NAME },
 	{ }
 };