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, 97 insertions, 1 deletions

diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
index 1c8c22c..7372958 100644
--- a/tools/testing/selftests/seccomp/seccomp_bpf.c
+++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
@@ -74,7 +74,12 @@
 #define SECCOMP_RET_ERRNO       0x00050000U /* returns an errno */
 #define SECCOMP_RET_TRACE       0x7ff00000U /* pass to a tracer or disallow */
 #define SECCOMP_RET_ALLOW       0x7fff0000U /* allow */
+#endif
+#ifndef SECCOMP_RET_LOG
+#define SECCOMP_RET_LOG       0x7ffc0000U /* allow after logging */
+#endif
 
+#ifndef SECCOMP_RET_ACTION
 /* Masks for the return value sections. */
 #define SECCOMP_RET_ACTION      0x7fff0000U
 #define SECCOMP_RET_DATA        0x0000ffffU
@@ -342,6 +347,28 @@ TEST(empty_prog)
 	EXPECT_EQ(EINVAL, errno);
 }
 
+TEST(log_all)
+{
+	struct sock_filter filter[] = {
+		BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_LOG),
+	};
+	struct sock_fprog prog = {
+		.len = (unsigned short)ARRAY_SIZE(filter),
+		.filter = filter,
+	};
+	long ret;
+	pid_t parent = getppid();
+
+	ret = prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
+	ASSERT_EQ(0, ret);
+
+	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog);
+	ASSERT_EQ(0, ret);
+
+	/* getppid() should succeed and be logged (no check for logging) */
+	EXPECT_EQ(parent, syscall(__NR_getppid));
+}
+
 TEST_SIGNAL(unknown_ret_is_kill_inside, SIGSYS)
 {
 	struct sock_filter filter[] = {
@@ -756,6 +783,7 @@ TEST_F(TRAP, handler)
 
 FIXTURE_DATA(precedence) {
 	struct sock_fprog allow;
+	struct sock_fprog log;
 	struct sock_fprog trace;
 	struct sock_fprog error;
 	struct sock_fprog trap;
@@ -767,6 +795,13 @@ FIXTURE_SETUP(precedence)
 	struct sock_filter allow_insns[] = {
 		BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_ALLOW),
 	};
+	struct sock_filter log_insns[] = {
+		BPF_STMT(BPF_LD|BPF_W|BPF_ABS,
+			offsetof(struct seccomp_data, nr)),
+		BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, __NR_getpid, 1, 0),
+		BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_ALLOW),
+		BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_LOG),
+	};
 	struct sock_filter trace_insns[] = {
 		BPF_STMT(BPF_LD|BPF_W|BPF_ABS,
 			offsetof(struct seccomp_data, nr)),
@@ -803,6 +838,7 @@ FIXTURE_SETUP(precedence)
 	memcpy(self->_x.filter, &_x##_insns, sizeof(_x##_insns)); \
 	self->_x.len = (unsigned short)ARRAY_SIZE(_x##_insns)
 	FILTER_ALLOC(allow);
+	FILTER_ALLOC(log);
 	FILTER_ALLOC(trace);
 	FILTER_ALLOC(error);
 	FILTER_ALLOC(trap);
@@ -813,6 +849,7 @@ FIXTURE_TEARDOWN(precedence)
 {
 #define FILTER_FREE(_x) if (self->_x.filter) free(self->_x.filter)
 	FILTER_FREE(allow);
+	FILTER_FREE(log);
 	FILTER_FREE(trace);
 	FILTER_FREE(error);
 	FILTER_FREE(trap);
@@ -830,6 +867,8 @@ TEST_F(precedence, allow_ok)
 
 	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->allow);
 	ASSERT_EQ(0, ret);
+	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->log);
+	ASSERT_EQ(0, ret);
 	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->trace);
 	ASSERT_EQ(0, ret);
 	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->error);
@@ -854,6 +893,8 @@ TEST_F_SIGNAL(precedence, kill_is_highest, SIGSYS)
 
 	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->allow);
 	ASSERT_EQ(0, ret);
+	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->log);
+	ASSERT_EQ(0, ret);
 	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->trace);
 	ASSERT_EQ(0, ret);
 	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->error);
@@ -885,6 +926,8 @@ TEST_F_SIGNAL(precedence, kill_is_highest_in_any_order, SIGSYS)
 	ASSERT_EQ(0, ret);
 	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->error);
 	ASSERT_EQ(0, ret);
+	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->log);
+	ASSERT_EQ(0, ret);
 	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->trace);
 	ASSERT_EQ(0, ret);
 	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->trap);
@@ -906,6 +949,8 @@ TEST_F_SIGNAL(precedence, trap_is_second, SIGSYS)
 
 	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->allow);
 	ASSERT_EQ(0, ret);
+	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->log);
+	ASSERT_EQ(0, ret);
 	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->trace);
 	ASSERT_EQ(0, ret);
 	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->error);
@@ -931,6 +976,8 @@ TEST_F_SIGNAL(precedence, trap_is_second_in_any_order, SIGSYS)
 	ASSERT_EQ(0, ret);
 	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->trap);
 	ASSERT_EQ(0, ret);
+	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->log);
+	ASSERT_EQ(0, ret);
 	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->trace);
 	ASSERT_EQ(0, ret);
 	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->error);
@@ -952,6 +999,8 @@ TEST_F(precedence, errno_is_third)
 
 	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->allow);
 	ASSERT_EQ(0, ret);
+	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->log);
+	ASSERT_EQ(0, ret);
 	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->trace);
 	ASSERT_EQ(0, ret);
 	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->error);
@@ -970,6 +1019,8 @@ TEST_F(precedence, errno_is_third_in_any_order)
 	ret = prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
 	ASSERT_EQ(0, ret);
 
+	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->log);
+	ASSERT_EQ(0, ret);
 	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->error);
 	ASSERT_EQ(0, ret);
 	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->trace);
@@ -992,6 +1043,8 @@ TEST_F(precedence, trace_is_fourth)
 
 	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->allow);
 	ASSERT_EQ(0, ret);
+	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->log);
+	ASSERT_EQ(0, ret);
 	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->trace);
 	ASSERT_EQ(0, ret);
 	/* Should work just fine. */
@@ -1013,12 +1066,54 @@ TEST_F(precedence, trace_is_fourth_in_any_order)
 	ASSERT_EQ(0, ret);
 	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->allow);
 	ASSERT_EQ(0, ret);
+	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->log);
+	ASSERT_EQ(0, ret);
 	/* Should work just fine. */
 	EXPECT_EQ(parent, syscall(__NR_getppid));
 	/* No ptracer */
 	EXPECT_EQ(-1, syscall(__NR_getpid));
 }
 
+TEST_F(precedence, log_is_fifth)
+{
+	pid_t mypid, parent;
+	long ret;
+
+	mypid = getpid();
+	parent = getppid();
+	ret = prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
+	ASSERT_EQ(0, ret);
+
+	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->allow);
+	ASSERT_EQ(0, ret);
+	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->log);
+	ASSERT_EQ(0, ret);
+	/* Should work just fine. */
+	EXPECT_EQ(parent, syscall(__NR_getppid));
+	/* Should also work just fine */
+	EXPECT_EQ(mypid, syscall(__NR_getpid));
+}
+
+TEST_F(precedence, log_is_fifth_in_any_order)
+{
+	pid_t mypid, parent;
+	long ret;
+
+	mypid = getpid();
+	parent = getppid();
+	ret = prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
+	ASSERT_EQ(0, ret);
+
+	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->log);
+	ASSERT_EQ(0, ret);
+	ret = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &self->allow);
+	ASSERT_EQ(0, ret);
+	/* Should work just fine. */
+	EXPECT_EQ(parent, syscall(__NR_getppid));
+	/* Should also work just fine */
+	EXPECT_EQ(mypid, syscall(__NR_getpid));
+}
+
 #ifndef PTRACE_O_TRACESECCOMP
 #define PTRACE_O_TRACESECCOMP	0x00000080
 #endif
@@ -2603,7 +2698,7 @@ TEST(get_action_avail)
 {
 	__u32 actions[] = { SECCOMP_RET_KILL,  SECCOMP_RET_TRAP,
 			    SECCOMP_RET_ERRNO, SECCOMP_RET_TRACE,
-			    SECCOMP_RET_ALLOW };
+			    SECCOMP_RET_LOG,   SECCOMP_RET_ALLOW };
 	__u32 unknown_action = 0x10000000U;
 	int i;
 	long ret;
@@ -2640,6 +2735,7 @@ TEST(get_action_avail)
  * - 64-bit arg prodding
  * - arch value testing (x86 modes especially)
  * - verify that FILTER_FLAG_LOG filters generate log messages
+ * - verify that RET_LOG generates log messages
  * - ...
  */