seccomp: Operation for checking if an action is available

Userspace code that needs to check if the kernel supports a given action
may not be able to use the /proc/sys/kernel/seccomp/actions_avail
sysctl. The process may be running in a sandbox and, therefore,
sufficient filesystem access may not be available. This patch adds an
operation to the seccomp(2) syscall that allows userspace code to ask
the kernel if a given action is available.

If the action is supported by the kernel, 0 is returned. If the action
is not supported by the kernel, -1 is returned with errno set to
-EOPNOTSUPP. If this check is attempted on a kernel that doesn't support
this new operation, -1 is returned with errno set to -EINVAL meaning
that userspace code will have the ability to differentiate between the
two error cases.

Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Suggested-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Kees Cook <keescook@chromium.org>

1 files changed, 36 insertions, 0 deletions

diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
index 2fb49d99588d..1f2888f6678b 100644
--- a/tools/testing/selftests/seccomp/seccomp_bpf.c
+++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
@@ -1731,6 +1731,10 @@ TEST_F_SIGNAL(TRACE_syscall, kill_after_ptrace, SIGSYS)
 #define SECCOMP_SET_MODE_FILTER 1
 #endif
 
+#ifndef SECCOMP_GET_ACTION_AVAIL
+#define SECCOMP_GET_ACTION_AVAIL 2
+#endif
+
 #ifndef SECCOMP_FILTER_FLAG_TSYNC
 #define SECCOMP_FILTER_FLAG_TSYNC 1
 #endif
@@ -2469,6 +2473,38 @@ TEST(syscall_restart)
                 _metadata->passed = 0;
 }
 
+TEST(get_action_avail)
+{
+        __u32 actions[] = { SECCOMP_RET_KILL,  SECCOMP_RET_TRAP,
+                            SECCOMP_RET_ERRNO, SECCOMP_RET_TRACE,
+                            SECCOMP_RET_ALLOW };
+        __u32 unknown_action = 0x10000000U;
+        int i;
+        long ret;
+
+        ret = seccomp(SECCOMP_GET_ACTION_AVAIL, 0, &actions[0]);
+        ASSERT_NE(ENOSYS, errno) {
+                TH_LOG("Kernel does not support seccomp syscall!");
+        }
+        ASSERT_NE(EINVAL, errno) {
+                TH_LOG("Kernel does not support SECCOMP_GET_ACTION_AVAIL operation!");
+        }
+        EXPECT_EQ(ret, 0);
+
+        for (i = 0; i < ARRAY_SIZE(actions); i++) {
+                ret = seccomp(SECCOMP_GET_ACTION_AVAIL, 0, &actions[i]);
+                EXPECT_EQ(ret, 0) {
+                        TH_LOG("Expected action (0x%X) not available!",
+                               actions[i]);
+                }
+        }
+
+        /* Check that an unknown action is handled properly (EOPNOTSUPP) */
+        ret = seccomp(SECCOMP_GET_ACTION_AVAIL, 0, &unknown_action);
+        EXPECT_EQ(ret, -1);
+        EXPECT_EQ(errno, EOPNOTSUPP);
+}
+
 /*
  * TODO:
  * - add microbenchmarks