seccomp: add unit test for seccomp filtering

The handling of some syscalls / libc function is quite subtle. For
example, 'fork' at a libc level doesn't always correspond to 'fork'
at a syscall level, since the 'clone' syscall is preferred usually.

The unit test will help to detect these kind of problems. A point of
difficulty in writing a test though is that the QEMU build process may
already be confined by seccomp. For example, if running inside a
container. Since we can't predict what filtering might have been applied
already, we are quite conservative and skip all tests if we see any kind
of seccomp filter active.

Acked-by: Eduardo Otubo <otubo@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

MAINTAINERS

@@ -2982,6 +2982,7 @@ M: Eduardo Otubo <otubo@redhat.com>
 S: Supported
 F: softmmu/qemu-seccomp.c
 F: include/sysemu/seccomp.h
+F: tests/unit/test-seccomp.c
 
 Cryptography
 M: Daniel P. Berrange <berrange@redhat.com>

tests/unit/meson.build

@@ -53,6 +53,10 @@ if have_system or have_tools
   tests += {
     'test-qmp-event': [testqapi],
   }
+
+  if seccomp.found()
+    tests += {'test-seccomp': ['../../softmmu/qemu-seccomp.c', seccomp]}
+  endif
 endif
 
 if have_block

tests/unit/test-seccomp.c

@@ -0,0 +1,270 @@
+/*
+ * QEMU seccomp test suite
+ *
+ * Copyright (c) 2021 Red Hat, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+#include "qemu/osdep.h"
+#include "qemu/config-file.h"
+#include "qemu/option.h"
+#include "sysemu/seccomp.h"
+#include "qapi/error.h"
+#include "qemu/module.h"
+
+#include <unistd.h>
+#include <sys/syscall.h>
+
+static void test_seccomp_helper(const char *args, bool killed,
+                                int errnum, int (*doit)(void))
+{
+    if (g_test_subprocess()) {
+        QemuOptsList *olist;
+        QemuOpts *opts;
+        int ret;
+
+        module_call_init(MODULE_INIT_OPTS);
+        olist = qemu_find_opts("sandbox");
+        g_assert(olist != NULL);
+
+        opts = qemu_opts_parse_noisily(olist, args, true);
+        g_assert(opts != NULL);
+
+        parse_sandbox(NULL, opts, &error_abort);
+
+        /* Running in a child process */
+        ret = doit();
+
+        if (errnum != 0) {
+            g_assert(ret != 0);
+            g_assert(errno == errnum);
+        } else {
+            g_assert(ret == 0);
+        }
+
+        _exit(0);
+    } else {
+        /* Running in main test process, spawning the child */
+        g_test_trap_subprocess(NULL, 0, 0);
+        if (killed) {
+            g_test_trap_assert_failed();
+        } else {
+            g_test_trap_assert_passed();
+        }
+    }
+}
+
+
+static void test_seccomp_killed(const char *args, int (*doit)(void))
+{
+    test_seccomp_helper(args, true, 0, doit);
+}
+
+static void test_seccomp_errno(const char *args, int errnum, int (*doit)(void))
+{
+    test_seccomp_helper(args, false, errnum, doit);
+}
+
+static void test_seccomp_passed(const char *args, int (*doit)(void))
+{
+    test_seccomp_helper(args, false, 0, doit);
+}
+
+#ifdef SYS_fork
+static int doit_sys_fork(void)
+{
+    int ret = syscall(SYS_fork);
+    if (ret < 0) {
+        return ret;
+    }
+    if (ret == 0) {
+        _exit(0);
+    }
+    return 0;
+}
+
+static void test_seccomp_sys_fork_on_nospawn(void)
+{
+    test_seccomp_killed("on,spawn=deny", doit_sys_fork);
+}
+
+static void test_seccomp_sys_fork_on(void)
+{
+    test_seccomp_passed("on", doit_sys_fork);
+}
+
+static void test_seccomp_sys_fork_off(void)
+{
+    test_seccomp_passed("off", doit_sys_fork);
+}
+#endif
+
+static int doit_fork(void)
+{
+    int ret = fork();
+    if (ret < 0) {
+        return ret;
+    }
+    if (ret == 0) {
+        _exit(0);
+    }
+    return 0;
+}
+
+static void test_seccomp_fork_on_nospawn(void)
+{
+    /* XXX fixme - should be killed */
+    test_seccomp_passed("on,spawn=deny", doit_fork);
+}
+
+static void test_seccomp_fork_on(void)
+{
+    test_seccomp_passed("on", doit_fork);
+}
+
+static void test_seccomp_fork_off(void)
+{
+    test_seccomp_passed("off", doit_fork);
+}
+
+static void *noop(void *arg)
+{
+    return arg;
+}
+
+static int doit_thread(void)
+{
+    pthread_t th;
+    int ret = pthread_create(&th, NULL, noop, NULL);
+    if (ret != 0) {
+        errno = ret;
+        return -1;
+    } else {
+        pthread_join(th, NULL);
+        return 0;
+    }
+}
+
+static void test_seccomp_thread_on(void)
+{
+    test_seccomp_passed("on", doit_thread);
+}
+
+static void test_seccomp_thread_on_nospawn(void)
+{
+    test_seccomp_passed("on,spawn=deny", doit_thread);
+}
+
+static void test_seccomp_thread_off(void)
+{
+    test_seccomp_passed("off", doit_thread);
+}
+
+static int doit_sched(void)
+{
+    struct sched_param param = { .sched_priority = 0 };
+    return sched_setscheduler(getpid(), SCHED_OTHER, &param);
+}
+
+static void test_seccomp_sched_on_nores(void)
+{
+    test_seccomp_errno("on,resourcecontrol=deny", EPERM, doit_sched);
+}
+
+static void test_seccomp_sched_on(void)
+{
+    test_seccomp_passed("on", doit_sched);
+}
+
+static void test_seccomp_sched_off(void)
+{
+    test_seccomp_passed("off", doit_sched);
+}
+
+static bool can_play_with_seccomp(void)
+{
+    g_autofree char *status = NULL;
+    g_auto(GStrv) lines = NULL;
+    size_t i;
+
+    if (!g_file_get_contents("/proc/self/status", &status, NULL, NULL)) {
+        return false;
+    }
+
+    lines = g_strsplit(status, "\n", 0);
+
+    for (i = 0; lines[i] != NULL; i++) {
+        if (g_str_has_prefix(lines[i], "Seccomp:")) {
+            /*
+             * "Seccomp: 1" or "Seccomp: 2" indicate we're already
+             * confined, probably as we're inside a container. In
+             * this case our tests might get unexpected results,
+             * so we can't run reliably
+             */
+            if (!strchr(lines[i], '0')) {
+                return false;
+            }
+
+            return true;
+        }
+    }
+
+    /* Doesn't look like seccomp is enabled in the kernel */
+    return false;
+}
+
+int main(int argc, char **argv)
+{
+    g_test_init(&argc, &argv, NULL);
+    if (can_play_with_seccomp()) {
+#ifdef SYS_fork
+        g_test_add_func("/softmmu/seccomp/sys-fork/on",
+                        test_seccomp_sys_fork_on);
+        g_test_add_func("/softmmu/seccomp/sys-fork/on-nospawn",
+                        test_seccomp_sys_fork_on_nospawn);
+        g_test_add_func("/softmmu/seccomp/sys-fork/off",
+                        test_seccomp_sys_fork_off);
+#endif
+
+        g_test_add_func("/softmmu/seccomp/fork/on",
+                        test_seccomp_fork_on);
+        g_test_add_func("/softmmu/seccomp/fork/on-nospawn",
+                        test_seccomp_fork_on_nospawn);
+        g_test_add_func("/softmmu/seccomp/fork/off",
+                        test_seccomp_fork_off);
+
+        g_test_add_func("/softmmu/seccomp/thread/on",
+                        test_seccomp_thread_on);
+        g_test_add_func("/softmmu/seccomp/thread/on-nospawn",
+                        test_seccomp_thread_on_nospawn);
+        g_test_add_func("/softmmu/seccomp/thread/off",
+                        test_seccomp_thread_off);
+
+        if (doit_sched() == 0) {
+            /*
+             * musl doesn't impl sched_setscheduler, hence
+             * we check above if it works first
+             */
+            g_test_add_func("/softmmu/seccomp/sched/on",
+                            test_seccomp_sched_on);
+            g_test_add_func("/softmmu/seccomp/sched/on-nores",
+                            test_seccomp_sched_on_nores);
+            g_test_add_func("/softmmu/seccomp/sched/off",
+                            test_seccomp_sched_off);
+        }
+    }
+    return g_test_run();
+}