seccomp: fix blocking of process spawning

When '-sandbox on,spawn=deny' is given, we are supposed to block the
ability to spawn processes. We naively blocked the 'fork' syscall,
forgetting that any modern libc will use the 'clone' syscall instead.

We can't simply block the 'clone' syscall though, as that will break
thread creation. We thus list the set of flags used to create threads
and block anything that doesn't match this exactly.

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

softmmu/qemu-seccomp.c

@@ -46,6 +46,82 @@ const struct scmp_arg_cmp sched_setscheduler_arg[] = {
     { .arg = 1, .op = SCMP_CMP_NE, .datum_a = SCHED_IDLE }
 };
 
+/*
+ * See 'NOTES' in 'man 2 clone' - s390 & cross have 'flags' in
+ *  different position to other architectures
+ */
+#if defined(HOST_S390X) || defined(HOST_S390) || defined(HOST_CRIS)
+#define CLONE_FLAGS_ARG 1
+#else
+#define CLONE_FLAGS_ARG 0
+#endif
+
+#ifndef CLONE_PIDFD
+# define CLONE_PIDFD 0x00001000
+#endif
+
+#define REQUIRE_CLONE_FLAG(flag) \
+    const struct scmp_arg_cmp clone_arg ## flag[] = { \
+    { .arg = CLONE_FLAGS_ARG, \
+      .op = SCMP_CMP_MASKED_EQ, \
+      .datum_a = flag, .datum_b = 0 } }
+
+#define FORBID_CLONE_FLAG(flag) \
+    const struct scmp_arg_cmp clone_arg ## flag[] = { \
+    { .arg = CLONE_FLAGS_ARG, \
+      .op = SCMP_CMP_MASKED_EQ, \
+      .datum_a = flag, .datum_b = flag } }
+
+#define RULE_CLONE_FLAG(flag) \
+    { SCMP_SYS(clone),                  QEMU_SECCOMP_SET_SPAWN, \
+      ARRAY_SIZE(clone_arg ## flag), clone_arg ## flag, SCMP_ACT_TRAP }
+
+/* If no CLONE_* flags are set, except CSIGNAL, deny */
+const struct scmp_arg_cmp clone_arg_none[] = {
+    { .arg = CLONE_FLAGS_ARG,
+      .op = SCMP_CMP_MASKED_EQ,
+      .datum_a = ~(CSIGNAL), .datum_b = 0 }
+};
+
+/*
+ * pthread_create should always set all of these.
+ */
+REQUIRE_CLONE_FLAG(CLONE_VM);
+REQUIRE_CLONE_FLAG(CLONE_FS);
+REQUIRE_CLONE_FLAG(CLONE_FILES);
+REQUIRE_CLONE_FLAG(CLONE_SIGHAND);
+REQUIRE_CLONE_FLAG(CLONE_THREAD);
+REQUIRE_CLONE_FLAG(CLONE_SYSVSEM);
+REQUIRE_CLONE_FLAG(CLONE_SETTLS);
+REQUIRE_CLONE_FLAG(CLONE_PARENT_SETTID);
+REQUIRE_CLONE_FLAG(CLONE_CHILD_CLEARTID);
+/*
+ * Musl sets this in pthread_create too, but it is
+ * obsolete and harmless since its behaviour is
+ * subsumed under CLONE_THREAD
+ */
+/*REQUIRE_CLONE_FLAG(CLONE_DETACHED);*/
+
+
+/*
+ * These all indicate an attempt to spawn a process
+ * instead of a thread, or other undesirable scenarios
+ */
+FORBID_CLONE_FLAG(CLONE_PIDFD);
+FORBID_CLONE_FLAG(CLONE_PTRACE);
+FORBID_CLONE_FLAG(CLONE_VFORK);
+FORBID_CLONE_FLAG(CLONE_PARENT);
+FORBID_CLONE_FLAG(CLONE_NEWNS);
+FORBID_CLONE_FLAG(CLONE_UNTRACED);
+FORBID_CLONE_FLAG(CLONE_NEWCGROUP);
+FORBID_CLONE_FLAG(CLONE_NEWUTS);
+FORBID_CLONE_FLAG(CLONE_NEWIPC);
+FORBID_CLONE_FLAG(CLONE_NEWUSER);
+FORBID_CLONE_FLAG(CLONE_NEWPID);
+FORBID_CLONE_FLAG(CLONE_NEWNET);
+FORBID_CLONE_FLAG(CLONE_IO);
+
+
 static const struct QemuSeccompSyscall denylist[] = {
     /* default set of syscalls that should get blocked */
     { SCMP_SYS(reboot),                 QEMU_SECCOMP_SET_DEFAULT,
@@ -143,6 +219,31 @@ static const struct QemuSeccompSyscall denylist[] = {
       0, NULL, SCMP_ACT_TRAP },
     { SCMP_SYS(execve),                 QEMU_SECCOMP_SET_SPAWN,
       0, NULL, SCMP_ACT_TRAP },
+    { SCMP_SYS(clone),                  QEMU_SECCOMP_SET_SPAWN,
+      ARRAY_SIZE(clone_arg_none), clone_arg_none, SCMP_ACT_TRAP },
+    RULE_CLONE_FLAG(CLONE_VM),
+    RULE_CLONE_FLAG(CLONE_FS),
+    RULE_CLONE_FLAG(CLONE_FILES),
+    RULE_CLONE_FLAG(CLONE_SIGHAND),
+    RULE_CLONE_FLAG(CLONE_THREAD),
+    RULE_CLONE_FLAG(CLONE_SYSVSEM),
+    RULE_CLONE_FLAG(CLONE_SETTLS),
+    RULE_CLONE_FLAG(CLONE_PARENT_SETTID),
+    RULE_CLONE_FLAG(CLONE_CHILD_CLEARTID),
+    /*RULE_CLONE_FLAG(CLONE_DETACHED),*/
+    RULE_CLONE_FLAG(CLONE_PIDFD),
+    RULE_CLONE_FLAG(CLONE_PTRACE),
+    RULE_CLONE_FLAG(CLONE_VFORK),
+    RULE_CLONE_FLAG(CLONE_PARENT),
+    RULE_CLONE_FLAG(CLONE_NEWNS),
+    RULE_CLONE_FLAG(CLONE_UNTRACED),
+    RULE_CLONE_FLAG(CLONE_NEWCGROUP),
+    RULE_CLONE_FLAG(CLONE_NEWUTS),
+    RULE_CLONE_FLAG(CLONE_NEWIPC),
+    RULE_CLONE_FLAG(CLONE_NEWUSER),
+    RULE_CLONE_FLAG(CLONE_NEWPID),
+    RULE_CLONE_FLAG(CLONE_NEWNET),
+    RULE_CLONE_FLAG(CLONE_IO),
     /* resource control */
     { SCMP_SYS(setpriority),            QEMU_SECCOMP_SET_RESOURCECTL,
       0, NULL, SCMP_ACT_ERRNO(EPERM) },

tests/unit/test-seccomp.c

@@ -126,8 +126,7 @@ static int doit_fork(void)
 
 static void test_seccomp_fork_on_nospawn(void)
 {
-    /* XXX fixme - should be killed */
-    test_seccomp_passed("on,spawn=deny", doit_fork);
+    test_seccomp_killed("on,spawn=deny", doit_fork);
 }
 
 static void test_seccomp_fork_on(void)