seccomp: Invalidate seccomp mode to catch death failures

stable inclusion
from linux-4.19.230
commit 255264d81da6edaf4cd4fab836d1ef3ba09af6aa

--------------------------------

commit 495ac306 upstream.

If seccomp tries to kill a process, it should never see that process
again. To enforce this proactively, switch the mode to something
impossible. If encountered: WARN, reject all syscalls, and attempt to
kill the process again even harder.

Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Will Drewry <wad@chromium.org>
Fixes: 8112c4f1 ("seccomp: remove 2-phase API")
Cc: stable@vger.kernel.org
Signed-off-by: NKees Cook <keescook@chromium.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NYongqiang Liu <liuyongqiang13@huawei.com>
Signed-off-by: NLaibin Qiu <qiulaibin@huawei.com>

kernel/seccomp.c

@@ -28,6 +28,9 @@
 #include <linux/syscalls.h>
 #include <linux/sysctl.h>
 
+/* Not exposed in headers: strictly internal use only. */
+#define SECCOMP_MODE_DEAD	(SECCOMP_MODE_FILTER + 1)
+
 #ifdef CONFIG_HAVE_ARCH_SECCOMP_FILTER
 #include <asm/syscall.h>
 #endif
@@ -629,6 +632,7 @@ static void __secure_computing_strict(int this_syscall)
 #ifdef SECCOMP_DEBUG
 	dump_stack();
 #endif
+	current->seccomp.mode = SECCOMP_MODE_DEAD;
 	seccomp_log(this_syscall, SIGKILL, SECCOMP_RET_KILL_THREAD, true);
 	do_exit(SIGKILL);
 }
@@ -743,6 +747,7 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
 	case SECCOMP_RET_KILL_THREAD:
 	case SECCOMP_RET_KILL_PROCESS:
 	default:
+		current->seccomp.mode = SECCOMP_MODE_DEAD;
 		seccomp_log(this_syscall, SIGSYS, action, true);
 		/* Dump core only if this is the last remaining thread. */
 		if (action == SECCOMP_RET_KILL_PROCESS ||
@@ -793,6 +798,11 @@ int __secure_computing(const struct seccomp_data *sd)
 		return 0;
 	case SECCOMP_MODE_FILTER:
 		return __seccomp_filter(this_syscall, sd, false);
+	/* Surviving SECCOMP_RET_KILL_* must be proactively impossible. */
+	case SECCOMP_MODE_DEAD:
+		WARN_ON_ONCE(1);
+		do_exit(SIGKILL);
+		return -1;
 	default:
 		BUG();
 	}