seccomp: Invalidate seccomp mode to catch death failures

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: 8112c4f140fa ("seccomp: remove 2-phase API")
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 4d8f44a177274f58f414c2d225498f648f4f88b1..db10e73d06e028371702b745bc2652f193828a7a 100644 (file)
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -29,6 +29,9 @@
 #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
@@ -1010,6 +1013,7 @@ 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);
 }
@@ -1261,6 +1265,7 @@ 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_THREAD ||
@@ -1309,6 +1314,11 @@ 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();
        }