seccomp fixes for v5.17-rc4

- Force HANDLER_EXIT even for SIGNAL_UNKILLABLE.
 - Make seccomp self-destruct after fatal filter results.
 - Update seccomp samples for easier behavioral demonstration.
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Merge tag 'seccomp-v5.17-rc4' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux

Pull seccomp fixes from Kees Cook:
 "This fixes a corner case of fatal SIGSYS being ignored since v5.15.
  Along with the signal fix is a change to seccomp so that seeing
  another syscall after a fatal filter result will cause seccomp to kill
  the process harder.

  Summary:

   - Force HANDLER_EXIT even for SIGNAL_UNKILLABLE

   - Make seccomp self-destruct after fatal filter results

   - Update seccomp samples for easier behavioral demonstration"

* tag 'seccomp-v5.17-rc4' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux:
  samples/seccomp: Adjust sample to also provide kill option
  seccomp: Invalidate seccomp mode to catch death failures
  signal: HANDLER_EXIT should clear SIGNAL_UNKILLABLE

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();
 	}

kernel/signal.c

@@ -1342,9 +1342,10 @@ force_sig_info_to_task(struct kernel_siginfo *info, struct task_struct *t,
 	}
 	/*
 	 * Don't clear SIGNAL_UNKILLABLE for traced tasks, users won't expect
-	 * debugging to leave init killable.
+	 * debugging to leave init killable. But HANDLER_EXIT is always fatal.
 	 */
-	if (action->sa.sa_handler == SIG_DFL && !t->ptrace)
+	if (action->sa.sa_handler == SIG_DFL &&
+	    (!t->ptrace || (handler == HANDLER_EXIT)))
 		t->signal->flags &= ~SIGNAL_UNKILLABLE;
 	ret = send_signal(sig, info, t, PIDTYPE_PID);
 	spin_unlock_irqrestore(&t->sighand->siglock, flags);

samples/seccomp/dropper.c

@@ -25,7 +25,7 @@
 #include <sys/prctl.h>
 #include <unistd.h>
 
-static int install_filter(int nr, int arch, int error)
+static int install_filter(int arch, int nr, int error)
 {
 	struct sock_filter filter[] = {
 		BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
@@ -42,6 +42,10 @@ static int install_filter(int nr, int arch, int error)
 		.len = (unsigned short)(sizeof(filter)/sizeof(filter[0])),
 		.filter = filter,
 	};
+	if (error == -1) {
+		struct sock_filter kill = BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL);
+		filter[4] = kill;
+	}
 	if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
 		perror("prctl(NO_NEW_PRIVS)");
 		return 1;
@@ -57,9 +61,10 @@ int main(int argc, char **argv)
 {
 	if (argc < 5) {
 		fprintf(stderr, "Usage:\n"
-			"dropper <syscall_nr> <arch> <errno> <prog> [<args>]\n"
+			"dropper <arch> <syscall_nr> <errno> <prog> [<args>]\n"
 			"Hint:	AUDIT_ARCH_I386: 0x%X\n"
 			"	AUDIT_ARCH_X86_64: 0x%X\n"
+			"	errno == -1 means SECCOMP_RET_KILL\n"
 			"\n", AUDIT_ARCH_I386, AUDIT_ARCH_X86_64);
 		return 1;
 	}