selftests/seccomp: Catch garbage on SECCOMP_IOCTL_NOTIF_RECV

This adds logic to the user_notification_basic test to set a member
of struct seccomp_notif to an invalid value to ensure that the kernel
returns EINVAL if any of the struct seccomp_notif members are set to
invalid values.
Signed-off-by: Sargun Dhillon <sargun@sargun.me>
Suggested-by: Christian Brauner <christian.brauner@ubuntu.com>
Link: https://lore.kernel.org/r/20191230203811.4996-1-sargun@sargun.me
Fixes: 6a21cc50 ("seccomp: add a return code to trap to userspace")
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>

tools/testing/selftests/seccomp/seccomp_bpf.c

@@ -3158,7 +3158,18 @@ TEST(user_notification_basic)
 	EXPECT_GT(poll(&pollfd, 1, -1), 0);
 	EXPECT_EQ(pollfd.revents, POLLIN);
 
-	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, &req), 0);
+	/* Test that we can't pass garbage to the kernel. */
+	memset(&req, 0, sizeof(req));
+	req.pid = -1;
+	errno = 0;
+	ret = ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, &req);
+	EXPECT_EQ(-1, ret);
+	EXPECT_EQ(EINVAL, errno);
+
+	if (ret) {
+		req.pid = 0;
+		EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, &req), 0);
+	}
 
 	pollfd.fd = listener;
 	pollfd.events = POLLIN | POLLOUT;