[BUG?] mm/secretmem: memory address mapped to memfd_secret can be used in write syscall.

["David Wang" <00107082@163.com>]

Hi,
According to https://lwn.net/Articles/865256/, 
the memory address got from memfd_secret/ftruncate/mmap should not be used by syscalls, since it is not accessible even by kernel.

But my test result shows that the "secret" memory could be used in  syscall write, is this expected behavior?
This is my test code:

int main() {
	int fd = syscall(__NR_memfd_secret, 0);
	if (fd < 0) {
		perror("Fail to create secret");
		return -1;
	}
	if (ftruncate(fd, 1024) < 0) {
		perror("Fail to size the secret");
		return -1;
	}
	char *key = mmap(NULL, 1024, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
	if (key == MAP_FAILED) {
		perror("Fail to mmap");
		return -1;
	}
	// should be some secure channel
	strcpy(key, "ThisIsAKey");
	// printf("[%d]key(%s) ready: %p\n", getpid(), key, key);
	// getchar();
	// make syscall, should err
	write(STDOUT_FILENO, key, strlen(key));  //<-- Here the key shows up on stdout.

	return 0;
}

Thanks
David