[PATCH v6 0/6] mm: introduce memfd_secret system call to create "secret" memory areas

[Hagen Paul Pfeifer]

* Mike Rapoport | 2020-09-24 16:28:58 [+0300]:

>This is an implementation of "secret" mappings backed by a file descriptor. 
>I've dropped the boot time reservation patch for now as it is not strictly
>required for the basic usage and can be easily added later either with or
>without CMA.

Isn't memfd_secret currently *unnecessarily* designed to be a "one task
feature"? memfd_secret fulfills exactly two (generic) features:

- address space isolation from kernel (aka SECRET_EXCLUSIVE, not in kernel's
  direct map) - hide from kernel, great
- disabling processor's memory caches against speculative-execution vulnerabilities
  (spectre and friends, aka SECRET_UNCACHED), also great

But, what about the following use-case: implementing a hardened IPC mechanism
where even the kernel is not aware of any data and optionally via SECRET_UNCACHED
even the hardware caches are bypassed! With the patches we are so close to
achieving this.

How? Shared, SECRET_EXCLUSIVE and SECRET_UNCACHED mmaped pages for IPC
involved tasks required to know this mapping (and memfd_secret fd). After IPC
is done, tasks can copy sensitive data from IPC pages into memfd_secret()
pages, un-sensitive data can be used/copied everywhere.

One missing piece is still the secure zeroization of the page(s) if the
mapping is closed by last process to guarantee a secure cleanup. This can
probably done as an general mmap feature, not coupled to memfd_secret() and
can be done independently ("reverse" MAP_UNINITIALIZED feature).

PS: thank you Mike for your effort!

See the following pseudo-code as an example:


// simple assume file-descriptor and mapping is inherited
// by child for simplicity, ptr is 
int fd = memfd_secret(SECRETMEM_UNCACHED);
ftruncate(fd, PAGE_SIZE);
uint32_t *ptr = mmap(NULL, PAGE_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

pid_t pid_other;

void signal_handler(int sig)
{
	// update IPC data on shared, uncachaed, exclusive mapped page
	*ptr += 1;
	// inform other
	sleep(1);
	kill(pid_other, SIGUSR1);
}

void ipc_loop(void)
{
	signal(SIGUSR1, signal_handler);
	while (1) {
		sleep(1);
	}
}

int main(void)
{
	pid_t child_pid;

	switch (child_pid = fork()) {
	case 0:
		pid_other = getppid();
		break;
	default:
		pid_other = child_pid
		break;
	}
	
	ipc_loop();
}


Hagen