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

From: Mike Rapoport <[email-masked]>
Date: Mon, 2 Nov 2020 17:40:28 +0200

On Sun, Nov 01, 2020 at 12:09:35PM +0100, Hagen Paul Pfeifer wrote:
> * 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.

As long as the task share the file descriptor, they can share the
secretmem pages, pretty much like normal memfd.

> 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).

There are "init_on_alloc" and "init_on_free" kernel parameters that
enable zeroing of the pages on alloc and on free globally.
Anyway, I'll add zeroing of the freed memory to secretmem.

> 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);
 
The ptr here will be visible to both parent and child.

> 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
> 

-- 
Sincerely yours,
Mike.

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