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Date:   Sun, 1 Nov 2020 12:09:35 +0100
From:   Hagen Paul Pfeifer <hagen@jauu.net>
To:     Mike Rapoport <rppt@kernel.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andy Lutomirski <luto@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>, Borislav Petkov <bp@alien8.de>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Christopher Lameter <cl@linux.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        David Hildenbrand <david@redhat.com>,
        Elena Reshetova <elena.reshetova@intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Idan Yaniv <idan.yaniv@ibm.com>,
        Ingo Molnar <mingo@redhat.com>,
        James Bottomley <jejb@linux.ibm.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Matthew Wilcox <willy@infradead.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
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        x86@kernel.org
Subject: Re: [PATCH v6 0/6] mm: introduce memfd_secret system call to create
 "secret" memory areas
Message-ID: <20201101110935.GA4105325@laniakea>
References: <20200924132904.1391-1-rppt@kernel.org>
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* 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