On 11/02/2020 4:40 PM Mike Rapoport [off-list ref] wrote:

quoted

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.

Including process_vm_readv() and process_vm_writev()? Let's take a hypothetical
"dbus-daemon-secure" service that receives data from process A and wants to
copy/distribute it to data areas of N other processes. Much like dbus but without
SOCK_DGRAM rather direct copy into secretmem/mmap pages (ring-buffer). Should be
possible, right?

quoted

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.

Great, this allows page-specific (thus runtime-performance-optimized) zeroing
of secured pages. init_on_free lowers the performance to much and is not precice
enough.

Hagen