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

[Hagen Paul Pfeifer]

> On 11/02/2020 4:40 PM Mike Rapoport <rppt at kernel.org> wrote:

> > 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?

> > 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