Re: [PATCH v18 0/9] mm: introduce memfd_secret system call to create "secret" memory areas

Andrew Morton
2021-05-05T19:08:56Z

On Wed, 3 Mar 2021 18:22:00 +0200 Mike Rapoport <rppt@kernel.org> wrote:

> This is an implementation of "secret" mappings backed by a file descriptor.
>
> The file descriptor backing secret memory mappings is created using a
> dedicated memfd_secret system call The desired protection mode for the
> memory is configured using flags parameter of the system call. The mmap()
> of the file descriptor created with memfd_secret() will create a "secret"
> memory mapping. The pages in that mapping will be marked as not present in
> the direct map and will be present only in the page table of the owning mm.
>
> Although normally Linux userspace mappings are protected from other users,
> such secret mappings are useful for environments where a hostile tenant is
> trying to trick the kernel into giving them access to other tenants
> mappings.

I continue to struggle with this and I don't recall seeing much
enthusiasm from others. Perhaps we're all missing the value point and
some additional selling is needed.

Am I correct in understanding that the overall direction here is to
protect keys (and perhaps other things) from kernel bugs? That if the
kernel was bug-free then there would be no need for this feature? If
so, that's a bit sad. But realistic I guess.

Is this intended to protect keys/etc after the attacker has gained the
ability to run arbitrary kernel-mode code? If so, that seems
optimistic, doesn't it?

I think that a very complete description of the threats which this
feature addresses would be helpful.