Re: [PATCH v5 0/5] mm: introduce memfd_secret system call to create "secret" memory areas

From: Andy Lutomirski
Date: Wed Sep 16 2020 - 14:50:54 EST

On Wed, Sep 16, 2020 at 12:28 AM Mike Rapoport <rppt@xxxxxxxxxx> wrote:
> From: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> Hi,
> 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.
> v5 changes:
> * rebase on v5.9-rc5
> * drop boot time memory reservation patch
> v4 changes:
> * rebase on v5.9-rc1
> * Do not redefine PMD_PAGE_ORDER in fs/dax.c, thanks Kirill
> * Make secret mappings exclusive by default and only require flags to
> memfd_secret() system call for uncached mappings, thanks again Kirill :)
> v3 changes:
> * Squash kernel-parameters.txt update into the commit that added the
> command line option.
> * Make uncached mode explicitly selectable by architectures. For now enable
> it only on x86.
> v2 changes:
> * Follow Michael's suggestion and name the new system call 'memfd_secret'
> * Add kernel-parameters documentation about the boot option
> * Fix i386-tinyconfig regression reported by the kbuild bot.
> CONFIG_SECRETMEM now depends on !EMBEDDED to disable it on small systems
> from one side and still make it available unconditionally on
> architectures that support SET_DIRECT_MAP.
> 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 have desired protection bits set in the user page
> table. For instance, current implementation allows uncached mappings.

I still have serious concerns with uncached mappings. I'm not saying
I can't be convinced, but I'm not currently convinced that we should
allow user code to create UC mappings on x86.