Re: [PATCH v4 0/6] mm: introduce memfd_secret system call to create "secret" memory areas

Mike Rapoport
2020-08-26T11:03:57Z

Any comments on this?

On Tue, Aug 18, 2020 at 05:15:48PM +0300, Mike Rapoport wrote:
> From: Mike Rapoport <rppt@linux.ibm.com>
>
> Hi,
>
> This is an implementation of "secret" mappings backed by a file descriptor.
>
> 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.
>
> 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.
>
> Additionally, the secret mappings may be used as a mean to protect guest
> memory in a virtual machine host.
>
> For demonstration of secret memory usage we've created a userspace library
> [1] that does two things: the first is act as a preloader for openssl to
> redirect all the OPENSSL_malloc calls to secret memory meaning any secret
> keys get automatically protected this way and the other thing it does is
> expose the API to the user who needs it. We anticipate that a lot of the
> use cases would be like the openssl one: many toolkits that deal with
> secret keys already have special handling for the memory to try to give
> them greater protection, so this would simply be pluggable into the
> toolkits without any need for user application modification.
>
> I've hesitated whether to continue to use new flags to memfd_create() or to
> add a new system call and I've decided to use a new system call after I've
> started to look into man pages update. There would have been two completely
> independent descriptions and I think it would have been very confusing.
>
> Hiding secret memory mappings behind an anonymous file allows (ab)use of
> the page cache for tracking pages allocated for the "secret" mappings as
> well as using address_space_operations for e.g. page migration callbacks.
>
> The anonymous file may be also used implicitly, like hugetlb files, to
> implement mmap(MAP_SECRET) and use the secret memory areas with "native" mm
> ABIs in the future.
>
> As the fragmentation of the direct map was one of the major concerns raised
> during the previous postings, I've added an amortizing cache of PMD-size
> pages to each file descriptor and an ability to reserve large chunks of the
> physical memory at boot time and then use this memory as an allocation pool
> for the secret memory areas.
>
> v3: https://lore.kernel.org/lkml/20200804095035.18778-1-rppt@kernel.org
> v2: https://lore.kernel.org/lkml/20200727162935.31714-1-rppt@kernel.org
> v1: https://lore.kernel.org/lkml/20200720092435.17469-1-rppt@kernel.org/
> rfc-v2: https://lore.kernel.org/lkml/20200706172051.19465-1-rppt@kernel.org/
> rfc-v1: https://lore.kernel.org/lkml/20200130162340.GA14232@rapoport-lnx/
>
> Mike Rapoport (6):
> mm: add definition of PMD_PAGE_ORDER
> mmap: make mlock_future_check() global
> mm: introduce memfd_secret system call to create "secret" memory areas
> arch, mm: wire up memfd_secret system call were relevant
> mm: secretmem: use PMD-size pages to amortize direct map fragmentation
> mm: secretmem: add ability to reserve memory at boot
>
> arch/Kconfig | 7 +
> arch/arm64/include/asm/unistd.h | 2 +-
> arch/arm64/include/asm/unistd32.h | 2 +
> arch/arm64/include/uapi/asm/unistd.h | 1 +
> arch/riscv/include/asm/unistd.h | 1 +
> arch/x86/Kconfig | 1 +
> arch/x86/entry/syscalls/syscall_32.tbl | 1 +
> arch/x86/entry/syscalls/syscall_64.tbl | 1 +
> fs/dax.c | 11 +-
> include/linux/pgtable.h | 3 +
> include/linux/syscalls.h | 1 +
> include/uapi/asm-generic/unistd.h | 7 +-
> include/uapi/linux/magic.h | 1 +
> include/uapi/linux/secretmem.h | 8 +
> kernel/sys_ni.c | 2 +
> mm/Kconfig | 4 +
> mm/Makefile | 1 +
> mm/internal.h | 3 +
> mm/mmap.c | 5 +-
> mm/secretmem.c | 451 +++++++++++++++++++++++++
> 20 files changed, 501 insertions(+), 12 deletions(-)
> create mode 100644 include/uapi/linux/secretmem.h
> create mode 100644 mm/secretmem.c
>
> --
> 2.26.2
>

--
Sincerely yours,
Mike.