[v12,00/10] mm: introduce memfd_secret system call to create "secret" memory areas

Message

Mike Rapoport Nov. 25, 2020, 9:21 a.m. UTC

From: Mike Rapoport <[e-mail]>

Hi,

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.

Additionally, in the future 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

https://git.kernel.org/pub/scm/linux/kernel/git/jejb/secret-memory-preloader.git

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.

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.

To limit fragmentation of the direct map to splitting only PUD-size pages,
I've added an amortizing cache of PMD-size pages to each file descriptor
that is used as an allocation pool for the secret memory areas.

As the memory allocated by secretmem becomes unmovable, we use CMA to back
large page caches so that page allocator won't be surprised by failing attempt
to migrate these pages.

v12:
* Add detection of whether set_direct_map has actual effect on arm64 and bail
  out of CMA allocation for secretmem and the memfd_secret() syscall if pages
  would not be removed from the direct map

v11: https://lore.kernel.org/lkml/[e-mail]
* Drop support for uncached mappings

v10: https://lore.kernel.org/lkml/[e-mail]
* Drop changes to arm64 compatibility layer
* Add Roman's Ack for memcg accounting

v9: https://lore.kernel.org/lkml/[e-mail]
* Fix build with and without CONFIG_MEMCG
* Update memcg accounting to avoid copying memcg_data, per Roman comments
* Fix issues in secretmem_fault(), thanks Matthew
* Do not wire up syscall in arm64 compatibility layer

v8: https://lore.kernel.org/lkml/[e-mail]
* Use CMA for all secretmem allocations as David suggested
* Update memcg accounting after transtion to CMA
* Prevent hibernation when there are active secretmem users
* Add zeroing of the memory before releasing it back to cma/page allocator
* Rebase on v5.10-rc2-mmotm-2020-11-07-21-40

Older history:
v7: https://lore.kernel.org/lkml/[e-mail]
v6: https://lore.kernel.org/lkml/[e-mail]
v5: https://lore.kernel.org/lkml/[e-mail]
v4: https://lore.kernel.org/lkml/[e-mail]
v3: https://lore.kernel.org/lkml/[e-mail]
v2: https://lore.kernel.org/lkml/[e-mail]
v1: https://lore.kernel.org/lkml/[e-mail]

Mike Rapoport (10):
  mm: add definition of PMD_PAGE_ORDER
  mmap: make mlock_future_check() global
  set_memory: allow set_direct_map_*_noflush() for multiple pages
  set_memory: allow querying whether set_direct_map_*() is actually enabled
  mm: introduce memfd_secret system call to create "secret" memory areas
  secretmem: use PMD-size pages to amortize direct map fragmentation
  secretmem: add memcg accounting
  PM: hibernate: disable when there are active secretmem users
  arch, mm: wire up memfd_secret system call were relevant
  secretmem: test: add basic selftest for memfd_secret(2)

 arch/arm64/include/asm/Kbuild             |   1 -
 arch/arm64/include/asm/cacheflush.h       |   6 -
 arch/arm64/include/asm/set_memory.h       |  17 +
 arch/arm64/include/uapi/asm/unistd.h      |   1 +
 arch/arm64/kernel/machine_kexec.c         |   1 +
 arch/arm64/mm/mmu.c                       |   6 +-
 arch/arm64/mm/pageattr.c                  |  23 +-
 arch/riscv/include/asm/set_memory.h       |   4 +-
 arch/riscv/include/asm/unistd.h           |   1 +
 arch/riscv/mm/pageattr.c                  |   8 +-
 arch/x86/Kconfig                          |   2 +-
 arch/x86/entry/syscalls/syscall_32.tbl    |   1 +
 arch/x86/entry/syscalls/syscall_64.tbl    |   1 +
 arch/x86/include/asm/set_memory.h         |   4 +-
 arch/x86/mm/pat/set_memory.c              |   8 +-
 fs/dax.c                                  |  11 +-
 include/linux/pgtable.h                   |   3 +
 include/linux/secretmem.h                 |  30 ++
 include/linux/set_memory.h                |  16 +-
 include/linux/syscalls.h                  |   1 +
 include/uapi/asm-generic/unistd.h         |   6 +-
 include/uapi/linux/magic.h                |   1 +
 kernel/power/hibernate.c                  |   5 +-
 kernel/power/snapshot.c                   |   4 +-
 kernel/sys_ni.c                           |   2 +
 mm/Kconfig                                |   5 +
 mm/Makefile                               |   1 +
 mm/filemap.c                              |   3 +-
 mm/gup.c                                  |  10 +
 mm/internal.h                             |   3 +
 mm/mmap.c                                 |   5 +-
 mm/secretmem.c                            | 439 ++++++++++++++++++++++
 mm/vmalloc.c                              |   5 +-
 scripts/checksyscalls.sh                  |   4 +
 tools/testing/selftests/vm/.gitignore     |   1 +
 tools/testing/selftests/vm/Makefile       |   3 +-
 tools/testing/selftests/vm/memfd_secret.c | 298 +++++++++++++++
 tools/testing/selftests/vm/run_vmtests    |  17 +
 38 files changed, 906 insertions(+), 51 deletions(-)
 create mode 100644 arch/arm64/include/asm/set_memory.h
 create mode 100644 include/linux/secretmem.h
 create mode 100644 mm/secretmem.c
 create mode 100644 tools/testing/selftests/vm/memfd_secret.c


base-commit: 9f8ce377d420db12b19d6a4f636fecbd88a725a5