Re: [PATCH v8 0/9] mm: introduce memfd_secret system call to create "secret" memory areas

Mike Rapoport
2020-11-12T14:56:51Z

Hi Andrew,

It'll be great if this can be pulled back to mmotm for wider exposure to
testing and fuzzing.

On Tue, Nov 10, 2020 at 05:14:35PM +0200, Mike Rapoport wrote:
> From: Mike Rapoport <rppt@linux.ibm.com>
>
> 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 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, 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.
>
> v8:
> * 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
>
> v7: https://lore.kernel.org/lkml/20201026083752.13267-1-rppt@kernel.org
> * Use set_direct_map() instead of __kernel_map_pages() to ensure error
> handling in case the direct map update fails
> * Add accounting of large pages used to reduce the direct map fragmentation
> * Teach get_user_pages() and frieds to refuse get/pin secretmem pages
>
> v6: https://lore.kernel.org/lkml/20200924132904.1391-1-rppt@kernel.org
> * Silence the warning about missing syscall, thanks to Qian Cai
> * Replace spaces with tabs in Kconfig additions, per Randy
> * Add a selftest.
>
> v5: https://lore.kernel.org/lkml/20200916073539.3552-1-rppt@kernel.org
> * rebase on v5.9-rc5
> * drop boot time memory reservation patch
>
> v4: https://lore.kernel.org/lkml/20200818141554.13945-1-rppt@kernel.org
> * 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: https://lore.kernel.org/lkml/20200804095035.18778-1-rppt@kernel.org
> * 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: https://lore.kernel.org/lkml/20200727162935.31714-1-rppt@kernel.org
> * 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.
>
> v1: https://lore.kernel.org/lkml/20200720092435.17469-1-rppt@kernel.org
>
> Mike Rapoport (9):
> mm: add definition of PMD_PAGE_ORDER
> mmap: make mlock_future_check() global
> set_memory: allow set_direct_map_*_noflush() for multiple pages
> 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/Kconfig | 7 +
> arch/arm64/include/asm/cacheflush.h | 4 +-
> arch/arm64/include/asm/unistd.h | 2 +-
> arch/arm64/include/asm/unistd32.h | 2 +
> arch/arm64/include/uapi/asm/unistd.h | 1 +
> arch/arm64/mm/pageattr.c | 10 +-
> arch/riscv/include/asm/set_memory.h | 4 +-
> arch/riscv/include/asm/unistd.h | 1 +
> arch/riscv/mm/pageattr.c | 8 +-
> arch/x86/Kconfig | 1 +
> 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 | 4 +-
> include/linux/syscalls.h | 1 +
> include/uapi/asm-generic/unistd.h | 6 +-
> include/uapi/linux/magic.h | 1 +
> include/uapi/linux/secretmem.h | 8 +
> kernel/power/hibernate.c | 5 +-
> kernel/power/snapshot.c | 4 +-
> kernel/sys_ni.c | 2 +
> mm/Kconfig | 5 +
> mm/Makefile | 1 +
> mm/filemap.c | 2 +-
> mm/gup.c | 10 +
> mm/internal.h | 3 +
> mm/mmap.c | 5 +-
> mm/secretmem.c | 451 ++++++++++++++++++++++
> 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, 895 insertions(+), 39 deletions(-)
> create mode 100644 include/linux/secretmem.h
> create mode 100644 include/uapi/linux/secretmem.h
> create mode 100644 mm/secretmem.c
> create mode 100644 tools/testing/selftests/vm/memfd_secret.c
>
>
> base-commit: 9f8ce377d420db12b19d6a4f636fecbd88a725a5
> --
> 2.28.0
>

--
Sincerely yours,
Mike.