| Message ID | 20201123095432.5860-1-rppt@kernel.org (mailing list archive) |
|---|---|
| Headers | show |
| Series | mm: introduce memfd_secret system call to create "secret" memory areas | expand |
On Mon, Nov 23, 2020 at 1:54 AM Mike Rapoport <rppt@kernel.org> 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. I'm still not ready to ACK uncached mappings on x86. I'm fine with the concept of allowing privileged users to create UC memory on x86 for testing and experimentation, but it's a big can of worms in general. The issues that immediately come to mind are: - Performance and DoS potential. UC will have bizarre, architecture- and platform-dependent performance characteristics. For all I know, even the access semantics might be architecture dependent. I'm not convinced it's possible to write portable code in C using the uncached feature. I'm also concerned that certain operation (unaligned locks, for example, and possibly any locked access) will trigger bus locks on x86, which, depending on CPU and kernel config will either DoS all other CPUs or send signals. (Or cause the hypervisor to terminate or otherwise penalize the the VM, which would be nasty.) - Correctness. I have reports that different x86 hypervisors do different things with UC mappings, including treating them as regular WB mappings. So the memory type you get out when you ask for "uncached" might not actually be uncached. UC is really an MMIO feature, not a "protect my data" feature. Abusing it to protect data is certainly interesting, but I'm far from convinced that it's wise. I'm especially unconvinced that monkey-patching a program to use uncached memory when it expects regular malloced memory is a reasonable thing to do.
On Mon, Nov 23, 2020 at 07:28:22AM -0800, Andy Lutomirski wrote: > On Mon, Nov 23, 2020 at 1:54 AM Mike Rapoport <rppt@kernel.org> 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. > > I'm still not ready to ACK uncached mappings on x86. I'm fine with > the concept of allowing privileged users to create UC memory on x86 > for testing and experimentation, but it's a big can of worms in > general. Ok, let's move forward without UC.
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. v10: * Drop changes to arm64 compatibility layer * Add Roman's Ack for memcg accounting v9: https://lore.kernel.org/lkml/20201117162932.13649-1-rppt@kernel.org * 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/20201110151444.20662-1-rppt@kernel.org * 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. Older history: v5: https://lore.kernel.org/lkml/20200916073539.3552-1-rppt@kernel.org v4: https://lore.kernel.org/lkml/20200818141554.13945-1-rppt@kernel.org 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 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/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 | 3 +- mm/gup.c | 10 + mm/internal.h | 3 + mm/mmap.c | 5 +- mm/secretmem.c | 446 ++++++++++++++++++++++ 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 + 36 files changed, 888 insertions(+), 38 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