The kernel has recently added support for shadow stacks, currently x86 only using their CET feature but both arm64 and RISC-V have equivalent features (GCS and Zicfiss respectively), I am actively working on GCS[1]. With shadow stacks the hardware maintains an additional stack containing only the return addresses for branch instructions which is not generally writeable by userspace and ensures that any returns are to the recorded addresses. This provides some protection against ROP attacks and making it easier to collect call stacks. These shadow stacks are allocated in the address space of the userspace process.
Our API for shadow stacks does not currently offer userspace any flexiblity for managing the allocation of shadow stacks for newly created threads, instead the kernel allocates a new shadow stack with the same size as the normal stack whenever a thread is created with the feature enabled. The stacks allocated in this way are freed by the kernel when the thread exits or shadow stacks are disabled for the thread. This lack of flexibility and control isn't ideal, in the vast majority of cases the shadow stack will be over allocated and the implicit allocation and deallocation is not consistent with other interfaces. As far as I can tell the interface is done in this manner mainly because the shadow stack patches were in development since before clone3() was implemented.
Since clone3() is readily extensible let's add support for specifying a shadow stack when creating a new thread or process in a similar manner to how the normal stack is specified, keeping the current implicit allocation behaviour if one is not specified either with clone3() or through the use of clone(). The user must provide a shadow stack address and size, this must point to memory mapped for use as a shadow stackby map_shadow_stack() with a shadow stack token at the top of the stack.
Please note that the x86 portions of this code are build tested only, I don't appear to have a system that can run CET avaible to me, I have done testing with an integration into my pending work for GCS. There is some possibility that the arm64 implementation may require the use of clone3() and explicit userspace allocation of shadow stacks, this is still under discussion.
Please further note that the token consumption done by clone3() is not currently implemented in an atomic fashion, Rick indicated that he would look into fixing this if people are OK with the implementation.
A new architecture feature Kconfig option for shadow stacks is added as here, this was suggested as part of the review comments for the arm64 GCS series and since we need to detect if shadow stacks are supported it seemed sensible to roll it in here.
[1] https://lore.kernel.org/r/20231009-arm64-gcs-v6-0-78e55deaa4dd@kernel.org/
Signed-off-by: Mark Brown broonie@kernel.org --- Changes in v6: - Rebase onto v6.10-rc3. - Ensure we don't try to free the parent shadow stack in error paths of x86 arch code. - Spelling fixes in userspace API document. - Additional cleanups and improvements to the clone3() tests to support the shadow stack tests. - Link to v5: https://lore.kernel.org/r/20240203-clone3-shadow-stack-v5-0-322c69598e4b@ker...
Changes in v5: - Rebase onto v6.8-rc2. - Rework ABI to have the user allocate the shadow stack memory with map_shadow_stack() and a token. - Force inlining of the x86 shadow stack enablement. - Move shadow stack enablement out into a shared header for reuse by other tests. - Link to v4: https://lore.kernel.org/r/20231128-clone3-shadow-stack-v4-0-8b28ffe4f676@ker...
Changes in v4: - Formatting changes. - Use a define for minimum shadow stack size and move some basic validation to fork.c. - Link to v3: https://lore.kernel.org/r/20231120-clone3-shadow-stack-v3-0-a7b8ed3e2acc@ker...
Changes in v3: - Rebase onto v6.7-rc2. - Remove stale shadow_stack in internal kargs. - If a shadow stack is specified unconditionally use it regardless of CLONE_ parameters. - Force enable shadow stacks in the selftest. - Update changelogs for RISC-V feature rename. - Link to v2: https://lore.kernel.org/r/20231114-clone3-shadow-stack-v2-0-b613f8681155@ker...
Changes in v2: - Rebase onto v6.7-rc1. - Remove ability to provide preallocated shadow stack, just specify the desired size. - Link to v1: https://lore.kernel.org/r/20231023-clone3-shadow-stack-v1-0-d867d0b5d4d0@ker...
--- Mark Brown (9): Documentation: userspace-api: Add shadow stack API documentation selftests: Provide helper header for shadow stack testing mm: Introduce ARCH_HAS_USER_SHADOW_STACK fork: Add shadow stack support to clone3() selftests/clone3: Remove redundant flushes of output streams selftests/clone3: Factor more of main loop into test_clone3() selftests/clone3: Explicitly handle child exits due to signals selftests/clone3: Allow tests to flag if -E2BIG is a valid error code selftests/clone3: Test shadow stack support
Documentation/userspace-api/index.rst | 1 + Documentation/userspace-api/shadow_stack.rst | 41 ++++ arch/x86/Kconfig | 1 + arch/x86/include/asm/shstk.h | 11 +- arch/x86/kernel/process.c | 2 +- arch/x86/kernel/shstk.c | 104 +++++++--- fs/proc/task_mmu.c | 2 +- include/linux/mm.h | 2 +- include/linux/sched/task.h | 13 ++ include/uapi/linux/sched.h | 13 +- kernel/fork.c | 76 ++++++-- mm/Kconfig | 6 + tools/testing/selftests/clone3/clone3.c | 225 ++++++++++++++++++---- tools/testing/selftests/clone3/clone3_selftests.h | 40 +++- tools/testing/selftests/ksft_shstk.h | 63 ++++++ 15 files changed, 512 insertions(+), 88 deletions(-) --- base-commit: 83a7eefedc9b56fe7bfeff13b6c7356688ffa670 change-id: 20231019-clone3-shadow-stack-15d40d2bf536
Best regards,