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d2907cbe9e
When SME is supported, Restoring SVE signal context can go wrong in a
few ways, including placing the task into an invalid state where the
kernel may read from out-of-bounds memory (and may potentially take a
fatal fault) and/or may kill the task with a SIGKILL.
(1) Restoring a context with SVE_SIG_FLAG_SM set can place the task into
an invalid state where SVCR.SM is set (and sve_state is non-NULL)
but TIF_SME is clear, consequently resuting in out-of-bounds memory
reads and/or killing the task with SIGKILL.
This can only occur in unusual (but legitimate) cases where the SVE
signal context has either been modified by userspace or was saved in
the context of another task (e.g. as with CRIU), as otherwise the
presence of an SVE signal context with SVE_SIG_FLAG_SM implies that
TIF_SME is already set.
While in this state, task_fpsimd_load() will NOT configure SMCR_ELx
(leaving some arbitrary value configured in hardware) before
restoring SVCR and attempting to restore the streaming mode SVE
registers from memory via sve_load_state(). As the value of
SMCR_ELx.LEN may be larger than the task's streaming SVE vector
length, this may read memory outside of the task's allocated
sve_state, reading unrelated data and/or triggering a fault.
While this can result in secrets being loaded into streaming SVE
registers, these values are never exposed. As TIF_SME is clear,
fpsimd_bind_task_to_cpu() will configure CPACR_ELx.SMEN to trap EL0
accesses to streaming mode SVE registers, so these cannot be
accessed directly at EL0. As fpsimd_save_user_state() verifies the
live vector length before saving (S)SVE state to memory, no secret
values can be saved back to memory (and hence cannot be observed via
ptrace, signals, etc).
When the live vector length doesn't match the expected vector length
for the task, fpsimd_save_user_state() will send a fatal SIGKILL
signal to the task. Hence the task may be killed after executing
userspace for some period of time.
(2) Restoring a context with SVE_SIG_FLAG_SM clear does not clear the
task's SVCR.SM. If SVCR.SM was set prior to restoring the context,
then the task will be left in streaming mode unexpectedly, and some
register state will be combined inconsistently, though the task will
be left in legitimate state from the kernel's PoV.
This can only occur in unusual (but legitimate) cases where ptrace
has been used to set SVCR.SM after entry to the sigreturn syscall,
as syscall entry clears SVCR.SM.
In these cases, the the provided SVE register data will be loaded
into the task's sve_state using the non-streaming SVE vector length
and the FPSIMD registers will be merged into this using the
streaming SVE vector length.
Fix (1) by setting TIF_SME when setting SVCR.SM. This also requires
ensuring that the task's sme_state has been allocated, but as this could
contain live ZA state, it should not be zeroed. Fix (2) by clearing
SVCR.SM when restoring a SVE signal context with SVE_SIG_FLAG_SM clear.
For consistency, I've pulled the manipulation of SVCR, TIF_SVE, TIF_SME,
and fp_type earlier, immediately after the allocation of
sve_state/sme_state, before the restore of the actual register state.
This makes it easier to ensure that these are always modified
consistently, even if a fault is taken while reading the register data
from the signal context. I do not expect any software to depend on the
exact state restored when a fault is taken while reading the context.
Fixes:
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Linux kernel ============ The Linux kernel is the core of any Linux operating system. It manages hardware, system resources, and provides the fundamental services for all other software. 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Start your kernel development journey here: * Getting Started: Documentation/process/development-process.rst * Your First Patch: Documentation/process/submitting-patches.rst * Coding Style: Documentation/process/coding-style.rst * Build System: Documentation/kbuild/index.rst * Development Tools: Documentation/dev-tools/index.rst * Kernel Hacking Guide: Documentation/kernel-hacking/hacking.rst * Core APIs: Documentation/core-api/index.rst Academic Researcher ------------------- Explore the kernel's architecture and internals: * Researcher Guidelines: Documentation/process/researcher-guidelines.rst * Memory Management: Documentation/mm/index.rst * Scheduler: Documentation/scheduler/index.rst * Networking Stack: Documentation/networking/index.rst * Filesystems: Documentation/filesystems/index.rst * RCU (Read-Copy Update): Documentation/RCU/index.rst * Locking Primitives: Documentation/locking/index.rst * Power Management: Documentation/power/index.rst Security Expert --------------- Security documentation and hardening guides: * Security Documentation: Documentation/security/index.rst * LSM Development: Documentation/security/lsm-development.rst * Self Protection: Documentation/security/self-protection.rst * Reporting Vulnerabilities: Documentation/process/security-bugs.rst * CVE Procedures: Documentation/process/cve.rst * Embargoed Hardware Issues: Documentation/process/embargoed-hardware-issues.rst * Security Features: Documentation/userspace-api/seccomp_filter.rst Backport/Maintenance Engineer ----------------------------- Maintain and stabilize kernel versions: * Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst * Backporting Guide: Documentation/process/backporting.rst * Applying Patches: Documentation/process/applying-patches.rst * Subsystem Profile: Documentation/maintainer/maintainer-entry-profile.rst * Git for Maintainers: Documentation/maintainer/configure-git.rst System Administrator -------------------- Configure, tune, and troubleshoot Linux systems: * Admin Guide: Documentation/admin-guide/index.rst * Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst * Sysctl Tuning: Documentation/admin-guide/sysctl/index.rst * Tracing/Debugging: Documentation/trace/index.rst * Performance Security: Documentation/admin-guide/perf-security.rst * Hardware Monitoring: Documentation/hwmon/index.rst Maintainer ---------- Lead kernel subsystems and manage contributions: * Maintainer Handbook: Documentation/maintainer/index.rst * Pull Requests: Documentation/maintainer/pull-requests.rst * Managing Patches: Documentation/maintainer/modifying-patches.rst * Rebasing and Merging: Documentation/maintainer/rebasing-and-merging.rst * Development Process: Documentation/process/maintainer-handbooks.rst * Maintainer Entry Profile: Documentation/maintainer/maintainer-entry-profile.rst * Git Configuration: Documentation/maintainer/configure-git.rst Hardware Vendor --------------- Write drivers and support new hardware: * Driver API Guide: Documentation/driver-api/index.rst * Driver Model: Documentation/driver-api/driver-model/driver.rst * Device Drivers: Documentation/driver-api/infrastructure.rst * Bus Types: Documentation/driver-api/driver-model/bus.rst * Device Tree Bindings: Documentation/devicetree/bindings/ * Power Management: Documentation/driver-api/pm/index.rst * DMA API: Documentation/core-api/dma-api.rst Distribution Maintainer ----------------------- Package and distribute the kernel: * Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst * ABI Documentation: Documentation/ABI/README * Kernel Configuration: Documentation/kbuild/kconfig.rst * Module Signing: Documentation/admin-guide/module-signing.rst * Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst * Tainted Kernels: Documentation/admin-guide/tainted-kernels.rst Communication and Support ========================= * Mailing Lists: https://lore.kernel.org/ * IRC: #kernelnewbies on irc.oftc.net * Bugzilla: https://bugzilla.kernel.org/ * MAINTAINERS file: Lists subsystem maintainers and mailing lists * Email Clients: Documentation/process/email-clients.rst