The arm64 Guarded Control Stack (GCS) feature provides support for hardware protected stacks of return addresses, intended to provide hardening against return oriented programming (ROP) attacks and to make it easier to gather call stacks for applications such as profiling.
When GCS is active a secondary stack called the Guarded Control Stack is maintained, protected with a memory attribute which means that it can only be written with specific GCS operations. The current GCS pointer can not be directly written to by userspace. When a BL is executed the value stored in LR is also pushed onto the GCS, and when a RET is executed the top of the GCS is popped and compared to LR with a fault being raised if the values do not match. GCS operations may only be performed on GCS pages, a data abort is generated if they are not.
The combination of hardware enforcement and lack of extra instructions in the function entry and exit paths should result in something which has less overhead and is more difficult to attack than a purely software implementation like clang's shadow stacks.
This series implements support for managing GCS for KVM guests.
Signed-off-by: Mark Brown broonie@kernel.org --- Changes in v16: - Rebase onto v6.17-rc3. - Also expose the feature to nested guests. - Implement emulation of EXLOCK when returning from nested guests. - Rename enter_exception_gcs() to compute_exlock(). - Move all ID_AA64PFR1_EL1 handling to the final kernel patch. - Drop unneeded forwarding of GCS exceptions. - Commit and cover message updates. - Link to v15: https://lore.kernel.org/r/20250820-arm64-gcs-v15-0-5e334da18b84@kernel.org
Changes in v15: - Rebase onto v6.17-rc1. - Link to v14: https://lore.kernel.org/r/20241005-arm64-gcs-v14-0-59060cd6092b@kernel.org
Changes in v14: - Rebase onto arm64/for-next/gcs which includes all the non-KVM support. - Manage the fine grained traps for GCS instructions. - Manage PSTATE.EXLOCK when delivering exceptions to KVM guests. - Link to v13: https://lore.kernel.org/r/20241001-arm64-gcs-v13-0-222b78d87eee@kernel.org
Changes in v13: - Rebase onto v6.12-rc1. - Allocate VM_HIGH_ARCH_6 since protection keys used all the existing bits. - Implement mm_release() and free transparently allocated GCSs there. - Use bit 32 of AT_HWCAP for GCS due to AT_HWCAP2 being filled. - Since we now only set GCSCRE0_EL1 on change ensure that it is initialised with GCSPR_EL0 accessible to EL0. - Fix OOM handling on thread copy. - Link to v12: https://lore.kernel.org/r/20240829-arm64-gcs-v12-0-42fec947436a@kernel.org
Changes in v12: - Clarify and simplify the signal handling code so we work with the register state. - When checking for write aborts to shadow stack pages ensure the fault is a data abort. - Depend on !UPROBES. - Comment cleanups. - Link to v11: https://lore.kernel.org/r/20240822-arm64-gcs-v11-0-41b81947ecb5@kernel.org
Changes in v11: - Remove the dependency on the addition of clone3() support for shadow stacks, rebasing onto v6.11-rc3. - Make ID_AA64PFR1_EL1.GCS writeable in KVM. - Hide GCS registers when GCS is not enabled for KVM guests. - Require HCRX_EL2.GCSEn if booting at EL1. - Require that GCSCR_EL1 and GCSCRE0_EL1 be initialised regardless of if we boot at EL2 or EL1. - Remove some stray use of bit 63 in signal cap tokens. - Warn if we see a GCS with VM_SHARED. - Remove rdundant check for VM_WRITE in fault handling. - Cleanups and clarifications in the ABI document. - Clean up and improve documentation of some sync placement. - Only set the EL0 GCS mode if it's actually changed. - Various minor fixes and tweaks. - Link to v10: https://lore.kernel.org/r/20240801-arm64-gcs-v10-0-699e2bd2190b@kernel.org
Changes in v10: - Fix issues with THP. - Tighten up requirements for initialising GCSCR*. - Only generate GCS signal frames for threads using GCS. - Only context switch EL1 GCS registers if S1PIE is enabled. - Move context switch of GCSCRE0_EL1 to EL0 context switch. - Make GCS registers unconditionally visible to userspace. - Use FHU infrastructure. - Don't change writability of ID_AA64PFR1_EL1 for KVM. - Remove unused arguments from alloc_gcs(). - Typo fixes. - Link to v9: https://lore.kernel.org/r/20240625-arm64-gcs-v9-0-0f634469b8f0@kernel.org
Changes in v9: - Rebase onto v6.10-rc3. - Restructure and clarify memory management fault handling. - Fix up basic-gcs for the latest clone3() changes. - Convert to newly merged KVM ID register based feature configuration. - Fixes for NV traps. - Link to v8: https://lore.kernel.org/r/20240203-arm64-gcs-v8-0-c9fec77673ef@kernel.org
Changes in v8: - Invalidate signal cap token on stack when consuming. - Typo and other trivial fixes. - Don't try to use process_vm_write() on GCS, it intentionally does not work. - Fix leak of thread GCSs. - Rebase onto latest clone3() series. - Link to v7: https://lore.kernel.org/r/20231122-arm64-gcs-v7-0-201c483bd775@kernel.org
Changes in v7: - Rebase onto v6.7-rc2 via the clone3() patch series. - Change the token used to cap the stack during signal handling to be compatible with GCSPOPM. - Fix flags for new page types. - Fold in support for clone3(). - Replace copy_to_user_gcs() with put_user_gcs(). - Link to v6: https://lore.kernel.org/r/20231009-arm64-gcs-v6-0-78e55deaa4dd@kernel.org
Changes in v6: - Rebase onto v6.6-rc3. - Add some more gcsb_dsync() barriers following spec clarifications. - Due to ongoing discussion around clone()/clone3() I've not updated anything there, the behaviour is the same as on previous versions. - Link to v5: https://lore.kernel.org/r/20230822-arm64-gcs-v5-0-9ef181dd6324@kernel.org
Changes in v5: - Don't map any permissions for user GCSs, we always use EL0 accessors or use a separate mapping of the page. - Reduce the standard size of the GCS to RLIMIT_STACK/2. - Enforce a PAGE_SIZE alignment requirement on map_shadow_stack(). - Clarifications and fixes to documentation. - More tests. - Link to v4: https://lore.kernel.org/r/20230807-arm64-gcs-v4-0-68cfa37f9069@kernel.org
Changes in v4: - Implement flags for map_shadow_stack() allowing the cap and end of stack marker to be enabled independently or not at all. - Relax size and alignment requirements for map_shadow_stack(). - Add more blurb explaining the advantages of hardware enforcement. - Link to v3: https://lore.kernel.org/r/20230731-arm64-gcs-v3-0-cddf9f980d98@kernel.org
Changes in v3: - Rebase onto v6.5-rc4. - Add a GCS barrier on context switch. - Add a GCS stress test. - Link to v2: https://lore.kernel.org/r/20230724-arm64-gcs-v2-0-dc2c1d44c2eb@kernel.org
Changes in v2: - Rebase onto v6.5-rc3. - Rework prctl() interface to allow each bit to be locked independently. - map_shadow_stack() now places the cap token based on the size requested by the caller not the actual space allocated. - Mode changes other than enable via ptrace are now supported. - Expand test coverage. - Various smaller fixes and adjustments. - Link to v1: https://lore.kernel.org/r/20230716-arm64-gcs-v1-0-bf567f93bba6@kernel.org
--- Mark Brown (6): arm64/gcs: Ensure FGTs for EL1 GCS instructions are disabled KVM: arm64: Manage GCS access and registers for guests KVM: arm64: Set PSTATE.EXLOCK when entering an exception KVM: arm64: Validate GCS exception lock when emulating ERET KVM: arm64: Allow GCS to be enabled for guests KVM: selftests: arm64: Add GCS registers to get-reg-list
arch/arm64/include/asm/el2_setup.h | 5 +++ arch/arm64/include/asm/kvm_emulate.h | 3 ++ arch/arm64/include/asm/kvm_host.h | 14 +++++++++ arch/arm64/include/asm/vncr_mapping.h | 2 ++ arch/arm64/include/uapi/asm/ptrace.h | 1 + arch/arm64/kvm/emulate-nested.c | 40 +++++++++++++++++++++++- arch/arm64/kvm/hyp/exception.c | 37 ++++++++++++++++++++++ arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h | 31 ++++++++++++++++++ arch/arm64/kvm/hyp/vhe/sysreg-sr.c | 10 ++++++ arch/arm64/kvm/nested.c | 7 +++-- arch/arm64/kvm/sys_regs.c | 32 +++++++++++++++++-- tools/testing/selftests/kvm/arm64/get-reg-list.c | 12 +++++++ 12 files changed, 188 insertions(+), 6 deletions(-) --- base-commit: 1b237f190eb3d36f52dffe07a40b5eb210280e00 change-id: 20230303-arm64-gcs-e311ab0d8729
Best regards, -- Mark Brown broonie@kernel.org