Guarded Control Stack support for AArch64 Linux

This document outlines briefly the interface provided to userspace by Linux in order to support use of the ARM Guarded Control Stack (GCS) feature.

This is an outline of the most important features and issues only and not intended to be exhaustive.

1. General

• GCS is an architecture feature intended to provide greater protection against return oriented programming (ROP) attacks and to simplify the implementation of features that need to collect stack traces such as profiling.

• When GCS is enabled a separate guarded control stack is maintained by the PE which is writeable only through specific GCS operations. This stores the call stack only, when a procedure call instruction is performed the current PC is pushed onto the GCS and on RET the address in the LR is verified against that on the top of the GCS.

• When active the current GCS pointer is stored in the system register GCSPR_EL0. This is readable by userspace but can only be updated via specific GCS instructions.

• The architecture provides instructions for switching between guarded control stacks with checks to ensure that the new stack is a valid target for switching.

• The functionality of GCS is similar to that provided by the x86 Shadow Stack feature, due to sharing of userspace interfaces the ABI refers to shadow stacks rather than GCS.

• Support for GCS is reported to userspace via HWCAP_GCS in the aux vector AT_HWCAP2 entry.

• GCS is enabled per thread. While there is support for disabling GCS at runtime this should be done with great care.

• GCS memory access faults are reported as normal memory access faults.

• GCS specific errors (those reported with EC 0x2d) will be reported as SIGSEGV with a si_code of SEGV_CPERR (control protection error).

• GCS is supported only for AArch64.

• On systems where GCS is supported GCSPR_EL0 is always readable by EL0 regardless of the GCS configuration for the thread.

• The architecture supports enabling GCS without verifying that return values in LR match those in the GCS, the LR will be ignored. This is not supported by Linux.

2. Enabling and disabling Guarded Control Stacks

• GCS is enabled and disabled for a thread via the PR_SET_SHADOW_STACK_STATUS prctl(), this takes a single flags argument specifying which GCS features should be used.

• When set PR_SHADOW_STACK_ENABLE flag allocates a Guarded Control Stack and enables GCS for the thread, enabling the functionality controlled by GCSCRE0_EL1.{nTR, RVCHKEN, PCRSEL}.

• When set the PR_SHADOW_STACK_PUSH flag enables the functionality controlled by GCSCRE0_EL1.PUSHMEn, allowing explicit GCS pushes.

• When set the PR_SHADOW_STACK_WRITE flag enables the functionality controlled by GCSCRE0_EL1.STREn, allowing explicit stores to the Guarded Control Stack.

• Any unknown flags will cause PR_SET_SHADOW_STACK_STATUS to return -EINVAL.

• PR_LOCK_SHADOW_STACK_STATUS is passed a bitmask of features with the same values as used for PR_SET_SHADOW_STACK_STATUS. Any future changes to the status of the specified GCS mode bits will be rejected.

• PR_LOCK_SHADOW_STACK_STATUS allows any bit to be locked, this allows userspace to prevent changes to any future features.

• There is no support for a process to remove a lock that has been set for it.

• PR_SET_SHADOW_STACK_STATUS and PR_LOCK_SHADOW_STACK_STATUS affect only the thread that called them, any other running threads will be unaffected.

• New threads inherit the GCS configuration of the thread that created them.

• GCS is disabled on exec().

• The current GCS configuration for a thread may be read with the PR_GET_SHADOW_STACK_STATUS prctl(), this returns the same flags that are passed to PR_SET_SHADOW_STACK_STATUS.

• If GCS is disabled for a thread after having previously been enabled then the stack will remain allocated for the lifetime of the thread. At present any attempt to reenable GCS for the thread will be rejected, this may be revisited in future.

• It should be noted that since enabling GCS will result in GCS becoming active immediately it is not normally possible to return from the function that invoked the prctl() that enabled GCS. It is expected that the normal usage will be that GCS is enabled very early in execution of a program.

3. Allocation of Guarded Control Stacks

• When GCS is enabled for a thread a new Guarded Control Stack will be allocated for it of half the standard stack size or 2 gigabytes, whichever is smaller.

• When a new thread is created by a thread which has GCS enabled then a new Guarded Control Stack will be allocated for the new thread with half the size of the standard stack.

• When a stack is allocated by enabling GCS or during thread creation then the top 8 bytes of the stack will be initialised to 0 and GCSPR_EL0 will be set to point to the address of this 0 value, this can be used to detect the top of the stack.

• Additional Guarded Control Stacks can be allocated using the map_shadow_stack() system call.

• Stacks allocated using map_shadow_stack() can optionally have an end of stack marker and cap placed at the top of the stack. If the flag SHADOW_STACK_SET_TOKEN is specified a cap will be placed on the stack, if SHADOW_STACK_SET_MARKER is not specified the cap will be the top 8 bytes of the stack and if it is specified then the cap will be the next 8 bytes. While specifying just SHADOW_STACK_SET_MARKER by itself is valid since the marker is all bits 0 it has no observable effect.

• Stacks allocated using map_shadow_stack() must have a size which is a multiple of 8 bytes larger than 8 bytes and must be 8 bytes aligned.

• An address can be specified to map_shadow_stack(), if one is provided then it must be aligned to a page boundary.

• When a thread is freed the Guarded Control Stack initially allocated for that thread will be freed. Note carefully that if the stack has been switched this may not be the stack currently in use by the thread.

4. Signal handling

• A new signal frame record gcs_context encodes the current GCS mode and pointer for the interrupted context on signal delivery. This will always be present on systems that support GCS.

• The record contains a flag field which reports the current GCS configuration for the interrupted context as PR_GET_SHADOW_STACK_STATUS would.

• The signal handler is run with the same GCS configuration as the interrupted context.

• When GCS is enabled for the interrupted thread a signal handling specific GCS cap token will be written to the GCS, this is an architectural GCS cap with the token type (bits 0..11) all clear. The GCSPR_EL0 reported in the signal frame will point to this cap token.

• The signal handler will use the same GCS as the interrupted context.

• When GCS is enabled on signal entry a frame with the address of the signal return handler will be pushed onto the GCS, allowing return from the signal handler via RET as normal. This will not be reported in the gcs_context in the signal frame.

5. Signal return

When returning from a signal handler:

• If there is a gcs_context record in the signal frame then the GCS flags and GCSPR_EL0 will be restored from that context prior to further validation.

• If there is no gcs_context record in the signal frame then the GCS configuration will be unchanged.

• If GCS is enabled on return from a signal handler then GCSPR_EL0 must point to a valid GCS signal cap record, this will be popped from the GCS prior to signal return.

• If the GCS configuration is locked when returning from a signal then any attempt to change the GCS configuration will be treated as an error. This is true even if GCS was not enabled prior to signal entry.

• GCS may be disabled via signal return but any attempt to enable GCS via signal return will be rejected.

6. ptrace extensions

• A new regset NT_ARM_GCS is defined for use with PTRACE_GETREGSET and PTRACE_SETREGSET.

• The GCS mode, including enable and disable, may be configured via ptrace. If GCS is enabled via ptrace no new GCS will be allocated for the thread.

• Configuration via ptrace ignores locking of GCS mode bits.

7. ELF coredump extensions

• NT_ARM_GCS notes will be added to each coredump for each thread of the dumped process. The contents will be equivalent to the data that would have been read if a PTRACE_GETREGSET of the corresponding type were executed for each thread when the coredump was generated.

8. /proc extensions

• Guarded Control Stack pages will include “ss” in their VmFlags in /proc/<pid>/smaps.