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@@ -0,0 +1,230 @@
+===============================================
+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 HWCAP2_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.
+
+* Due to the complexity surrounding allocation and deallocation of stacks and
+ lack of practical application it is not possible to enable GCS via ptrace.
+ GCS may be disabled via the ptrace interface.
+
+* Other GCS modes may be configured via ptrace.
+
+* 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.
@@ -15,6 +15,7 @@ ARM64 Architecture
cpu-feature-registers
cpu-hotplug
elf_hwcaps
+ gcs
hugetlbpage
kdump
legacy_instructions