[PATCH 04/35] arm64/gcs: Document the ABI for Guarded Control Stacks

[Mark Brown]

Add some documentation of the userspace ABI for Guarded Control Stacks.

Signed-off-by: Mark Brown <broonie@xxxxxxxxxx>
---
 Documentation/arch/arm64/gcs.rst   | 216 +++++++++++++++++++++++++++++++++++++
 Documentation/arch/arm64/index.rst |   1 +
 2 files changed, 217 insertions(+)

diff --git a/Documentation/arch/arm64/gcs.rst b/Documentation/arch/arm64/gcs.rst
new file mode 100644
index 000000000000..27ba72d27952
--- /dev/null
+++ b/Documentation/arch/arm64/gcs.rst
@@ -0,0 +1,216 @@
+===============================================
+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 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.
+
+* EL0 GCS entries with bit 63 set are reserved for use, one such use is defined
+  below for signals and should be ignored when parsing the stack if not
+  understood.
+
+
+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 for
+  and enables GCS for the thread, enabling the functionality controlled by
+  GCSPRE0_EL1.{nTR, RVCHKEN, PCRSEL}.
+
+* When set the PR_SHADOW_STACK_PUSH flag enables the functionality controlled
+  by GCSCRE0_EL1.PUSHMEn, allowing explicit GCS push and pops.
+
+* When set the PR_SHADOW_STACK_WRITE flag enables the functionality controlled
+  by GCSCRE0_EL1.STREn, allowing explicit stores to the Guarded Control Stack.
+
+* When set the PR_SHADOW_STACK_LOCK flag prevents any further configuration of
+  the GCS settings for the thread, further attempts to configure GCS will
+  return -EBUSY.
+
+* Any unknown flags will cause PR_SET_SHADOW_STACK_STATUS to return -EINVAL.
+
+* PR_SET_SHADOW_STACK_STATUS affects only the thread the called it, 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 size RLIMIT_STACK / 2 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() will have the top 8 bytes
+  set to 0 and the 8 bytes below that initialised with an architecturally
+  valid GCS cap value, this allows switching to these stacks using the
+  stack switch instructions provided by the architecture.
+
+* When GCS is disabled for a thread 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
+  token with bit 63 set.  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.
+
+
+7.  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 stakcs and
+  lack of practical application changes to the GCS configuration via ptrace
+  are not supported.
+
+
+
+8.  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.
+
+
+
+9.  /proc extensions
+--------------------
+
+* Guarded Control Stack pages will include "ss" in their VmFlags in
+  /proc/<pid>/smaps.
diff --git a/Documentation/arch/arm64/index.rst b/Documentation/arch/arm64/index.rst
index d08e924204bf..dcf3ee3eb8c0 100644
--- a/Documentation/arch/arm64/index.rst
+++ b/Documentation/arch/arm64/index.rst
@@ -14,6 +14,7 @@ ARM64 Architecture
     booting
     cpu-feature-registers
     elf_hwcaps
+    gcs
     hugetlbpage
     kdump
     legacy_instructions

-- 
2.30.2