Re: [PATCH v10 05/40] arm64/gcs: Document the ABI for Guarded Control Stacks

[Catalin Marinas <catalin.marinas@arm.com>]

On Thu, Aug 01, 2024 at 01:06:32PM +0100, Mark Brown wrote:
> +1.  General
> +-----------
[...]
> +* EL0 GCS entries with bit 63 set are reserved for use, one such use is defined

Maybe "reserved for specific uses". The proposed sentenced feels like
it's missing something.

> +  below for signals and should be ignored when parsing the stack if not
> +  understood.
[...]
> +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 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.

Is the half size still the case? It also seems a bit inconsistent to
have RLIMIT_STACK when GCS is enabled and half the stack size when a new
thread is created.

[...]
> +* 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.

Is this true for shadow stacks explicitly allocated by the user with
map_shadow_stack()?

> +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 and 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.

I assume this is true even with sigaltstack. Not easy to have
alternative shadow stack without additional ABI.

-- 
Catalin