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.