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
