arm64/signal: Set up and restore the GCS context for signal handlers

arm64/signal: Set up and restore the GCS context for signal handlers

When invoking a signal handler we use the GCS configuration and stack
for the current thread.

Since we implement signal return by calling the signal handler with a
return address set up pointing to a trampoline in the vDSO we need to
also configure any active GCS for this by pushing a frame for the
trampoline onto the GCS.  If we do not do this then signal return will
generate a GCS protection fault.

In order to guard against attempts to bypass GCS protections via signal
return we only allow returning with GCSPR_EL0 pointing to an address
where it was previously preempted by a signal.  We do this by pushing a
cap onto the GCS, this takes the form of an architectural GCS cap token
with the top bit set and token type of 0 which we add on signal entry
and validate and pop off on signal return.  The combination of the top
bit being set and the token type mean that this can't be interpreted as
a valid token or address.

Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20241001-arm64-gcs-v13-25-222b78d87eee@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>