[PATCH v11 24/39] arm64/signal: Set up and restore the GCS context for signal handlers

[Catalin Marinas]

On Thu, Aug 22, 2024 at 02:15:27AM +0100, Mark Brown wrote:
> 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 at linaro.org>
> Signed-off-by: Mark Brown <broonie at kernel.org>

Reviewed-by: Catalin Marinas <catalin.marinas at arm.com>