Subject: Re: [PATCH v10 02/43] arm64: RME: Handle Granule Protection Faults (GPFs)

From: Catalin Marinas <[email-masked]>
Date: Fri, 29 Aug 2025 12:38:29 +0100

On Wed, Aug 20, 2025 at 03:55:22PM +0100, Steven Price wrote:
> If the host attempts to access granules that have been delegated for use
> in a realm these accesses will be caught and will trigger a Granule
> Protection Fault (GPF).
> 
> A fault during a page walk signals a bug in the kernel and is handled by
> oopsing the kernel. A non-page walk fault could be caused by user space
> having access to a page which has been delegated to the kernel and will
> trigger a SIGBUS to allow debugging why user space is trying to access a
> delegated page.
> 
> Reviewed-by: Suzuki K Poulose <[email-masked]>
> Reviewed-by: Gavin Shan <[email-masked]>
> Signed-off-by: Steven Price <[email-masked]>
> ---
> Changes since v2:
>  * Include missing "Granule Protection Fault at level -1"
> ---
>  arch/arm64/mm/fault.c | 31 +++++++++++++++++++++++++------
>  1 file changed, 25 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index d816ff44faff..e4237637cd8f 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -854,6 +854,25 @@ static int do_tag_check_fault(unsigned long far, unsigned long esr,
>  	return 0;
>  }
>  
> +static int do_gpf_ptw(unsigned long far, unsigned long esr, struct pt_regs *regs)
> +{
> +	const struct fault_info *inf = esr_to_fault_info(esr);
> +
> +	die_kernel_fault(inf->name, far, esr, regs);
> +	return 0;
> +}

This is fine, it's irrelevant whether the fault happened at EL0 or EL1.

> +static int do_gpf(unsigned long far, unsigned long esr, struct pt_regs *regs)
> +{
> +	const struct fault_info *inf = esr_to_fault_info(esr);
> +
> +	if (!is_el1_instruction_abort(esr) && fixup_exception(regs, esr))
> +		return 0;
> +
> +	arm64_notify_die(inf->name, regs, inf->sig, inf->code, far, esr);
> +	return 0;
> +}

The end result is somewhat similar but why not just return 1 and avoid
the arm64_notify_die() call? Let do_mem_abort() handle the oops vs user
signal. With die_kernel_fault() we print the "Unable to handle
kernel..." message and some more information.

-- 
Catalin