Re: [PATCH v16 07/45] arm64: mm: Handle Granule Protection Faults (GPFs)

On Mon, Aug 03, 2026 at 02:43:23PM +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 protected]>
> Reviewed-by: Gavin Shan <[email protected]>
> Signed-off-by: Steven Price <[email protected]>
> ---
> Changes since v10:
>  * Don't call arm64_notify_die() in do_gpf() but simply return 1.
> Changes since v2:
>  * Include missing "Granule Protection Fault at level -1"
> ---
>  arch/arm64/mm/fault.c | 28 ++++++++++++++++++++++------
>  1 file changed, 22 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index 85e23388f9bb..ea3ae0ca7dba 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -909,6 +909,22 @@ 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;
> +}
> +
> +static int do_gpf(unsigned long far, unsigned long esr, struct pt_regs *regs)
> +{
> +	if (!is_el1_instruction_abort(esr) && fixup_exception(regs, esr))
> +		return 0;
> +
> +	return 1;
> +}

Is there a valid case for fixup_exception() here? IOW, do we ever have a
valid user mapping of the pages delegated to a guest? If the above is
considered a kernel bug, I'd not silently ignore this (like return less
bytes copied or -EFAULT to user) but rather warn, potentially
rate-limited.

If there is a real use-case for this, what's preventing GUP + memcpy()
from triggering a similar fault with no fixup available?

-- 
Catalin