Re: [PATCH v18] arm64: mm: Handle Granule Protection Faults (GPFs)

[Will Deacon <will@kernel.org>]

On Sun, Sep 13, 2026 at 08:04:58AM +0100, Suzuki K Poulose wrote:
> From: Steven Price <steven.price@arm.com>
> 
> 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.
> 
> There is work in progress to unmap the guest_memfd backed private pages from the
> linear map. Until we get that support, we could get spurious GPFs from within
> the kernel, e.g., load_unaligned_zeropad(). So, try to fix them up for now.
> 
> Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> Reviewed-by: Gavin Shan <gshan@redhat.com>
> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
> Signed-off-by: Steven Price <steven.price@arm.com>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> ---
> Changes since v17:
>  * Pass untagged address to die_kernel_fault() - Sashiko
>  * Explicitly check !user_mode() for fixups - Catalin
>  * Switch to BUS_OBJERR for si_code from SI_KERNEL - Catalin
>  * Clarify the commit description about the upcoming work on
>    unmapping guest_memfd backed pages from linear map
> Changes since v16:
>  * Update the commit description to indicate why we try to fixup GPFs
> 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 | 30 ++++++++++++++++++++++++------
>  1 file changed, 24 insertions(+), 6 deletions(-)

I still don't think we should do this, given that the plan is to unmap
the memory from the linear map. If this thing fires, it's a kernel bug
and it should be fatal.

Will