Re: [PATCH v2 14/15] arm64: drtm: Call UNPROTECT_MEMORY

[Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>]

On Fri,  2 Oct 2026 20:31:51 -0300
Jason Gunthorpe <jgg@nvidia.com> wrote:

> The stub always requests ARM_DRTM_LAUNCH_DMA_COMPLETE. On a system
> with an SMMU the COMPLETE option is sanely implemented using the SMMU
> global abort register. DMA_COMPLETE should be mostly a no-op. It
> seems to be designed for the region protection mode that Linux does
> not use.
> 
> However, the FW can still do something interesting under it and we
> should call it. Choose late_initcall as we know the SMMU drivers are
> loaded by then. If some platform does actually block DMA beyond the
> SMMU, then this would result in a noisy failure of built-in driver
> DMA, not a silent security gap.
> 
> The platforms I'm aware of are OK with this placement, and it is a
> reasonable place to start.
> 
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> ---
>  arch/arm64/kernel/drtm.c | 39 +++++++++++++++++++++++++++++++++++++++
>  1 file changed, 39 insertions(+)
> 
> diff --git a/arch/arm64/kernel/drtm.c b/arch/arm64/kernel/drtm.c
> index 14493948a27f0d..6062980a5d9e53 100644
> --- a/arch/arm64/kernel/drtm.c
> +++ b/arch/arm64/kernel/drtm.c
> @@ -3,8 +3,47 @@
>   *
>   * Support functions for ARM DEN 0113 "DRTM Architecture for Arm"
>   */
> +#include <linux/bug.h>
>  #include <linux/efi.h>
> +#include <linux/init.h>
> +#include <linux/printk.h>
>  
>  #include <asm/drtm.h>
>  
>  struct arm64_drtm_handoff arm64_drtm_handoff __efi_data_handoff;
> +
> +static int __init arm64_drtm_unprotect_memory(void)
> +{
> +	s64 status;
> +
> +	if (!arm64_drtm_handoff.drtm_enabled)
> +		return 0;
> +
> +	/*
> +	 * Currently Linux can only fully support a DRTM implementation that
> +	 * relies only the SMMU.

That sentence doesn't parse.  relies on maybe?

> This should be called directly after attaching
> +	 * a driver to every SMMU but before binding any drivers that want to
> +	 * use devices attached to the SMMU. Our boot flow does not have a way
> +	 * to do that, so for now call it here.
> +	 *
> +	 * For SMMU based implementations their UNPROTECT_MEMORY is probably a
> +	 * NOP since touching the SMMU here is forbidden, but they may still do
> +	 * something interesting so be sure to call it at least.
> +	 *
> +	 * The stub excludes the most likely case of non-SMMU by only using
> +	 * ARM_DRTM_DMA_PROTECTION_COMPLETE, which is defined as:
> +	 *    Complete DMA protection is hardware-based enforcement at the SMMU
> +	 *    that blocks all DMA from Non- secure devices."
> +	 *
> +	 * For now if people have such systems they cannot include built-in
> +	 * drivers for any DMA devices, those have to be modules to be ordered
> +	 * after this.
> +	 */
> +	status = arm_drtm_unprotect_memory();
> +	WARN(status != ARM_DRTM_SUCCESS,
> +	     "DRTM: failed to unprotect memory (x0=%lld)", status);
> +	if (status == ARM_DRTM_SUCCESS)
> +		pr_info("DRTM: launch completed\n");
> +	return 0;
> +}
> +late_initcall_sync(arm64_drtm_unprotect_memory);