On Fri, 2 Oct 2026 20:31:51 -0300 Jason Gunthorpe <[email protected]> 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 <[email protected]> > --- > 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);
