Re: [PATCH 3/5] ArmPkg/ArmMmuLib AARCH64: implement support for EFI_MEMORY_RP permissions

[Leif Lindholm <leif.lindholm@linaro.org>]

On Mon, Jan 07, 2019 at 08:15:02AM +0100, Ard Biesheuvel wrote:
> Wire up the access flag (AF) page table attribute to the EFI_MEMORY_RP
> permission attribute, so that attempts to read from such a region will
> trigger an access flag fault.
> 
> Note that this is a stronger notion than just read protection, since
> it now implies that any write or execute attempt is trapped as well.
> However, this does not really matter in practice since we never assume
> that a read protected page is writable or executable, and StackGuard
> and HeapGuard (which are the primary users of this facility) certainly
> don't care.

So ... I'm cautiously positive to this patch.
But this use does contradict the UEFI spec (2.7a, 2.3.6.1 Memory
types), which says EFI_MEMORY_RP is "not used or defined" for AArch64.

Charles?

/
    Leif

> Contributed-under: TianoCore Contribution Agreement 1.1
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> ---
>  ArmPkg/Drivers/CpuDxe/AArch64/Mmu.c              |  5 +++--
>  ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c | 14 +++++++++++---
>  2 files changed, 14 insertions(+), 5 deletions(-)
> 
> diff --git a/ArmPkg/Drivers/CpuDxe/AArch64/Mmu.c b/ArmPkg/Drivers/CpuDxe/AArch64/Mmu.c
> index 3e216c7cb235..e62e3fa87112 100644
> --- a/ArmPkg/Drivers/CpuDxe/AArch64/Mmu.c
> +++ b/ArmPkg/Drivers/CpuDxe/AArch64/Mmu.c
> @@ -223,8 +223,9 @@ EfiAttributeToArmAttribute (
>      ArmAttributes = TT_ATTR_INDX_MASK;
>    }
>  
> -  // Set the access flag to match the block attributes
> -  ArmAttributes |= TT_AF;
> +  if ((EfiAttributes & EFI_MEMORY_RP) == 0) {
> +    ArmAttributes |= TT_AF;
> +  }
>  
>    // Determine protection attributes
>    if (EfiAttributes & EFI_MEMORY_RO) {
> diff --git a/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c b/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c
> index e1fabfcbea14..b59c081a7e49 100644
> --- a/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c
> +++ b/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c
> @@ -102,6 +102,10 @@ PageAttributeToGcdAttribute (
>      GcdAttributes |= EFI_MEMORY_XP;
>    }
>  
> +  if ((PageAttributes & TT_AF) == 0) {
> +    GcdAttributes |= EFI_MEMORY_RP;
> +  }
> +
>    return GcdAttributes;
>  }
>  
> @@ -451,7 +455,11 @@ GcdAttributeToPageAttribute (
>      PageAttributes |= TT_AP_RO_RO;
>    }
>  
> -  return PageAttributes | TT_AF;
> +  if ((GcdAttributes & EFI_MEMORY_RP) == 0) {
> +    PageAttributes |= TT_AF;
> +  }
> +
> +  return PageAttributes;
>  }
>  
>  EFI_STATUS
> @@ -474,9 +482,9 @@ ArmSetMemoryAttributes (
>      // No memory type was set in Attributes, so we are going to update the
>      // permissions only.
>      //
> -    PageAttributes &= TT_AP_MASK | TT_UXN_MASK | TT_PXN_MASK;
> +    PageAttributes &= TT_AP_MASK | TT_UXN_MASK | TT_PXN_MASK | TT_AF;
>      PageAttributeMask = ~(TT_ADDRESS_MASK_BLOCK_ENTRY | TT_AP_MASK |
> -                          TT_PXN_MASK | TT_XN_MASK);
> +                          TT_PXN_MASK | TT_XN_MASK | TT_AF);
>    }
>  
>    TranslationTable = ArmGetTTBR0BaseAddress ();
> -- 
> 2.20.1
>