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

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

On Mon, Jan 14, 2019 at 03:59:08PM +0100, Ard Biesheuvel wrote:
> On Mon, 14 Jan 2019 at 15:29, Leif Lindholm <leif.lindholm@linaro.org> wrote:
> >
> > 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?
> 
> Not defined by the spec means we can use it do whatever we bloody want
> with it, at least that is what a typical compiler engineer will tell
> you :-)

Not defined, yes. Not used, less so. For a reciprocal compiler
engineer analogy, think pointer tagging :)

> I think there was a pending ECR to update the AArch64 binding code to
> reflect reality, but I don't think we included EFI_MEMORY_RP.

Right.

Anyway, I'm reasonably happy with stretching the rules slightly here,
and I believe it to be safe, but I do want Charles' take on it.

/
    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
> > >