Re: [PATCH 2/3] ArmPkg/CpuDxe: Perform preliminary NX remap of free memory

["Marvin Häuser" <mhaeuser@posteo.de>]

Thanks!! :) Comments inline.

> On 8. Feb 2023, at 18:58, Ard Biesheuvel <ardb@kernel.org> wrote:
> 
> The DXE core implementation of PcdDxeNxMemoryProtectionPolicy already
> contains an assertion that EfiConventionalMemory and EfiBootServicesData
> are subjected to the same policy when it comes to the use of NX
> permissions. The reason for this is that we may otherwise end up with
> unbounded recursion in the page table code, given that allocating a page
> table would then involve a permission attribute change, and this could
> result in the need for a block entry to be split, which would trigger
> the allocation of a page table recursively.
> 
> For the same reason, a shortcut exists in ApplyMemoryProtectionPolicy()
> where, instead of setting the memory attributes unconditionally, we
> compare the NX policies and avoid touching the page tables if they are
> the same for the old and the new memory types. Without this shortcut, we
> may end up in a situation where, as the CPU arch protocol DXE driver is
> ramping up, the same unbounded recursion is triggered, due to the fact
> that the NX policy for EfiConventionalMemory has not been applied yet.
> 
> To break this cycle, let's remap all EfiConventionalMemory regions
> according to the NX policy for EfiBootServicesData before exposing the
> CPU arch protocol to the DXE core and other drivers. This ensures that
> creating EfiBootServicesData allocations does not result in memory
> attribute changes, and therefore no recursion.
> 
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
> ArmPkg/Drivers/CpuDxe/CpuDxe.c   | 77 ++++++++++++++++++++
> ArmPkg/Drivers/CpuDxe/CpuDxe.inf |  2 +
> 2 files changed, 79 insertions(+)
> 
> diff --git a/ArmPkg/Drivers/CpuDxe/CpuDxe.c b/ArmPkg/Drivers/CpuDxe/CpuDxe.c
> index d04958e79e52..83fd6fd4e476 100644
> --- a/ArmPkg/Drivers/CpuDxe/CpuDxe.c
> +++ b/ArmPkg/Drivers/CpuDxe/CpuDxe.c
> @@ -11,6 +11,8 @@
> 
> #include <Guid/IdleLoopEvent.h>
> 
> +#include <Library/MemoryAllocationLib.h>
> +
> BOOLEAN  mIsFlushingGCD;
> 
> /**
> @@ -227,6 +229,69 @@ InitializeDma (
>   CpuArchProtocol->DmaBufferAlignment = ArmCacheWritebackGranule ();
> }
> 
> +STATIC
> +VOID
> +RemapUnusedMemoryNx (
> +  VOID
> +  )
> +{
> +  UINT64                     TestBit;
> +  UINTN                      MemoryMapSize;
> +  UINTN                      MapKey;
> +  UINTN                      DescriptorSize;
> +  UINT32                     DescriptorVersion;
> +  EFI_MEMORY_DESCRIPTOR      *MemoryMap;
> +  EFI_MEMORY_DESCRIPTOR      *MemoryMapEntry;
> +  EFI_MEMORY_DESCRIPTOR      *MemoryMapEnd;
> +  EFI_STATUS                 Status;
> +
> +  TestBit = LShiftU64 (1, EfiBootServicesData);
> +  if ((PcdGet64 (PcdDxeNxMemoryProtectionPolicy) & TestBit) == 0) {
> +    return;
> +  }
> +
> +  MemoryMapSize = 0;
> +  MemoryMap     = NULL;
> +
> +  Status = gBS->GetMemoryMap (
> +                  &MemoryMapSize,
> +                  MemoryMap,
> +                  &MapKey,
> +                  &DescriptorSize,
> +                  &DescriptorVersion
> +                  );
> +  ASSERT (Status == EFI_BUFFER_TOO_SMALL);
> +  do {
> +    MemoryMap = (EFI_MEMORY_DESCRIPTOR *)AllocatePool (MemoryMapSize);

nitpick: *If* there is a V2, maybe drop the cast?

> +    ASSERT (MemoryMap != NULL);

I know ASSERTing for NULL is a common pattern for some reason, but I'd rather not have more code with this added.

> +    Status = gBS->GetMemoryMap (
> +                    &MemoryMapSize,
> +                    MemoryMap,
> +                    &MapKey,
> +                    &DescriptorSize,
> +                    &DescriptorVersion
> +                    );

Another nitpick, isn't it common practice to call the Core* functions directly within *Core? I know there is code that doesn't, but the former should be more efficient.

> +    if (EFI_ERROR (Status)) {
> +      FreePool (MemoryMap);
> +    }
> +  } while (Status == EFI_BUFFER_TOO_SMALL);

Is this guaranteed to terminate? I mean, I get the idea - try again with the larger size and when allocating the bigger buffer, its potential new entry will already be accounted for. However, I saw downstream code that tried something like this (they actually added a constant overhead ahead of time) bounded by something like 5 iterations. Might just have been defensive programming - probably was -, but it's not trivial to verify, I think, is it?

Regarding the added constant, the spec actually says the following, which obviously is just to shortcut a second round of GetMemoryMap(), but still:
"The actual size of the buffer allocated for the consequent call to GetMemoryMap() should be bigger then the value returned in MemoryMapSize"

It appears the spec did not define a canonical way to call GetMemoryMap(), did it? :(

> +
> +  ASSERT_EFI_ERROR (Status);
> +
> +  MemoryMapEntry = MemoryMap;
> +  MemoryMapEnd   = (EFI_MEMORY_DESCRIPTOR *)((UINT8 *)MemoryMap + MemoryMapSize);
> +  while ((UINTN)MemoryMapEntry < (UINTN)MemoryMapEnd) {
> +    if (MemoryMapEntry->Type == EfiConventionalMemory) {
> +      ArmSetMemoryRegionNoExec (
> +        MemoryMapEntry->PhysicalStart,
> +        EFI_PAGES_TO_SIZE (MemoryMapEntry->NumberOfPages)
> +        );
> +    }
> +
> +    MemoryMapEntry = NEXT_MEMORY_DESCRIPTOR (MemoryMapEntry, DescriptorSize);
> +  }
> +}
> +
> EFI_STATUS
> CpuDxeInitialize (
>   IN EFI_HANDLE        ImageHandle,
> @@ -240,6 +305,18 @@ CpuDxeInitialize (
> 
>   InitializeDma (&mCpu);
> 
> +  //
> +  // Once we install the CPU arch protocol, the DXE core's memory
> +  // protection routines will invoke them to manage the permissions of page
> +  // allocations as they are created. Given that this includes pages
> +  // allocated for page tables by this driver, we must ensure that unused
> +  // memory is mapped with the same permissions as boot services data
> +  // regions. Otherwise, we may end up with unbounded recursion, due to the
> +  // fact that updating permissions on a newly allocated page table may trigger
> +  // a block entry split, which triggers a page table allocation, etc etc
> +  //
> +  RemapUnusedMemoryNx ();

Hmm. I might be too tired, but why is this not already done by InitializeDxeNxMemoryProtectionPolicy(), assuming BS_Data and ConvMem have the same XP configuration?

This reassures me that the CPU Arch protocol producers should be linked into DxeCore rather than loaded at some arbitrary point in time... Unrelated to the patch, of course.

Best regards,
Marvin

> +
>   Status = gBS->InstallMultipleProtocolInterfaces (
>                   &mCpuHandle,
>                   &gEfiCpuArchProtocolGuid,
> diff --git a/ArmPkg/Drivers/CpuDxe/CpuDxe.inf b/ArmPkg/Drivers/CpuDxe/CpuDxe.inf
> index e732e21cb94a..8fd0f4133088 100644
> --- a/ArmPkg/Drivers/CpuDxe/CpuDxe.inf
> +++ b/ArmPkg/Drivers/CpuDxe/CpuDxe.inf
> @@ -48,6 +48,7 @@ [LibraryClasses]
>   DefaultExceptionHandlerLib
>   DxeServicesTableLib
>   HobLib
> +  MemoryAllocationLib
>   PeCoffGetEntryPointLib
>   UefiDriverEntryPoint
>   UefiLib
> @@ -64,6 +65,7 @@ [Guids]
> 
> [Pcd.common]
>   gArmTokenSpaceGuid.PcdVFPEnabled
> +  gEfiMdeModulePkgTokenSpaceGuid.PcdDxeNxMemoryProtectionPolicy
> 
> [FeaturePcd.common]
>   gArmTokenSpaceGuid.PcdDebuggerExceptionSupport
> -- 
> 2.39.1
>