Re: [edk2-devel] [PATCH 04/10] MdePkg/PiFirmwareFile: fix undefined behavior in FFS_FILE_SIZE

["Michael D Kinney" <michael.d.kinney@intel.com>]

Laszlo,

I have been following this thread.  I think the style
used here to access the 3 array elements to build the
24-bit size value is the best approach.  I prefer this
over adding the union.

I agree there is a read overrun issue when using UINT32 to
read the Size[3] array contents.

I do not think this is a real issue in practice, because the
Size[3] array accessed is part of the larger
EFI_COMMON_SECTION_HEADER structure.  However, we always should
clean up code to not do any read/write overruns without this
type of analysis and the need to keep track of exceptions.

There is a related set of code in the BaseLib for Read/Write
Unaligned24().

UINT32
EFIAPI
ReadUnaligned24 (
  IN CONST UINT32              *Buffer
  );

UINT32
EFIAPI
WriteUnaligned24 (
  OUT UINT32                    *Buffer,
  IN  UINT32                    Value
  );

This API does not get flagged for read overrun issues because
a UINT32 is passed in.  However, for CPU archs that required aligned
access, the 24-bit value must be read in pieces.  This is why there
are 2 different implementations:

IA32/X64
========
UINT32
EFIAPI
ReadUnaligned24 (
  IN CONST UINT32              *Buffer
  )
{
  ASSERT (Buffer != NULL);

  return *Buffer & 0xffffff;
}


ARM/AARCH64
============
UINT32
EFIAPI
ReadUnaligned24 (
  IN CONST UINT32              *Buffer
  )
{
  ASSERT (Buffer != NULL);

  return (UINT32)(
            ReadUnaligned16 ((UINT16*)Buffer) |
            (((UINT8*)Buffer)[2] << 16)
            );
}

The ARM/ARCH64 implementation is clean because it does
not do a read overrun of the 24-bit field.  The IA32/X64
implementation may have an issue because it reads a 32-bit
value and strips the upper 8 bits.

If we apply the same technique to the Size field of
EFI_COMMON_SECTION_HEADER, then the 24-bit value would be
built from reading only the 3 bytes of the array.

Best regards,
 
Mike

> -----Original Message-----
> From: devel@edk2.groups.io [mailto:devel@edk2.groups.io]
> On Behalf Of Laszlo Ersek
> Sent: Friday, April 12, 2019 4:31 PM
> To: edk2-devel-groups-io <devel@edk2.groups.io>
> Cc: Gao, Liming <liming.gao@intel.com>; Kinney, Michael
> D <michael.d.kinney@intel.com>
> Subject: [edk2-devel] [PATCH 04/10]
> MdePkg/PiFirmwareFile: fix undefined behavior in
> FFS_FILE_SIZE
> 
> Accessing "EFI_FFS_FILE_HEADER.Size", which is of type
> UINT8[3], through a
> (UINT32*), is undefined behavior. Fix it by accessing
> the array elements
> individually.
> 
> (We can't use a union here, unfortunately, as easily as
> with
> "EFI_COMMON_SECTION_HEADER", given the fields in
> "EFI_FFS_FILE_HEADER".)
> 
> Cc: Liming Gao <liming.gao@intel.com>
> Cc: Michael D Kinney <michael.d.kinney@intel.com>
> Bugzilla:
> https://bugzilla.tianocore.org/show_bug.cgi?id=1710
> Signed-off-by: Laszlo Ersek <lersek@redhat.com>
> ---
>  MdePkg/Include/Pi/PiFirmwareFile.h | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/MdePkg/Include/Pi/PiFirmwareFile.h
> b/MdePkg/Include/Pi/PiFirmwareFile.h
> index 4fce8298d1c0..0668f3fa9af4 100644
> --- a/MdePkg/Include/Pi/PiFirmwareFile.h
> +++ b/MdePkg/Include/Pi/PiFirmwareFile.h
> @@ -174,18 +174,26 @@ typedef struct {
>    /// If FFS_ATTRIB_LARGE_FILE is not set then
> EFI_FFS_FILE_HEADER is used.
>    ///
>    UINT64                    ExtendedSize;
>  } EFI_FFS_FILE_HEADER2;
> 
>  #define IS_FFS_FILE2(FfsFileHeaderPtr) \
>      (((((EFI_FFS_FILE_HEADER *) (UINTN)
> FfsFileHeaderPtr)->Attributes) & FFS_ATTRIB_LARGE_FILE)
> == FFS_ATTRIB_LARGE_FILE)
> 
> +#define FFS_FILE_SIZE_ARRAY(FfsFileHeaderPtr) \
> +    (((EFI_FFS_FILE_HEADER *) (UINTN)
> (FfsFileHeaderPtr))->Size)
> +
> +#define FFS_FILE_SIZE_ELEMENT(FfsFileHeaderPtr, Index)
> \
> +    ((UINT32) FFS_FILE_SIZE_ARRAY
> (FfsFileHeaderPtr)[(Index)])
> +
>  #define FFS_FILE_SIZE(FfsFileHeaderPtr) \
> -    ((UINT32) (*((UINT32 *) ((EFI_FFS_FILE_HEADER *)
> (UINTN) FfsFileHeaderPtr)->Size) & 0x00ffffff))
> +    ((FFS_FILE_SIZE_ELEMENT ((FfsFileHeaderPtr), 0) <<
> 0) | \
> +     (FFS_FILE_SIZE_ELEMENT ((FfsFileHeaderPtr), 1) <<
> 8) | \
> +     (FFS_FILE_SIZE_ELEMENT ((FfsFileHeaderPtr), 2) <<
> 16))
> 
>  #define FFS_FILE2_SIZE(FfsFileHeaderPtr) \
>      ((UINT32) (((EFI_FFS_FILE_HEADER2 *) (UINTN)
> FfsFileHeaderPtr)->ExtendedSize))
> 
>  typedef UINT8 EFI_SECTION_TYPE;
> 
>  ///
>  /// Pseudo type. It is used as a wild card when
> retrieving sections.
> --
> 2.19.1.3.g30247aa5d201
> 
> 
> 
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