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

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

Laszlo,

We should also be able to do a consistent fix without
adding any new unions or macros:

#define FFS_FILE_SIZE(FfsFileHeaderPtr) ((UINT32)( \
  (((EFI_FFS_FILE_HEADER *) (UINTN) FfsFileHeaderPtr)->Size[0]       ) | \
  (((EFI_FFS_FILE_HEADER *) (UINTN) FfsFileHeaderPtr)->Size[1] << 8  ) | \
  (((EFI_FFS_FILE_HEADER *) (UINTN) FfsFileHeaderPtr)->Size[2] << 16 ) ))

#define SECTION_SIZE(SectionHeaderPtr) ((UINT32)( \
  (((EFI_COMMON_SECTION_HEADER *) (UINTN) SectionHeaderPtr)->Size[0]       ) | \
  (((EFI_COMMON_SECTION_HEADER *) (UINTN) SectionHeaderPtr)->Size[1] << 8  ) | \
  (((EFI_COMMON_SECTION_HEADER *) (UINTN) SectionHeaderPtr)->Size[2] << 16 ) ))

Best regards,

Mike

> -----Original Message-----
> From: Kinney, Michael D
> Sent: Wednesday, April 17, 2019 10:52 AM
> To: devel@edk2.groups.io; lersek@redhat.com; Kinney,
> Michael D <michael.d.kinney@intel.com>
> Cc: Gao, Liming <liming.gao@intel.com>
> Subject: RE: [edk2-devel] [PATCH 04/10]
> MdePkg/PiFirmwareFile: fix undefined behavior in
> FFS_FILE_SIZE
> 
> 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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