On Apr 17, 2019, at 10:52 AM, Michael D Kinney <michael.d.kinney@intel.com> wrote:

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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