Re: [edk2-devel] [PATCH 02/10] MdePkg/PiFirmwareFile: fix undefined behavior in SECTION_SIZE

["Laszlo Ersek" <lersek@redhat.com>]

On 04/14/19 09:19, Jordan Justen wrote:
> On 2019-04-12 16:31:20, Laszlo Ersek wrote:
>> RH covscan justifiedly reports that accessing
>> "EFI_COMMON_SECTION_HEADER.Size", which is of type UINT8[3], through a
>> (UINT32*), is undefined behavior:
>>
>>> Error: OVERRUN (CWE-119):
>>> edk2-89910a39dcfd/OvmfPkg/Sec/SecMain.c:178: overrun-local: Overrunning
>>> array of 3 bytes at byte offset 3 by dereferencing pointer
>>> "(UINT32 *)((EFI_COMMON_SECTION_HEADER *)(UINTN)Section)->Size".
>>> #  176|       Section = (EFI_COMMON_SECTION_HEADER*)(UINTN) CurrentAddress;
>>> #  177|
>>> #  178|->     Size = SECTION_SIZE (Section);
>>> #  179|       if (Size < sizeof (*Section)) {
>>> #  180|         return EFI_VOLUME_CORRUPTED;
>>
>> Fix this by introducing EFI_COMMON_SECTION_HEADER_UNION, and expressing
>> SECTION_SIZE() in terms of "EFI_COMMON_SECTION_HEADER_UNION.Uint32".
>>
>> 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
>> Issue: scan-1007.txt
>> 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 a9f3bcc4eb8e..4fce8298d1c0 100644
>> --- a/MdePkg/Include/Pi/PiFirmwareFile.h
>> +++ b/MdePkg/Include/Pi/PiFirmwareFile.h
>> @@ -229,16 +229,24 @@ typedef struct {
>>    ///
>>    UINT8             Size[3];
>>    EFI_SECTION_TYPE  Type;
>>    ///
>>    /// Declares the section type.
>>    ///
>>  } EFI_COMMON_SECTION_HEADER;
>>  
>> +///
>> +/// Union that permits accessing EFI_COMMON_SECTION_HEADER as a UINT32 object.
>> +///
>> +typedef union {
>> +  EFI_COMMON_SECTION_HEADER Hdr;
>> +  UINT32                    Uint32;
>> +} EFI_COMMON_SECTION_HEADER_UNION;
>> +
>>  typedef struct {
>>    ///
>>    /// A 24-bit unsigned integer that contains the total size of the section in bytes,
>>    /// including the EFI_COMMON_SECTION_HEADER.
>>    ///
>>    UINT8             Size[3];
>>  
>>    EFI_SECTION_TYPE  Type;
>> @@ -476,17 +484,17 @@ typedef struct {
>>    /// A UINT16 that represents a particular build. Subsequent builds have monotonically
>>    /// increasing build numbers relative to earlier builds.
>>    ///
>>    UINT16                        BuildNumber;
>>    CHAR16                        VersionString[1];
>>  } EFI_VERSION_SECTION2;
>>  
>>  #define SECTION_SIZE(SectionHeaderPtr) \
>> -    ((UINT32) (*((UINT32 *) ((EFI_COMMON_SECTION_HEADER *) (UINTN) SectionHeaderPtr)->Size) & 0x00ffffff))
>> +    (((EFI_COMMON_SECTION_HEADER_UNION *) (UINTN) (SectionHeaderPtr))->Uint32 & 0x00ffffff)
> 
> Mike, all,
> 
> Can we add a typedef for EFI_COMMON_SECTION_HEADER_UNION if it's not
> in the PI spec?
> 
> If it's not allowed, I think something like this might work too:
> 
> #define SECTION_SIZE(SectionHeaderPtr) \
>     (*((UINT32*)(UINTN)(SectionHeaderPtr)) & 0x00ffffff)

(Less importantly:)

It might shut up the static analyzer, but regarding the C standard, it's
equally undefined behavior.

Anyway I don't feel too strongly about this, given that we disable the
strict aliasing / effective type rules in "tools_def.template"
("-fno-strict-aliasing").

> Then again, I see SECTION_SIZE is not in the spec, so maybe it's ok to
> add the typedef.

(More importantly:)

Indeed the doubt you voice about ..._UNION crossed my mind, but then I
too searched the PI spec for SECTION_SIZE, with no hits.

Beyond that, I searched both the PI and UEFI specs, for "_UNION" --
again no hits, despite our definitions of:

- EFI_IMAGE_OPTIONAL_HEADER_UNION
- EFI_GRAPHICS_OUTPUT_BLT_PIXEL_UNION

in

- "MdePkg/Include/IndustryStandard/PeImage.h"
- "MdePkg/Include/Protocol/GraphicsOutput.h"

respectively.

Thanks,
Laszlo

> 
> -Jordan
>