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

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

Hi Laszlo,

On 4/16/19 12:59 PM, Laszlo Ersek wrote:
> On 04/16/19 11:04, Jordan Justen wrote:
>> On 2019-04-15 09:15:31, Laszlo Ersek wrote:
>>> 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.
>>
>> I think you are still accessing it through a UINT32*, since you are
>> using a pointer to a union, and an field of type UINT32 within the
>> union.
> 
> Using a union makes the behavior well-defined.
> 
>> 6.2.7 Compatible type and composite type
>>
>> 1 Two types have compatible type if their types are the same.
>>   Additional rules for determining whether two types are compatible
>>   are described in [...]
> 
>> 6.5 Expressions
>>
>> 6 The /effective type/ of an object for an access to its stored value
>>   is the declared type of the object, if any. [...]
>>
>> 7 An object shall have its stored value accessed only by an lvalue
>>   expression that has one of the following types:
>>
>>   — a type compatible with the effective type of the object,
>>   — a qualified version of a type compatible with the effective type
>>     of the object,
>>   — a type that is the signed or unsigned type corresponding to the
>>     effective type of the object,
>>   — a type that is the signed or unsigned type corresponding to a
>>     qualified version of the effective type of the object,
>>   — an aggregate or union type that includes one of the aforementioned
>>     types among its members (including, recursively, a member of a
>>     subaggregate or contained union), or
>>   — a character type.
> 
> - Regarding 6.5p6, the original object we intend to access has
> (declared) type EFI_COMMON_SECTION_HEADER. Therefore the effective type
> is EFI_COMMON_SECTION_HEADER.
> 
> - Based on 6.2.7p1, EFI_COMMON_SECTION_HEADER is compatible with
> EFI_COMMON_SECTION_HEADER. (Because they are the same.)
> 
> - Based on 6.5p7 item #5, EFI_COMMON_SECTION_HEADER can be accessed as
> EFI_COMMON_SECTION_HEADER_UNION, because EFI_COMMON_SECTION_HEADER_UNION
> includes "a type compatible with the effective type of the object" (#1)
> among its members -- namely an EFI_COMMON_SECTION_HEADER, which is
> compatible with EFI_COMMON_SECTION_HEADER, because they are the same.
> 
>> I guess it might more well defined to shift the bytes, like is
>> sometimes done with the FFS file sizes.
> 
> I did that (i.e. byte-shifting) in the other patch:
> 
>   [edk2-devel] [PATCH 04/10]
>   MdePkg/PiFirmwareFile: fix undefined behavior in FFS_FILE_SIZE
> 
> but for SECTION_SIZE, the union is well-defined too.

Why not use a 8-bit access macro similar to FFS_FILE_SIZE(), the one you
added in patch 4/10 of this series?

> 
> Thanks,
> Laszlo
> 
>>
>> -Jordan
>>
>>> 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
>>>>
>>>