Re: [edk2-devel] [PATCH v1 2/6] ShellPkg: acpiview: GTDT: Prevent buffer overruns

["Sami Mujawar" <sami.mujawar@arm.com>]

Hi Zhichao,

Please see my response inline.

Regards,

Sami Mujawar

-----Original Message-----
From: devel@edk2.groups.io <devel@edk2.groups.io> On Behalf Of Gao, Zhichao via Groups.Io
Sent: 05 August 2019 08:23 AM
To: devel@edk2.groups.io; Krzysztof Koch <Krzysztof.Koch@arm.com>
Cc: Carsey, Jaben <jaben.carsey@intel.com>; Ni, Ray <ray.ni@intel.com>; Sami Mujawar <Sami.Mujawar@arm.com>; Matteo Carlini <Matteo.Carlini@arm.com>; nd <nd@arm.com>
Subject: Re: [edk2-devel] [PATCH v1 2/6] ShellPkg: acpiview: GTDT: Prevent buffer overruns

One confusion below:

> -----Original Message-----
> From: devel@edk2.groups.io [mailto:devel@edk2.groups.io] On Behalf Of 
> Krzysztof Koch
> Sent: Thursday, August 1, 2019 4:44 PM
> To: devel@edk2.groups.io
> Cc: Carsey, Jaben <jaben.carsey@intel.com>; Ni, Ray 
> <ray.ni@intel.com>; Gao, Zhichao <zhichao.gao@intel.com>; 
> Sami.Mujawar@arm.com; Matteo.Carlini@arm.com; nd@arm.com
> Subject: [edk2-devel] [PATCH v1 2/6] ShellPkg: acpiview: GTDT: Prevent 
> buffer overruns
> 
> Modify the GTDT table parsing logic to prevent reading past the ACPI 
> buffer lengths provided and to make it consistent with other table 
> parsers. This includes converting the do-while loop in ParseAcpiGtdt() into a while loop.
> 
> Remove a check which ensures that the entire Platform GT Block 
> Structure buffer has been parsed. The ACPI specification does not ban 
> from defining buffers which are larger than the size indicated by the 
> count and sizes of substructures which constitute it.
> 
> Change the data type of the Length parameter to the DumpGTBlock() 
> function to reflect the width of the respective ACPI structure's field.
> 
> References:
> - ACPI 6.3, January 2019, Table 5-124
> 
> Signed-off-by: Krzysztof Koch <krzysztof.koch@arm.com>
> ---
> 
> Notes:
>     v1:
>     - Prevent buffer overruns in GTDT acpiview parser [Krzysztof]
> 
>
> ShellPkg/Library/UefiShellAcpiViewCommandLib/Parsers/Gtdt/GtdtParser.c
> | 147 ++++++++++----------
>  1 file changed, 76 insertions(+), 71 deletions(-)
> 
> diff --git
> a/ShellPkg/Library/UefiShellAcpiViewCommandLib/Parsers/Gtdt/GtdtParser
> .c
> b/ShellPkg/Library/UefiShellAcpiViewCommandLib/Parsers/Gtdt/GtdtParser
> .c
> index
> 1e5b5764f50a2d29aa904c889bc89af5bdc3af5c..57174e14c80072f12b90e1996e
> be8f0002d0c404 100644
> ---
> a/ShellPkg/Library/UefiShellAcpiViewCommandLib/Parsers/Gtdt/GtdtParser
> .c
> +++
> b/ShellPkg/Library/UefiShellAcpiViewCommandLib/Parsers/Gtdt/GtdtPars
> +++ er.c
> @@ -23,7 +23,6 @@ STATIC CONST UINT8*  PlatformTimerType;  STATIC 
> CONST UINT16* PlatformTimerLength;  STATIC CONST UINT32* 
> GtBlockTimerCount;  STATIC CONST UINT32* GtBlockTimerOffset; -STATIC 
> CONST UINT16* GtBlockLength;  STATIC ACPI_DESCRIPTION_HEADER_INFO 
> AcpiHdrInfo;
> 
>  /**
> @@ -127,7 +126,7 @@ STATIC CONST ACPI_PARSER 
> GtPlatformTimerHeaderParser[] = {  **/  STATIC CONST ACPI_PARSER 
> GtBlockParser[] = {
>    {L"Type", 1, 0, L"%d", NULL, NULL, NULL, NULL},
> -  {L"Length", 2, 1, L"%d", NULL, (VOID**)&GtBlockLength, NULL, NULL},
> +  {L"Length", 2, 1, L"%d", NULL, NULL, NULL, NULL},
>    {L"Reserved", 1, 3, L"%x", NULL, NULL, NULL, NULL},
>    {L"Physical address (CntCtlBase)", 8, 4, L"0x%lx", NULL, NULL, NULL, NULL},
>    {L"Timer Count", 4, 12, L"%d", NULL, (VOID**)&GtBlockTimerCount, @@ 
> -
> 168,56 +167,43 @@ STATIC CONST ACPI_PARSER SBSAGenericWatchdogParser[] 
> = {
>  /**
>    This function parses the Platform GT Block.
> 
> -  @param [in] Ptr     Pointer to the start of the GT Block data.
> -  @param [in] Length  Length of the GT Block structure.
> +  @param [in] Ptr       Pointer to the start of the GT Block data.
> +  @param [in] Length    Length of the GT Block structure.
>  **/
>  STATIC
>  VOID
>  DumpGTBlock (
>    IN UINT8* Ptr,
> -  IN UINT32 Length
> +  IN UINT16 Length
>    )
>  {
>    UINT32 Index;
>    UINT32 Offset;
> -  UINT32 GTBlockTimerLength;
> 
> -  Offset = ParseAcpi (
> -             TRUE,
> -             2,
> -             "GT Block",
> -             Ptr,
> -             Length,
> -             PARSER_PARAMS (GtBlockParser)
> -             );
> -  GTBlockTimerLength = (*GtBlockLength - Offset) / 
> (*GtBlockTimerCount);
> -  Length -= Offset;
> +  ParseAcpi (
> +    TRUE,
> +    2,
> +    "GT Block",
> +    Ptr,
> +    Length,
> +    PARSER_PARAMS (GtBlockParser)
> +    );
> 
> -  if (*GtBlockTimerCount != 0) {
> -    Ptr += (*GtBlockTimerOffset);
> -    Index = 0;
> -    while ((Index < (*GtBlockTimerCount)) && (Length >=
> GTBlockTimerLength)) {
> -      Offset = ParseAcpi (
> -                 TRUE,
> -                 2,
> -                 "GT Block Timer",
> -                 Ptr,
> -                 GTBlockTimerLength,
> -                 PARSER_PARAMS (GtBlockTimerParser)
> -                 );
> -      // Increment by GT Block Timer structure size
> -      Ptr += Offset;
> -      Length -= Offset;
> -      Index++;
> -    }
> +  Offset = *GtBlockTimerOffset;
> +  Index = 0;
> 
> -    if (Length != 0) {
> -      IncrementErrorCount ();
> -      Print (
> -        L"ERROR:GT Block Timer length mismatch. Unparsed %d bytes.\n",
> -        Length
> -        );
> -    }
> +  // Parse the specified number of GT Block Timer Structures or the 
> + GT Block  // Structure buffer length. Whichever is minimum.
> +  while ((Index++ < *GtBlockTimerCount) &&
> +         (Offset < Length)) {
> +    Offset += ParseAcpi (
> +                TRUE,
> +                2,
> +                "GT Block Timer",
> +                Ptr + Offset,
> +                Length - Offset,
> +                PARSER_PARAMS (GtBlockTimerParser)
> +                );

The above confuse me a lot. ACPI Spec 6.3 Table 5-124:
Length - "20+n*40, where n is the number of timers implemented in the GT Block"
GT Block Timer Structure[] - Byte Offset: GT Block Timer Offset - "Array of GT Block Timer Structures. See Table 5-125"

Why the 'Byte Offset' is not 20?
'Length - Offset' would be 'Length - 20' == 'n * 40'. If the 'GT Block Timer Offset' is not 20, its value should be lager 20. Then 'Length - Offset' would always less than 'n * 40'. It may miss  some info of the GtBlockTimer.
Maybe all the platforms' GT block table's 'GT Block Timer Offset' is always 20. If so, then there is no problem with this above section.

Did I misunderstand something?
[SAMI] The '20+n*40' in the 'Description' column in the ACPI Spec 6.3 Table 5-124 is an example of how the length field may be computed. 
The 'GT Block Timer Offset' field could technically allow unused bytes between the 'GT Block Timer Offset' field and the start of the first 'GT Block Timer Structure'. An OS is expected to read the 'GT Block Timer Offset' field to know where the 'GT Block Timer Structure' data starts. Conversely the firmware must encode the length field appropriately. In this case the length field would be '20+UnusedByteCount+(n*40)'.

The 'Length - Offset' is used to ensure we don't run past the buffer. This also reinforces the fact that ACPIview would be providing a view of the tables as an OS might see.

I will check if this can be made clearer in the specification.
[/SAMI]

Thanks,
Zhichao

>    }
>  }
> 
> @@ -270,6 +256,7 @@ ParseAcpiGtdt (
>    )
>  {
>    UINT32 Index;
> +  UINT32 Offset;
>    UINT8* TimerPtr;
> 
>    if (!Trace) {
> @@ -285,36 +272,54 @@ ParseAcpiGtdt (
>      PARSER_PARAMS (GtdtParser)
>      );
> 
> -  if (*GtdtPlatformTimerCount != 0) {
> -    TimerPtr = Ptr + (*GtdtPlatformTimerOffset);
> -    Index = 0;
> -    do {
> -      // Parse the Platform Timer Header
> -      ParseAcpi (
> -        FALSE,
> -        0,
> -        NULL,
> -        TimerPtr,
> -        4,  // GT Platform Timer structure header length.
> -        PARSER_PARAMS (GtPlatformTimerHeaderParser)
> +  TimerPtr = Ptr + *GtdtPlatformTimerOffset;  Offset = 
> + *GtdtPlatformTimerOffset;  Index = 0;
> +
> +  // Parse the specified number of Platform Timer Structures or the 
> + GTDT  // buffer length. Whichever is minimum.
> +  while ((Index++ < *GtdtPlatformTimerCount) &&
> +         (Offset < AcpiTableLength)) {
> +    // Parse the Platform Timer Header to obtain Length and Type
> +    ParseAcpi (
> +      FALSE,
> +      0,
> +      NULL,
> +      TimerPtr,
> +      AcpiTableLength - Offset,
> +      PARSER_PARAMS (GtPlatformTimerHeaderParser)
> +      );
> +
> +    // Make sure the Platform Timer is inside the table.
> +    if ((Offset + *PlatformTimerLength) > AcpiTableLength) {
> +      IncrementErrorCount ();
> +      Print (
> +        L"ERROR: Invalid Platform Timer Structure length. " \
> +          L"PlatformTimerLength = %d. RemainingTableBufferLength = %d. " \
> +          L"GTDT parsing aborted.\n",
> +        *PlatformTimerLength,
> +        AcpiTableLength - Offset
>          );
> -      switch (*PlatformTimerType) {
> -        case EFI_ACPI_6_2_GTDT_GT_BLOCK:
> -          DumpGTBlock (TimerPtr, *PlatformTimerLength);
> -          break;
> -        case EFI_ACPI_6_2_GTDT_SBSA_GENERIC_WATCHDOG:
> -          DumpWatchdogTimer (TimerPtr, *PlatformTimerLength);
> -          break;
> -        default:
> -          IncrementErrorCount ();
> -          Print (
> -            L"ERROR: INVALID Platform Timer Type = %d\n",
> -            *PlatformTimerType
> -            );
> -          break;
> -      } // switch
> -      TimerPtr += (*PlatformTimerLength);
> -      Index++;
> -    } while (Index < *GtdtPlatformTimerCount);
> -  }
> +      return;
> +    }
> +
> +    switch (*PlatformTimerType) {
> +      case EFI_ACPI_6_3_GTDT_GT_BLOCK:
> +        DumpGTBlock (TimerPtr, *PlatformTimerLength);
> +        break;
> +      case EFI_ACPI_6_3_GTDT_SBSA_GENERIC_WATCHDOG:
> +        DumpWatchdogTimer (TimerPtr, *PlatformTimerLength);
> +        break;
> +      default:
> +        IncrementErrorCount ();
> +        Print (
> +          L"ERROR: Invalid Platform Timer Type = %d\n",
> +          *PlatformTimerType
> +          );
> +        break;
> +    } // switch
> +
> +    TimerPtr += *PlatformTimerLength;
> +    Offset += *PlatformTimerLength;
> +  } // while
>  }
> --
> 'Guid(CE165669-3EF3-493F-B85D-6190EE5B9759)'
> 
> 
> 
>