An attacker with local privileged access can exploit this vulnerability to read the contents of the physical memory and use this information to exploit other vulnerabilities in DXE. A malicious code installed as a result of the vulnerability exploitation in a DXE driver could survive across an operating system (OS) boot process and runtime or modify NVRAM area on SPI flash storage (to gain persistence on target platform). Additionally, this vulnerability potentially could be used by threat actors to bypass OS security mechanisms (modify privileged memory or runtime variables), influence on the OS boot process, and in some cases would allow an attacker to hook or modify EFI Runtime services.
Binarly REsearch Team has discovered a memory contents leak / information disclosure vulnerability. Lack of synchronization between PaletteSize calculated from BitsPerPixel in BMP Image Header and PaletteIndex calculated from data in BMP Image allows OOB Read.
An attacker with local privileged access can exploit this vulnerability to read the contents of the physical memory and use this information to exploit other vulnerabilities in DXE. A malicious code installed as a result of the vulnerability exploitation in a DXE driver could survive across an operating system (OS) boot process and runtime or modify NVRAM area on SPI flash storage (to gain persistence on target platform). Additionally, this vulnerability potentially could be used by threat actors to bypass OS security mechanisms (modify privileged memory or runtime variables), influence on the OS boot process, and in some cases would allow an attacker to hook or modify EFI Runtime services.
The pseudocode of the vulnerable function is shown below:
// local variable allocation has failed, the output may be wrong!
EFI_STATUS __fastcall DbDecodeImage(BUFFER_INDICATOR *Bi, EFI_IMAGE_INPUT *ImageInput)
{
UINTN BufferSize; // rcx
BMP_COLOR_PALETTE *Palette; // r15
EFI_STATUS Status; // rsi
void *Start; // rax
void *Current; // rcx
char *Start_plus_BMP_FILE_HEADER; // rax
char *Start_plus_BufferSize; // rcx
size_t ImageSize; // rdi
__int64 BmpHeaderSize; // r12
__int64 ColorAmount; // r14
EFI_GRAPHICS_OUTPUT_BLT_PIXEL *mem; // rax
UINT16 BitsPerPixel; // cx
size_t PaletteSize; // r14 MAPDST
void *Start_plus_BmpHeaderSize; // rax
void *Start_plus_BufferSize_1; // rcx
void *ImageBuffer; // rax MAPDST
void *Start_plus_ImageOffset; // rcx
UINT32 Height; // er8
int i; // ebx
EFI_GRAPHICS_OUTPUT_BLT_PIXEL *PixelPtr; // rdi
__int64 ImageIndex; // r9
UINT32 Width; // er11
UINT32 j; // er10
unsigned __int64 offset; // rcx
EFI_GRAPHICS_OUTPUT_BLT_PIXEL v29; // eax
UINT8 v30; // al
__int64 index; // rcx
__int64 v32; // rcx
unsigned __int64 v33; // rcx
__int64 v34; // rax
unsigned __int64 v35; // rcx
__int64 BitIndex; // r12
unsigned __int64 PaletteIndex; // rdx
__int64 v38; // rax
int v39; // eax
BMP_FILE_HEADER Bfh; // [rsp+20h] [rbp-40h]
BMP_INFO_HEADER Bih; // [rsp+30h] [rbp-30h]
UINT64 ScratchedBufferSize; // [rsp+A0h] [rbp+40h] OVERLAPPED
EFI_GRAPHICS_OUTPUT_BLT_PIXEL Color; // [rsp+A0h] [rbp+40h] FORCED
BufferSize = Bi->BufferSize;
Bfh.ImageOffset = 0;
Palette = 0i64;
Bih.BitsPerPixel = 0;
Status = 0i64;
Bih.ImageSize = 0;
*(_QWORD *)&Bih.HeaderSize = 0i64;
Bih.Height = 0;
if ( BufferSize <= 0x36 )
return Status;
Start = Bi->Start;
Bi->Current = Bi->Start;
ScratchedBufferSize = sizeof(BMP_FILE_HEADER);
if ( (char *)Start + sizeof(BMP_FILE_HEADER) > (char *)Start + BufferSize )
return Status;
if ( BiReadBuffer(Bi, &Bfh, &ScratchedBufferSize, (char *)Start + sizeof(BMP_FILE_HEADER)) )
return Status;
Current = Bi->Start;
ScratchedBufferSize = sizeof(BMP_INFO_HEADER);
Start_plus_BMP_FILE_HEADER = (char *)Current + sizeof(BMP_FILE_HEADER);
Start_plus_BufferSize = (char *)Current + Bi->BufferSize;
Bi->Current = Start_plus_BMP_FILE_HEADER;
if ( Start_plus_BMP_FILE_HEADER + sizeof(BMP_INFO_HEADER) > Start_plus_BufferSize
|| BiReadBuffer(Bi, &Bih, &ScratchedBufferSize, Start_plus_BMP_FILE_HEADER + sizeof(BMP_INFO_HEADER))
|| Bfh.BmpId[0] != 'B'
|| Bfh.BmpId[1] != 'M' )
{
return Status;
}
ImageSize = Bih.ImageSize;
BmpHeaderSize = Bih.HeaderSize + sizeof(BMP_FILE_HEADER);
ColorAmount = 1 << SLOBYTE(Bih.BitsPerPixel);
ScratchedBufferSize = Bih.ImageSize;
mem = (EFI_GRAPHICS_OUTPUT_BLT_PIXEL *)malloc(4i64 * Bih.Height * Bih.Width);
ImageInput->Bitmap = mem;
if ( !mem )
return EFI_OUT_OF_RESOURCES;
BitsPerPixel = Bih.BitsPerPixel;
ImageInput->Height = Bih.Height;
ImageInput->Width = Bih.Width;
if ( !ImageSize )
{
ImageSize = Bih.Height * ((((BitsPerPixel * (unsigned __int64)Bih.Width) >> 3) + 3) & 0xFFFFFFFC);
ScratchedBufferSize = Bih.Height * ((((BitsPerPixel * (unsigned __int64)Bih.Width) >> 3) + 3) & 0xFFFFFFFC);
}
if ( (unsigned __int16)(BitsPerPixel - 1) <= 7u )
{
PaletteSize = 4 * ColorAmount;
Palette = (BMP_COLOR_PALETTE *)malloc(PaletteSize);// Root cause #2: OOB index. Lack of synchronization between PaletteSize and PaletteIndex.
if ( !Palette )
return EFI_OUT_OF_RESOURCES;
Start_plus_BmpHeaderSize = (char *)Bi->Start + BmpHeaderSize;
Start_plus_BufferSize_1 = (char *)Bi->Start + Bi->BufferSize;
Bi->Current = Start_plus_BmpHeaderSize;
if ( (char *)Start_plus_BmpHeaderSize + PaletteSize <= Start_plus_BufferSize_1 )
BiReadBuffer(Bi, Palette, &PaletteSize, (char *)Start_plus_BmpHeaderSize + PaletteSize);
}
ImageBuffer = malloc(ImageSize);
if ( ImageBuffer )
{
Start_plus_ImageOffset = (char *)Bi->Start + Bfh.ImageOffset;
Bi->Current = Start_plus_ImageOffset;
if ( (void *)((unsigned __int64)Start_plus_ImageOffset + ImageSize) <= (char *)Bi->Start + Bi->BufferSize )
BiReadBuffer(Bi, ImageBuffer, &ScratchedBufferSize, (char *)Start_plus_ImageOffset + ImageSize);
Height = Bih.Height;
i = 0;
PixelPtr = ImageInput->Bitmap;
ImageIndex = 0i64;
Color.Reserved = 0;
if ( !Bih.Height )
goto exit;
Width = Bih.Width;
while ( 1 )
{
j = 0;
if ( Width )
break;
LABEL_40:
if ( ImageIndex & 3 )
v39 = 4 - (ImageIndex & 3);
else
v39 = 0;
ImageIndex = (unsigned int)(v39 + ImageIndex);
if ( ++i >= Height )
goto exit;
}
while ( 1 )
{
if ( Bih.BitsPerPixel == 1 ) // 2 Color Mode
// CRASH area
{
BitIndex = 7i64;
while ( j < Width )
{
PaletteIndex = ((unsigned __int64)*((unsigned __int8 *)ImageBuffer + ImageIndex) >> BitIndex) & 1;// OOB read Root Cause #3
*(_WORD *)&Color.Green = *(_WORD *)&Palette[PaletteIndex].Green;// OOB read Root Cause #2
// ^ Both Red + Green in Color and Palette objects
Color.Blue = Palette[PaletteIndex].Blue;// OOB read Root Cause #2
v38 = j++;
--BitIndex;
PixelPtr[v38 + Width * (Height - i - 1)] = Color;
Height = Bih.Height;
Width = Bih.Width;
}
--j;
ImageIndex = (unsigned int)(ImageIndex + 1);
goto LABEL_39;
}
if ( Bih.BitsPerPixel == 4 ) // 16 Color Mode
break;
switch ( Bih.BitsPerPixel )
{
case 8u: // 256 Color Mode
// CRASH case x2
v32 = *((unsigned __int8 *)ImageBuffer + ImageIndex);
ImageIndex = (unsigned int)(ImageIndex + 1);
*(_WORD *)&Color.Green = *(_WORD *)&Palette[v32].Green;// OOB read Root Cause #2
// ^ Both Red + Green in Color and Palette objects
v30 = Palette[v32].Blue; // OOB read Root Cause #2
LABEL_30:
index = Width * (Height - i - 1);
LABEL_33:
Color.Blue = v30;
offset = j + index;
v29 = Color;
goto LABEL_34;
case 0x18u: // 16M Color Mode
*(_WORD *)&Color.Green = *(_WORD *)((char *)ImageBuffer + ImageIndex + 1);
v30 = *((_BYTE *)ImageBuffer + ImageIndex);
ImageIndex = (unsigned int)(ImageIndex + 3);
goto LABEL_30;
case 0x20u:
offset = j + (unsigned __int64)(Width * (Height - i - 1));
v29 = *(EFI_GRAPHICS_OUTPUT_BLT_PIXEL *)((char *)ImageBuffer + ImageIndex);
ImageIndex = (unsigned int)(ImageIndex + 4);
LABEL_34:
PixelPtr[offset] = v29;
Height = Bih.Height;
Width = Bih.Width;
break;
}
LABEL_39:
if ( ++j >= Width )
goto LABEL_40;
}
v33 = (unsigned __int64)*((unsigned __int8 *)ImageBuffer + ImageIndex) >> 4;
*(_WORD *)&Color.Green = *(_WORD *)&Palette[v33].Green;// OOB read Root Cause #2
// ^ Both Red + Green in Color and Palette objects
Color.Blue = Palette[v33].Blue; // OOB read Root Cause #2
v34 = j++;
PixelPtr[v34 + Width * (Height - i - 1)] = Color;
LOBYTE(v33) = *((_BYTE *)ImageBuffer + ImageIndex);
ImageIndex = (unsigned int)(ImageIndex + 1);
v35 = v33 & 0xF;
*(_WORD *)&Color.Green = *(_WORD *)&Palette[v35].Green;
v30 = Palette[v35].Blue;
index = Bih.Width * (Bih.Height - i - 1);
goto LABEL_33;
}
Status = EFI_OUT_OF_RESOURCES;
exit:
if ( Palette )
gEfiBootServices->FreePool(Palette);
if ( ImageBuffer )
gEfiBootServices->FreePool(ImageBuffer);
return Status;
}
As we can see from the pseudocode, PaletteSize and PaletteIndex are calculated is independently from each other, which leads to OOB read cases.
This bug is subject to a 90 day disclosure deadline. After 90 days elapsed or a patch has been made broadly available (whichever is earlier), the bug report will become visible to the public.
Binarly REsearch Team
