SMM callout vulnerability in combined DXE/SMM on Fujitsu device (SMM arbitrary code execution).
BINARLY efiXplorer team identified a SMM callout in a Fujitsu device, which allows an attacker to access the System Management Mode and execute arbitrary code.
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Potential Impact
An attacker can exploit this vulnerability to elevate privileges from ring 0 to ring -2, execute arbitrary code in System Management Mode - an environment more privileged than operating system (OS) and completely isolated from it. Running arbitrary code in SMM additionally bypasses SMM-based SPI flash protections against modifications, which can help an attacker to install a firmware backdoor/implant into the BIOS. Such a malicious firmware code in the BIOS could persist across operating system re-installs. Additionally, this vulnerability could potentially be used by threat actors to bypass security mechanisms provided by UEFI firmware (for example, Secure Boot and some types of memory isolation for hypervisors).
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Vulnerability Information
- BINARLY internal vulnerability identifier: BRLY-2021-018
- CERT/CC assigned case number: VU#796611
- Insyde PSIRT assigned CVE identifier: CVE-2021-41841
- CVSS v3.1: 8.2 High AV:L/AC:L/PR:H/UI:N/S:C/C:H/I:H/A:H
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Affected Fujitsu firmware
| Device name | Driver name | Driver SHA256 | File GUID |
|---|---|---|---|
Fujitsu LIFEBOOK E459/E449 | AhciBusDxe | 4143f3774dd14fcfc1afc3d7ee774f97e4ca556032bbc7c3133f5ba9b0045402 | 3ACC966D-8E33-45C6-B4FE-62724BCD15A9 |
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Vulnerability description
In the function at offset 0x1068 (image sha256: 4143f3774dd14fcfc1afc3d7ee774f97e4ca556032bbc7c3133f5ba9b0045402), the child software System Management Interrupt (SWSMI) handler with the GUID 56947330-585c-4470-a95d-c55c529feb47 is registered:
if ( SmstIsNotNull() )
{
Handle1 = 0;
gBS_E738->InstallProtocolInterface(&Handle1, &ProprietaryProtocol_D8C0, EFI_NATIVE_INTERFACE, 0);
gHandle = Handle;
return gSmst_E718->SmiHandlerRegister(SmiHandler_1C10, &gSmiHandlerGuid, &DispatchHandle);
}
Find below the decompiled SWSMI handler code:
EFI_STATUS __fastcall SmiHandler_1C10(
EFI_HANDLE DispatchHandle,
const void *Context,
void *CommBuffer,
UINTN *CommBufferSize)
{
// [COLLAPSED LOCAL DECLARATIONS. PRESS KEYPAD CTRL-"+" TO EXPAND]
if ( CommBuffer && CommBufferSize )
{
if ( *CommBuffer == 1 )
{
Buffer = 0;
if ( gBS_E738->LocateHandleBuffer(ByProtocol, &EFI_ATA_PASS_THRU_PROTOCOL_GUID_D660, 0, &NoHandles, &Buffer) )
{
Status = EFI_NOT_FOUND;
}
else
{
Buffer = 0;
BufferSize = 0;
v6 = gSmst_E718->SmmLocateHandle(ByProtocol, &EFI_ATA_PASS_THRU_PROTOCOL_GUID_D660, 0, &BufferSize, 0);
Status = EFI_NOT_FOUND;
if ( v6 )
{
if ( v6 == EFI_BUFFER_TOO_SMALL )
{
gSmst_E718->SmmAllocatePool(EfiRuntimeServicesData, BufferSize, &Buffer);
if ( !Buffer )
{
Status = 0x8000000000000009;
goto _WriteStatus;
}
v6 = gSmst_E718->SmmLocateHandle(
ByProtocol,
&EFI_ATA_PASS_THRU_PROTOCOL_GUID_D660,
0,
&BufferSize,
Buffer);
}
else if ( v6 != EFI_NOT_FOUND )
{
goto _WriteStatus;
}
}
if ( v6 == EFI_NOT_FOUND )
v7 = 0;
else
v7 = BufferSize >> 3;
gSmst_E718->SmmFreePool(Buffer);
if ( v7 == NoHandles )
goto _SetEfiSuccessStatus;
if ( !Handle )
{
Status = EFI_NOT_READY;
goto _WriteStatus;
}
Status = sub_1E6C();
if ( !Status )
{
Status = gBS_E738->HandleProtocol(Handle, &EFI_LOADED_IMAGE_PROTOCOL_GUID_D710, &Interface);
if ( !Status )
{
Interface->Unload = sub_2C74;
if ( !gBS_E738->LocateProtocol(&EFI_ACPI_SUPPORT_PROTOCOL_GUID_D6E0, 0, &v15) )
{
v8 = 0;
do
{
v9 = (v15->GetAcpiTable)(v15, v8, &v16, &v18, v17);
if ( !v9 )
{
if ( *v16 == 'PCAF' )
{
v9 = EFI_ABORTED;
dword_E6E0 = *(v16 + 19);
}
gBS_E738->FreePool(v16);
}
++v8;
}
while ( !v9 );
}
}
goto _WriteStatus;
}
if ( Status == EFI_NOT_STARTED )
_SetEfiSuccessStatus:
Status = EFI_SUCCESS;
}
}
else
{
Status = EFI_UNSUPPORTED;
}
_WriteStatus:
*(CommBuffer + 1) = Status;
}
return 0;
}
If CommBuffer and CommBufferSize pointers are not-NULL and first QWORD from CommBuffer is equal to 0x01, the following code will be executed:
Buffer = 0;
if ( gBS_E738->LocateHandleBuffer(ByProtocol, &EFI_ATA_PASS_THRU_PROTOCOL_GUID_D660, 0, &NoHandles, &Buffer) )
{
Status = EFI_NOT_FOUND;
}
Using services from EFI_BOOT_SERVICES (in this case LocateHandleBuffer()) inside a SW SMI handler is unsafe and may lead to arbitrary code execution in SMM.
In addition to this code, this SMI handler uses many other services from the EFI_BOOT_SERVICES and EFI_RUNTIME_SERVICES tables.
An example of using the LocateProtocol() (from EFI_BOOT_SERVICES) and GetVariable() (from EFI_RUNTIME_SERVICES) services is shown below (SmiHandler_1C10() -> sub_1E6C()):
EFI_STATUS sub_1E6C()
{
// [COLLAPSED LOCAL DECLARATIONS. PRESS KEYPAD CTRL-"+" TO EXPAND]
Interface = 0;
v0 = 0;
if ( !gBS_E738->LocateProtocol(&EFI_SMM_RUNTIME_PROTOCOL_GUID_D630, 0, &Interface) )
(*Interface)(&EFI_SMM_RUNTIME_PROTOCOL_GUID_D630, 0, &Interface);
v1 = ::Buffer;
v2 = 0;
if ( ::Buffer )
{
DataSize = 0;
Status = gRT_E740->GetVariable(VariableName, &EFI_GENERIC_VARIABLE_GUID_D540, 0, &DataSize, 0);
if ( Status == EFI_BUFFER_TOO_SMALL )
{
Data = SmmAllocatePool(DataSize);
v2 = Data;
if ( Data )
Status = (gRT_E740->GetVariable)(VariableName, &EFI_GENERIC_VARIABLE_GUID_D540, 0, &DataSize, Data);
}
...
}
...
return result;
}
Each of these services can be targeted by a potential attacker.
To exploit this vulnerability is enough to:
- overwrite the service address in the
EFI_BOOT_SERVICES(orEFI_RUNTIME_SERVICES) table with the shellcode address - trigger the SWSMI handler (SwSmi number is specified in UEFI ACPI table)
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Disclosure timeline
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.
| Disclosure Activity | Date |
|---|---|
Fujitsu PSIRT is notified | 2021-09-10 |
Fujitsu PSIRT is confirmed issue | 2021-09-14 |
CERT/CC created a case | 2021-09-27 |
Insyde PSIRT assigned CVE number | 2021-11-01 |
Insyde PSIRT provide patch release | 2021-11-09 |
BINARLY public disclosure date | 2022-02-01 |
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Acknowledgements
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