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The BINARLY efiXplorer team has discovered an SMM callout vulnerability on a Gigabyte device allowing a potential attacker to hijack execution flow of code running in the System Management Mode. Exploitation of this issue could lead to escalation of privileges to SMM.
According to AMD, this vulnerability is a rediscovery of CVE-2021-26317 reported by Jiawei Yin.
An attacker can exploit this vulnerability to elevate privileges from ring 0 to ring -2, resulting in the execution of 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. This can help an attacker to install a firmware backdoor/implant into the BIOS. Such 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 the UEFI firmware (for example, Secure Boot and some types of memory isolation for hypervisors).
The vulnerability exists in the SW SMI handlers registered with SwSmiInputValue 0xBC and 0xBA (and located at offsets 0x319C and 0x32B0 correspondingly).
The handlers is registered as follows:
...
gSmst->SmmLocateProtocol(&EFI_SMM_SW_DISPATCH2_PROTOCOL_GUID, 0, &EfiSmmSwDispatch2Protocol);
RegisterContext.SwSmiInputValue = 0xBC;
result = EfiSmmSwDispatch2Protocol->Register(
EfiSmmSwDispatch2Protocol,
SwSmiHandler, // vulnerable handler, offset: 0x319C
&RegisterContext,
&DispatchHandle);
if ( result >= 0 )
{
RegisterContext.SwSmiInputValue = 0xBA;
result = EfiSmmSwDispatch2Protocol->Register(
EfiSmmSwDispatch2Protocol,
SwSmiHandler_0, // vulnerable handler, offset: 0x32B0
&RegisterContext,
&DispatchHandle);
if ( result >= 0 )
{
if ( value > 0xC )
return 0;
check_val = 0x1B20;
if ( !_bittest(&check_val, value) )
return 0;
RegisterContext.SwSmiInputValue = 0xB9;
result = EfiSmmSwDispatch2Protocol->Register(
EfiSmmSwDispatch2Protocol,
SwSmiHandler_1,
&RegisterContext,
&DispatchHandle);
if ( result >= 0 )
return 0;
}
}
...Each of the handlers mentioned contains many function calls from the interfaces, which has been located as follows:
EfiSmmBase2Protocol->InSmm(EfiSmmBase2Protocol, &InSmram);
if ( InSmram )
{
// AMD_CPM_TABLE_SMM_PROTOCOL_GUID = af6efacf-7a13-45a3-b1a5-aafc061c4b79
result = gBS->LocateProtocol(&AMD_CPM_TABLE_SMM_PROTOCOL_GUID, 0, &gAmdCpmTableSmmProtocol);
if ( result < 0 )
return result;
// AMD_CPM_DISPLAY_FEATURE_PROTOCOL_GUID = db4a79ac-5bbb-4625-6a9e-febf9d6d95eb
if ( gBS->LocateProtocol(&AMD_CPM_DISPLAY_FEATURE_PROTOCOL_GUID, 0, &gAmdCpmDisplayFeatureProtocol) != EFI_SUCCESS )
return EFI_ABORTED;
...Below are some examples of functions calls from the mentioned protocols interfaces that are used within SMI handlers:
// from SwSmiHandler located at offset 0x319C
v6 = (*(gAmdCpmTableSmmProtocol + 113))(gAmdCpmTableSmmProtocol, '31A$');
// from sub_1818
...
(*(gAmdCpmTableSmmProtocol + 0x24))(gAmdCpmTableSmmProtocol, a2, a3, a4);
LOBYTE(v7) = *(a1 + 2 * v4 + 37);
LOBYTE(v8) = *(a1 + 2 * v4 + 36) & 7;
LOBYTE(v9) = *(a1 + 2 * v4 + 36) >> 3;
(*(gAmdCpmTableSmmProtocol + 0x24))(gAmdCpmTableSmmProtocol, v7, v9, v8);
LOBYTE(v10) = *(a1 + 2 * v4 + 37);
LOBYTE(v11) = *(a1 + 2 * v4 + 36) & 7;
LOBYTE(v12) = *(a1 + 2 * v4 + 36) >> 3;
(*(gAmdCpmTableSmmProtocol + 0x24))(gAmdCpmTableSmmProtocol, v10, v12, v11);
LOBYTE(v13) = *(a1 + 2 * v4 + 37);
LOBYTE(v14) = *(a1 + 2 * v4 + 36) & 7;
LOBYTE(v15) = *(a1 + 2 * v4 + 36) >> 3;
(*(gAmdCpmTableSmmProtocol + 0x24))(gAmdCpmTableSmmProtocol, v13, v15, v14);
LOBYTE(v16) = *(a1 + 2 * v4 + 37);
LOBYTE(v17) = *(a1 + 2 * v4 + 36) & 7;
LOBYTE(v18) = *(a1 + 2 * v4 + 36) >> 3;
(*(gAmdCpmTableSmmProtocol + 0x24))(gAmdCpmTableSmmProtocol, v16, v18, v17);
...The AMD_CPM_TABLE_SMM_PROTOCOL_GUID (af6efacf-7a13-45a3-b1a5-aafc061c4b79) protocol installation routine shown below (code from AmdCpmInitSmm module):
EfiSmmBase2Protocol->InSmm(EfiSmmBase2Protocol, &InSmram);
if ( !InSmram )
return res;
Status = gSmst->SmmAllocatePool(EfiRuntimeServicesData, 0x410, &Buffer);
if ( Status >= 0 )
{
Status = gBS->LocateProtocol(&AMD_CPM_TABLE_PROTOCOL_GUID, 0, &Interface);
if ( Status >= 0 )
{
*Buffer = *Interface;
*(Buffer + 1) = *(Interface + 1);
InterfaceBuffer = Buffer;
*(Buffer + 0x71) = sub_13A0;
InterfaceBuffer[0x72] = &loc_13A8;
InterfaceBuffer[0x78] = sub_14E4;
InterfaceBuffer[0x79] = sub_14B4;
sub_2DE4(Buffer);
sub_3A00(Buffer);
sub_5FC0(Buffer);
Handle[0] = 0;
return gBS->InstallProtocolInterface(Handle, &AMD_CPM_TABLE_SMM_PROTOCOL_GUID, EFI_NATIVE_INTERFACE, Buffer);
...
As we can see, the interface address (located with gBS->LocateProtocol()) is located inside SMRAM (since Buffer allocated with gSmst->SmmAllocatePool() function), but installed with gBS->InstallProtocolInterface() service.
The vulnerability cannot be exploited from the operating system. But an attacker capable of executing code in DXE could control af6efacf-7a13-45a3-b1a5-aafc061c4b79 protocol interface address.
In order to safely use the functions from this protocol interface within the SMI handler, this protocol must be installed with EFI_SMM_SYSTEM_TABLE2::SmmInstallProtocolInterface() and located with EFI_SMM_SYSTEM_TABLE2::SmmLocateProtocol().
This vulnerability is subject to a 90 day disclosure deadline. After 90 days elapsed or a patch has been made broadly available (whichever is earlier), the vulnerability report will be made publicly available.
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