[BRLY-2022-119] Memory contents leak / information disclosure vulnerability in DXE driver on Dell platform.

Summary

BINARLY efiXplorer team has discovered a memory contents leak / information disclosure vulnerability that allows a potential attacker to dump stack memory or global memory into an NVRAM variable. This in turn could help building a successful attack vector based on exploiting a memory corruption vulnerability.

Vulnerability Information

• BINARLY internal vulnerability identifier: BRLY-2022-119
• Dell PSIRT assigned CVE identifier: CVE-2023-25938
• DSA identifier: DSA-2023-099/DSA-2023-204
• CVSS v3.1: 4.9 Medium AV:P/AC:L/PR:H/UI:N/S:C/C:H/I:N/A:N

Affected Dell firmware with confirmed impact by Binarly team

Potential impact

An attacker with high physical access can exploit this vulnerability to read the contents of stack memory or global memory. This information could help with explotation of other vulnerabilities in DXE to elevate privileges from ring 3 or ring 0 (depends on the operating system) to a DXE driver and execute arbitrary code. Malicious code installed as a result of this exploitation could survive operating system (OS) boot process and runtime, or modify NVRAM area on the SPI flash storage (to gain persistence). Additionally, threat actors could use this vulnerability to bypass OS security mechanisms (modify privileged memory or runtime variables), influence OS boot process, and in some cases allow an attacker to hook or modify EFI Runtime services.

Vulnerability description

Let's take XPS 17 9710, Precision 5760's firmware (version: 0.1.14.0, module sha256: 4b32444ad888c2597de681707449065e75e4b620aa50c2f2e875eca97b875d73) as an example.

The following code in the module actually allows leaking memory:

• a call to a gRT->GetVariable() offset: 0x103d
• a call to a gRT->SetVariable() offset: 0x1074

__int64 __fastcall sub_E74()
{
  char v0; // bl
  __int64 result; // rax
  unsigned __int8 v2; // al
  __int64 (__fastcall **v3)(__int64); // rax
  bool v4; // zf
  int v5; // eax
  UINTN v6; // [rsp+30h] [rbp-D0h] BYREF
  UINTN DataSize; // [rsp+38h] [rbp-C8h] BYREF
  char Data[91]; // [rsp+40h] [rbp-C0h] BYREF
  int v9; // [rsp+9Bh] [rbp-65h]
  int v10; // [rsp+9Fh] [rbp-61h]
  int v11; // [rsp+A9h] [rbp-57h]
  char v12[2048]; // [rsp+2D0h] [rbp+1D0h] BYREF
  UINT32 Attributes; // [rsp+AE0h] [rbp+9E0h] BYREF
  UINT32 v14; // [rsp+AE8h] [rbp+9E8h] BYREF
  void *Interface; // [rsp+AF0h] [rbp+9F0h] BYREF
  void *v16; // [rsp+AF8h] [rbp+9F8h] BYREF

  Interface = 0i64;
  v16 = 0i64;
  v0 = 0;
  result = gBS->LocateProtocol(&PLATFORM_NVS_AREA_PROTOCOL_GUID, 0i64, &Interface);
  if ( result >= 0 )
  {
    if ( (gBS->LocateProtocol(&UNKNOWN_PROTOCOL_GUID_0, 0i64, &v16) & 0x8000000000000000ui64) == 0i64 )
    {
      while ( 1 )
      {
        (*((void (__fastcall **)(void *, __int64, __int64))v16 + 3))(v16, 166i64, 1i64);
        __outbyte(0x910u, 0x11u);
        v2 = __inbyte(0x911u);
        if ( ((v2 + 1) & 0xFE) != 0 )
          break;
        if ( (unsigned __int8)++v0 >= 3u )
          goto LABEL_7;
      }
      switch ( v2 )
      {
        case 1u:
LABEL_7:
          *(_BYTE *)(*(_QWORD *)Interface + 2079i64) = 0;
          break;
        case 2u:
          *(_BYTE *)(*(_QWORD *)Interface + 2079i64) = 1;
          break;
        case 4u:
          *(_BYTE *)(*(_QWORD *)Interface + 2079i64) = 2;
          break;
        case 8u:
          *(_BYTE *)(*(_QWORD *)Interface + 2079i64) = 3;
          break;
      }
    }
    DataSize = 654i64;
    result = gRT->GetVariable((CHAR16 *)L"CpuSetup", &CPU_SETUP_VARIABLE_GUID, &Attributes, &DataSize, Data);
    if ( result >= 0 )
    {
      v3 = (__int64 (__fastcall **)(__int64))sub_1824();
      v4 = (v3[1](306i64) & 1) == 0;
      v5 = 100000;
      if ( v4 )
        v5 = 109000;
      v11 = v5;
      v10 = v5;
      v9 = v5;
      gRT->SetVariable((CHAR16 *)L"CpuSetup", &CPU_SETUP_VARIABLE_GUID, Attributes, 0x28Eui64, Data);
      v6 = 2028i64;
      gRT->GetVariable(                         // <= first call (we can rewrite DataSize here)
        (CHAR16 *)L"PchSetup",
        &PCH_SETUP_VARIABLE_GUID,
        &v14,
        &v6,
        v12);
      v12[1712] = 0;
      return gRT->SetVariable(                  // <= second call
               (CHAR16 *)L"PchSetup",
               &PCH_SETUP_VARIABLE_GUID,
               v14,
               v6,
               v12);
    }
  }
  return result;
}

The gRT->SetVariable() service is called with the DataSize as an argument, which will be overwritten inside the gRT->GetVariable() service if the length of PchSetup NVRAM variable is greater than 2028.

Thus, a potential attacker can dump X - 2028 bytes from the stack (or global memory) into PchSetup NVRAM variable by setting PchSetup NVRAM variable's size to X > 2028.

To fix this vulnerability the DataSize must be re-initialized with the size of PchSetup before calling gRT->SetVariable().

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.

Acknowledgements

BINARLY efiXplorer team