[BRLY-2022-101] 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-2022-101
• Dell PSIRT assigned CVE identifier: CVE-2023-25937
• 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 Latitude 3300's firmware (version: 0.1.17.0, module sha256: 242a5ea5ff855aed77a41207d3f43546b13a32bb5ceab4e333e7b5f0958afb8a) as an example.

The following code in the module actually allows leaking memory:

• a call to a gRT->GetVariable() offset: 0xe7cc
• a call to a gRT->SetVariable() offset: 0xe9be

__int64 __fastcall sub_E790()
{
  __int64 result; // rax
  unsigned int v1; // eax
  unsigned __int64 i; // rbx
  unsigned int v3; // edx
  unsigned int v4; // ecx
  char v5; // dl
  bool v6; // cl
  int v7; // edi
  int v8; // esi
  unsigned __int64 v9; // rcx
  __int64 v10[7]; // [rsp+30h] [rbp-38h] BYREF
  int v11; // [rsp+70h] [rbp+8h] BYREF
  unsigned int v12; // [rsp+78h] [rbp+10h] BYREF
  unsigned int v13; // [rsp+80h] [rbp+18h] BYREF
  __int64 v14; // [rsp+88h] [rbp+20h] BYREF

  v10[0] = 5360i64;
  gRT->GetVariable(                             // <= first call (we can rewrite DataSize here)
    (CHAR16 *)L"CpuSetup",
    &CPU_SETUP_VARIABLE_GUID,
    (UINT32 *)&v14,
    (UINTN *)v10,
    &unk_2E570);
  result = sub_21B94(3i64, -2147483624, (char *)&v13, &v11);
  if ( !v11 )
  {
    v1 = v13;
    for ( i = 0i64; i <= 3; ++i )
    {
      if ( i )
      {
        if ( i == 1 )
        {
          v3 = v1 >> 5;
          v4 = v1 >> 9;
        }
        else if ( i == 2 )
        {
          v3 = v1 >> 15;
          v4 = v1 >> 19;
        }
        else
        {
          v3 = v1 >> 20;
          v4 = HIBYTE(v1);
        }
      }
      else
      {
        LOBYTE(v3) = v1;
        v4 = v1 >> 4;
      }
      v5 = v3 & 0xF;
      v6 = (v4 & 1) == 0;
      if ( *((_BYTE *)&unk_2E570 + i + 1438) == 1 && v6 )
      {
        v7 = v5 & 0xF;
        v8 = v7 << 16;
        sub_21B94(3i64, (v7 << 16) | 0x80000118, (char *)&v12, &v11);
        if ( !v11 )
        {
          v9 = (100000 * ((unsigned __int64)v12 >> 16) + 0x80000) >> 20;
          *((_WORD *)&unk_2E570 + i + 721) = (100000 * (unsigned __int64)(unsigned __int16)v12 + 0x80000) >> 20;
          *((_WORD *)&unk_2E570 + i + 725) = v9;
        }
        sub_21B94(3i64, v8 | 0x80000518, (char *)&v12, &v11);
        if ( !v11 )
          *((_WORD *)&unk_2E570 + i + 753) = v12;
        sub_21B94(3i64, v8 | 0x80000718, (char *)&v12, &v11);
        if ( !v11 )
          *((_WORD *)&unk_2E570 + i + 757) = (1000 * v12 + 4096) >> 13;
        sub_21B94(3i64, (v7 << 8) | 0x80000019, (char *)&v12, &v11);
        if ( !v11 )
          *((_WORD *)&unk_2E570 + i + 761) = v12 & 0x7FFF;
        v1 = v13;
      }
    }
    return gRT->SetVariable(                    // <= second call
             (CHAR16 *)L"CpuSetup",
             &CPU_SETUP_VARIABLE_GUID,
             v14,
             v10[0],
             &unk_2E570);
  }
  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 CpuSetup NVRAM variable is greater than 5360.

Thus, a potential attacker can dump X - 5360 bytes from the stack (or global memory) into CpuSetup NVRAM variable by setting CpuSetup NVRAM variable's size to X > 5360.

To fix this vulnerability the DataSize must be re-initialized with the size of CpuSetup 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