Multiple SMM memory corruption vulnerabilities in the SMM module on AMI-based devices (SMRAM write)

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

Vulnerability Information

• BINARLY internal vulnerability identifier: BRLY-DVA-2025-005
• AMI PSIRT assigned CVE identifier: CVE-2025-33043
• CVSS v3.1: 8.2 High AV:L/AC:L/PR:H/UI:N/S:C/C:H/I:H/A:H

Affected firmware with confirmed impact by BINARLY team

Vulnerability description

Let's consider the module 2b228b2a38932e2250f24ae04e626709026d12f0a52f5a5b86089dabc6221109.

The pseudocode of the vulnerable function is shown below:

EFI_STATUS SwSmiHandler(
        EFI_HANDLE DispatchHandle,
        const void *Context,
        EFI_SMM_SW_CONTEXT *CommBuffer,
        UINTN *CommBufferSize)
{
  UINT8 FuncIndex;
  SMI_UC_DWORD_REGS *Buffer;

  Buffer = 0;
  if ( !gInit )
  {
    gInit = 1;
    gInitError = InitMicrocodeVariables() == 0;
  }
  gEfiSmmCpuProtocol->ReadSaveState(
    gEfiSmmCpuProtocol,
    4,
    EFI_SMM_SAVE_STATE_REGISTER_RSI,
    CommBuffer->SwSmiCpuIndex,
    &Buffer);
  if ( LOWORD(Buffer->EAX) == 0xD042 )
  {
    FuncIndex = Buffer->EBX;
    Buffer->EFLAGS &= ~1; // SMRAM write
    LOWORD(Buffer->EAX) = 0; // SMRAM write
    if ( gInitError || FuncIndex >= 4 )
    {
      Buffer->EFLAGS |= 1; // SMRAM write
      LOWORD(Buffer->EAX) = 0x8600; // SMRAM write
    }
    else
    {
      (gMicrocodeUpdateAPI[FuncIndex])(Buffer);
    }
  }
  return EFI_SUCCESS;
}

As we can see from the pseudocode, Buffer is an attacker-controlled pointer (obtained via EFI_SMM_SAVE_STATE_REGISTER_RSI). Buffer is not validated against overlapping with SMRAM.

Buffer has the following structure:

00000000 struct SMI_UC_DWORD_REGS
00000000 {
00000000     UINT32 EAX;
00000004     UINT32 EBX;
00000008     UINT32 ECX;
0000000C     UINT32 EDX;
00000010     UINT32 ESI;
00000014     UINT32 EDI;
00000018     UINT32 EFLAGS;
0000001C     UINT16 ES;
0000001E     UINT16 CS;
00000020     UINT16 SS;
00000022     UINT16 DS;
00000024     UINT16 FS;
00000026     UINT16 GS;
00000028     UINT32 EBP;
0000002C };

If LOWORD(Buffer->EAX) is equal to 0xD042, unvalidated writes will occur in SwSmiHandler function and in functions from gMicrocodeUpdateAPI table:

.data:0000000000006518 gMicrocodeUpdateAPI dq offset PresenceTest
.data:0000000000006518                                         ; DATA XREF: SwSmiHandler+8B↑o
.data:0000000000006518                                         ; SwSmiHandler+92↑r
.data:0000000000006520                 dq offset WriteUpdateData
.data:0000000000006528                 dq offset UpdateControl
.data:0000000000006530                 dq offset ReadUpdateData

UINT64 PresenceTest(SMI_UC_DWORD_REGS *Regs)
{
  UINT64 NumMcodeBlks;

  // Regs is attacker-controlled

  NumMcodeBlks = gNumMcodeBlks;
  qmemcpy(&Regs->EBX, "ETNIPEPL", 8); // SMRAM write
  LOWORD(Regs->ESI) = NumMcodeBlks; // SMRAM write
  Regs->EDX = 1; // SMRAM write
  return NumMcodeBlks;
}

void UpdateControl(SMI_UC_DWORD_REGS *Regs)
{
  // Regs is attacker-controlled

  LOBYTE(Regs->EBX) = 1; // SMRAM write
}

The check if ( LOWORD(Buffer->EAX) == 0xD042 ) makes it harder to exploit, but does not mitigate the vulnerability, since an attacker can at least write code at the location of the check itself:

.text:000000000000208E 41 BB 42 D0 00                 mov     r11d, 0D042h
.text:000000000000208E 00
.text:0000000000002094 66 44 39 18                    cmp     [rax], r11w

Disclosure timeline

This vulnerability is subject to a 90 day disclosure period. After 90 days or when a patch has been made generally available (whichever comes first) the advisory will be publicly disclosed.

Acknowledgements

BINARLY REsearch team