.webp)
An attacker can exploit this vulnerability to elevate privileges from ring 0 to ring -2, execute 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, which can help an attacker to install a firmware backdoor/implant into BIOS. Such a malicious firmware code in BIOS could persist across operating system re-installs. Additionally, this vulnerability potentially could be used by malicious actors to bypass security mechanisms provided by UEFI firmware (for example, Secure Boot and some types of memory isolation for hypervisors).
Binarly REsearch Team has discovered SMM memory corruption vulnerability on Intel platforms allowing a possible attacker to write fixed or predictable data to SMRAM. Exploiting this issue could lead to escalating privileges to SMM.
An attacker can exploit this vulnerability to elevate privileges from ring 0 to ring -2, 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 BIOS. Such a malicious firmware code in BIOS could persist across operating system re-installs. Additionally, this vulnerability potentially could be used by malicious actors to bypass security mechanisms provided by UEFI firmware (for example, Secure Boot and some types of memory isolation for hypervisors).
The vulnerability exists in SW SMI handler located at offset 0x1480 in the binary.The pseudocode for this handler is shown below:
__int64 __fastcall SwSmiHandlerB2(
EFI_HANDLE DispatchHandle,
const void *Context,
void *CommBuffer,
UINTN *CommBufferSize)
{
// Local variables initialization
Res = 0;
v43 = 0;
v44 = 0;
v45 = 0;
DataSize = 0x75;
v26 = 0;
v30 = 0;
Status = gEfiSmmVariableProtocol->SmmGetVariable(L"OcSetup", &gVariableGuid, &Attributes, &DataSize, Data);
if ( Status < 0 )
return Status;
if ( !Data[39] )
return Status;
CpuIndex = 0xFFFFFFFFFFFFFFFF;
if ( CommBuffer && CommBufferSize )
CpuIndex = *CommBuffer;
if ( CpuIndex == -1 )
return EFI_UNSUPPORTED;
Status = (SmmCpuProtocol->ReadSaveState)(SmmCpuProtocol, 4, EFI_SMM_SAVE_STATE_REGISTER_RBX, CpuIndex, &RbxValue);
Status = (SmmCpuProtocol->ReadSaveState)(SmmCpuProtocol, 4, EFI_SMM_SAVE_STATE_REGISTER_RCX, CpuIndex, &RcxValue);
Ptr = RbxValue;
Ptr16 = (RbxValue + 16);
v27 = sub_3544();
v29 = sub_352C();
v28 = sub_3514();
if ( RcxValue >= 3 )
{
Res = 0x8004;
result = (SmmCpuProtocol->WriteSaveState)(SmmCpuProtocol, 4, EFI_SMM_SAVE_STATE_REGISTER_RBX, CpuIndex, &Res);
Status = result;
return result;
}
if ( !RcxValue )
{
v46 = 12;
v26 = 456;
v30 = 55;
if ( v27 )
{
v26 += 8;
++v30;
if ( v29 )
{
v26 += 32;
v30 += 4;
}
if ( v28 )
{
v26 += 8 * word_40E8;
v30 += word_40E8;
}
}
if ( *Ptr != '2DB$' )
{
if ( *Ptr == '$DB$' )
{
*Ptr = '2DB$';
*(Ptr + 4) = v26;
*(Ptr + 8) = 2;
*(Ptr + 10) = 0;
Res = 1;
}
else
{
Res = 0x8001;
}
result = (SmmCpuProtocol->WriteSaveState)(SmmCpuProtocol, 4, 39, CpuIndex, &Res);
Status = result;
return result;
}
if ( *(Ptr + 4) <= v26 )
{
if ( *(Ptr + 4) < v46 )
{
Res = 0x8003;
result = (SmmCpuProtocol->WriteSaveState)(SmmCpuProtocol, 4, 39, CpuIndex, &Res);
Status = result;
return result;
}
if ( *(Ptr + 4) < v26 )
{
*(Ptr + 4) = v26;
*(Ptr + 8) = 2;
*(Ptr + 10) = 0;
Res = 0x8002;
result = (SmmCpuProtocol->WriteSaveState)(SmmCpuProtocol, 4, 39, CpuIndex, &Res);
Status = result;
return result;
}
Res = 0;
Status = (SmmCpuProtocol->WriteSaveState)(SmmCpuProtocol, 4, 39, CpuIndex, &Res);
*(Ptr + 8) = 2;
*(Ptr + 10) = 0;
}
else
{
*(Ptr + 4) = v26;
*(Ptr + 8) = 2;
*(Ptr + 10) = 0;
Res = 2;
Status = (SmmCpuProtocol->WriteSaveState)(SmmCpuProtocol, 4, 39, CpuIndex, &Res);
}
*(Ptr + 12) = v30;
*Ptr16 = 0x29;
Ptr16[1] = Data[40];
Ptr16[2] = 0;
Ptr16[3] = Data[41];
Ptr16[4] = 7;
Ptr16[5] = Data[1];
Ptr16[6] = 8;
Ptr16[7] = Data[2];
Ptr16[8] = 10;
Ptr16[9] = *&Data[3];
Ptr16[10] = 11;
if ( Data[5] )
Ptr16[11] = Data[5];
else
Ptr16[11] = -2;
Ptr16[12] = 21;
Ptr16[13] = *&Data[6];
Ptr16[14] = 22;
Ptr16[15] = Data[8];
Ptr16[16] = 23;
Ptr16[17] = Data[9];
Ptr16[18] = 25;
Ptr16[19] = Data[10];
Ptr16[20] = 40;
Ptr16[21] = *&Data[11];
Ptr16[22] = 24;
if ( Data[14] )
Ptr16[23] = Data[14];
else
Ptr16[23] = -2;
Ptr16[24] = 19;
if ( Data[16] == Data[15] )
Ptr16[25] = -2;
else
Ptr16[25] = Data[16];
Ptr16[26] = 5;
Ptr16[27] = *&Data[18];
Ptr16[28] = 1;
Ptr16[29] = *&Data[21];
Ptr16[30] = 59;
Ptr16[31] = Data[25];
Ptr16[32] = 101;
Ptr16[33] = Data[99];
Ptr16[34] = 88;
Ptr16[35] = Data[44];
Ptr16[36] = 284;
Ptr16[37] = Data[108];
Ptr16[38] = 102;
Ptr16[39] = *&Data[95];
Ptr16[40] = 114;
Ptr16[41] = Data[97];
Ptr16[42] = 115;
Ptr16[43] = Data[98];
v5 = 22;
if ( Data[44] )
{
if ( Data[44] == 1 )
{
Ptr16[44] = 2;
Ptr16[45] = *&Data[45];
v5 = 23;
}
}
else
{
Ptr16[44] = 2;
Ptr16[45] = *&Data[47];
v5 = 23;
}
if ( Data[51] == 1 )
v33 = 1000 - *&Data[49];
else
v33 = *&Data[49] + 1000;
Ptr16[2 * v5] = 34;
Ptr16[2 * v5 + 1] = v33;
v6 = v5 + 1;
Ptr16[2 * v6] = 66;
Ptr16[2 * v6++ + 1] = Data[61];
Ptr16[2 * v6] = 64;
Ptr16[2 * v6++ + 1] = Data[26];
Ptr16[2 * v6] = 73;
Ptr16[2 * v6 + 1] = Data[27];
v7 = v6 + 1;
if ( v27 )
{
Ptr16[2 * v7] = 26;
Ptr16[2 * v7++ + 1] = Data[62];
if ( v28 )
{
Ptr16[2 * v7] = 29;
Ptr16[2 * v7 + 1] = Data[63];
v8 = v7 + 1;
Ptr16[2 * v8] = 30;
Ptr16[2 * v8 + 1] = Data[64];
v7 = v8 + 1;
if ( word_40E8 > 2u )
{
Ptr16[2 * v7] = 31;
Ptr16[2 * v7 + 1] = Data[65];
v9 = v7 + 1;
Ptr16[2 * v9] = 32;
Ptr16[2 * v9 + 1] = Data[66];
v7 = v9 + 1;
if ( word_40E8 > 4u )
{
Ptr16[2 * v7] = 42;
Ptr16[2 * v7 + 1] = Data[67];
v10 = v7 + 1;
Ptr16[2 * v10] = 43;
Ptr16[2 * v10 + 1] = Data[68];
v7 = v10 + 1;
if ( word_40E8 > 6u )
{
Ptr16[2 * v7] = 96;
Ptr16[2 * v7 + 1] = Data[69];
v11 = v7 + 1;
Ptr16[2 * v11] = 97;
Ptr16[2 * v11 + 1] = Data[70];
v7 = v11 + 1;
}
}
}
}
if ( v29 )
{
Ptr16[2 * v7] = 50;
Ptr16[2 * v7 + 1] = Data[71];
v12 = v7 + 1;
Ptr16[2 * v12] = 49;
Ptr16[2 * v12 + 1] = Data[80];
v13 = v12 + 1;
Ptr16[2 * v13] = 47;
if ( *&Data[81] == *&Data[85] )
Ptr16[2 * v13 + 1] = -2;
else
Ptr16[2 * v13 + 1] = *&Data[81];
v14 = v13 + 1;
Ptr16[2 * v14] = 48;
if ( *&Data[72] == *&Data[76] )
Ptr16[2 * v14 + 1] = -2;
else
Ptr16[2 * v14 + 1] = *&Data[72];
v7 = v14 + 1;
}
}
Ptr16[2 * v7] = 76;
Ptr16[2 * v7 + 1] = Data[52];
v15 = v7 + 1;
v32 = (Data[89] & 1) == 0;
Ptr16[2 * v15] = 80;
Ptr16[2 * v15++ + 1] = v32;
Ptr16[2 * v15] = 82;
Ptr16[2 * v15 + 1] = Data[28];
v16 = v15 + 1;
if ( Data[28] )
{
if ( Data[28] == 1 )
{
Ptr16[2 * v16] = 81;
Ptr16[2 * v16++ + 1] = *&Data[32];
}
}
else
{
Ptr16[2 * v16] = 81;
Ptr16[2 * v16++ + 1] = *&Data[34];
}
if ( Data[31] == 1 )
v24 = 1000 - *&Data[29];
else
v24 = *&Data[29] + 1000;
Ptr16[2 * v16] = 83;
Ptr16[2 * v16 + 1] = v24;
v17 = v16 + 1;
Ptr16[2 * v17] = 98;
Ptr16[2 * v17 + 1] = Data[100];
v18 = v17 + 1;
if ( Data[100] )
{
if ( Data[100] == 1 )
{
Ptr16[2 * v18] = 99;
Ptr16[2 * v18++ + 1] = *&Data[104];
}
}
else
{
Ptr16[2 * v18] = 99;
Ptr16[2 * v18++ + 1] = *&Data[106];
}
if ( Data[103] == 1 )
v24 = 1000 - *&Data[101];
else
v24 = *&Data[101] + 1000;
Ptr16[2 * v18] = 100;
Ptr16[2 * v18 + 1] = v24;
v19 = v18 + 1;
if ( Data[38] == 1 )
v34 = 1000 - *&Data[36];
else
v34 = *&Data[36] + 1000;
Ptr16[2 * v19] = 85;
Ptr16[2 * v19 + 1] = v34;
}
if ( RcxValue == 1 )
{
...
}
return Status;
}As we can see, pointer extracted from RBX value with SmmCpuProtocol->ReadSaveState is not validated.It meen that potential attacker can point RBX value to SMRAM content and rewrite some data inside SMRAM.
But the limitation is that RBX must point to content that starts with 2DB$ or $DB$. However, we can still point to this function code or to the memory right before SMRAM.
To protect against exploitation, you can check that the buffer pointed to by RBX does not overlap with SMRAM.
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.
Binarly REsearch Team
.svg)
