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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.
An attacker with high local 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.
Let's take Edge Gateway 5200's firmware (version: 105, module sha256: 224640325a6bd17bfdffec43c6f676fa4ddc6d0bd74147b906e7f98a3a641175) as an example.
The following code in the module actually allows leaking memory:
gRT->GetVariable() offset: 0x2aa4gRT->SetVariable() offset: 0x2ae5__int64 sub_2890()
{
__int64 result; // rax
char v1; // al
__int64 v2; // rax
char v3; // bl
char v4; // di
char v5; // si
signed __int64 v6; // rbx
int v7; // eax
__int64 v8; // rdx
__int16 v9; // [rsp+30h] [rbp-98h] BYREF
int v10; // [rsp+32h] [rbp-96h]
int v11; // [rsp+36h] [rbp-92h]
int v12; // [rsp+3Ah] [rbp-8Eh]
int v13; // [rsp+3Eh] [rbp-8Ah]
int v14; // [rsp+42h] [rbp-86h]
char Destination; // [rsp+48h] [rbp-80h] BYREF
char v16; // [rsp+49h] [rbp-7Fh]
unsigned __int8 v17; // [rsp+4Bh] [rbp-7Dh]
char v18; // [rsp+50h] [rbp-78h]
char v19; // [rsp+51h] [rbp-77h]
char v20; // [rsp+52h] [rbp-76h]
char v21; // [rsp+55h] [rbp-73h]
char v22; // [rsp+56h] [rbp-72h]
char v23; // [rsp+58h] [rbp-70h]
char v24; // [rsp+5Ah] [rbp-6Eh]
void *v25; // [rsp+70h] [rbp-58h] BYREF
__int64 v26; // [rsp+78h] [rbp-50h] BYREF
__int64 (**v27)(void); // [rsp+80h] [rbp-48h] BYREF
void *Interface; // [rsp+88h] [rbp-40h] BYREF
EFI_EVENT Event; // [rsp+90h] [rbp-38h] BYREF
char Data; // [rsp+D0h] [rbp+8h] BYREF
UINT32 Attributes; // [rsp+D8h] [rbp+10h] BYREF
void *v32; // [rsp+E0h] [rbp+18h] BYREF
UINTN DataSize; // [rsp+E8h] [rbp+20h] BYREF
Attributes = 3;
DataSize = 1i64;
v26 = 0i64;
Data = 0;
if ( (gBS->LocateProtocol(&SETUP_GUID, 0i64, &v27) & 0x8000000000000000ui64) == 0i64 )
return (*v27)();
result = gBS->LocateProtocol(&EFI_TPM_MP_DRIVER_PROTOCOL_GUID, 0i64, &Interface);
if ( result >= 0 )
{
result = gBS->LocateProtocol(&EFI_TCG_PROTOCOL_GUID, 0i64, &v25);
if ( result >= 0 )
{
result = gBS->LocateProtocol(&TCG_PLATFORM_SETUP_POLICY_GUID, 0i64, &v32);
if ( result >= 0 )
{
v10 = 369098752;
v9 = -16128;
v12 = 0x4000000;
v11 = 1702887424;
if ( !sub_52F4() )
v11 = 1694498816;
v13 = 0x4000000;
v14 = 134283264;
(*(v25 + 3))(v25, 22i64, &v9, 256i64, &unk_6BD0);
v9 = -16128;
v10 = 369098752;
v1 = sub_52F4();
v12 = 83886080;
v13 = 0x4000000;
v11 = v1 != 0 ? 1702887424 : 1694498816;
v14 = 285278208;
v2 = (*(v25 + 3))(v25, 22i64, &v9, 512i64, &unk_70F0);
v3 = byte_70FE;
if ( v2 < 0 )
v3 = 0;
gBS->CopyMem(&Destination, v32 + 1, 0x1Bui64);
v4 = v23;
v5 = v24;
v20 = v3;
v18 = v23;
v19 = v24;
v21 = 0;
if ( (gRT->GetVariable(L"TpmOldvar", &TC_EFI_GLOBAL_VARIABLE_GUID, &Attributes, &DataSize, &Data) & 0x8000000000000000ui64) == 0i64 )
{
if ( Data == v16 )
{
if ( v16 != (((v4 | v5) & 1) == 0) )
v22 = 1;
}
else
{
Data = v16;
sub_55F0(L"TpmOldvar", &TC_EFI_GLOBAL_VARIABLE_GUID, Attributes, DataSize, &Data);
}
}
else
{
Data = v16;
gRT->SetVariable( // <= second call
L"TpmOldvar",
&TC_EFI_GLOBAL_VARIABLE_GUID,
Attributes,
DataSize,
&Data);
}
if ( v22 )
{
v17 = 0;
v22 = 0;
v16 = ((v4 | v5) & 1) == 0;
Data = v16;
sub_55F0(L"TpmOldvar", &TC_EFI_GLOBAL_VARIABLE_GUID, Attributes, DataSize, &Data);
}
byte_7342 = Destination;
v6 = sub_593C(&v26);
if ( v6 < 0 )
v6 = gBS->CreateEventEx(0x200u, 8ui64, sub_260C, 0i64, &EFI_ACPI_20_TABLE_GUID, &Event);
else
sub_260C(0i64, 0i64);
(*(v32 + 4))(&Destination, 0i64);
if ( v16 == (((v4 | v5) & 1) == 0) )
{
if ( !v17 )
return v6;
v7 = sub_1D24(v17);
v17 = 0;
}
else
{
v7 = sub_1D24(7 - (v16 != 0));
}
if ( v7 )
{
v21 = 1;
(*(v32 + 4))(&Destination, 0i64);
return 0i64;
}
LOBYTE(v8) = 1;
(*(v32 + 4))(&Destination, v8);
sub_E64(0);
return v6;
}
}
}
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 TpmOldvar NVRAM variable is greater than 1.
Thus, a potential attacker can dump X - 1 bytes from the stack (or global memory) into TpmOldvar NVRAM variable by setting TpmOldvar NVRAM variable's size to X > 1.
To fix this vulnerability the DataSize must be re-initialized with the size of TpmOldvar before calling gRT->SetVariable().
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 efiXplorer team
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