[BRLY-2021-023] SMM callout vulnerability in combined DXE/SMM driver on BullSequana Edge server

Summary

Binarly REsearch Team has discovered a SMM callout vulnerability on a BullSequana Edge server allowing a possible attacker to hijack the execution flow of a code running in System Management Mode. Exploiting this issue could lead to escalating privileges to SMM.

Vulnerability Information

• BINARLY internal vulnerability identifier: BRLY-2021-023
• CERT/CC assigned case number: VU#796611
• Insyde PSIRT assigned CVE identifier: CVE-2021-42113
• CVSS v3.1: 8.2 High AV:L/AC:L/PR:H/UI:N/S:C/C:H/I:H/A:H

Affected BullSequana Edge servers firmwares with confirmed impact by Binarly REsearch Team

Potential impact

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 the BIOS. Such a malicious firmware code in the BIOS could persist across operating system re-installs. Additionally, this vulnerability could potentially be used by threat actors to bypass security mechanisms provided by the UEFI firmware (for example, Secure Boot and some types of memory isolation for hypervisors).

Vulnerabilities description

The vulnerability exists in the child SW SMI handler registered with the GUID 1d3de7f0-0807-424f-aa69-11a54e19a46f and located at offset 0x1E0C in the driver:

EFI_STATUS SmiHandler(EFI_HANDLE DispatchHandle, const void *Context, void *CommBuffer, UINTN *CommBufferSize)
{
    if ( CommBuffer && CommBufferSize )
    {
        if ( *(_QWORD *)CommBuffer == 1 )
        {
            LOBYTE(Context) = 1;
            Status = sub_80001508(*((_QWORD *)CommBuffer + 2), (char)Context);
            ...
        }
...

As we can see in case 1 is passed as the first QWORD inside a Communication Buffer the sub_80001508() function is called. The second argument of this routine indicates whether the function is executed in SMM (from SMRAM) or not, thus the routine contains the following constructions:

EFI_STATUS sub_80001508(__int64 Pointer, char InSmm)
{
    if ( InSmm )
    {
        // usage of gSmst
        ...
    }
    else
    {
        // usage of gBS
        ...
    }
}

Still, at least one EFI_BOOT_SERVICES dereferencing operation is left without any dependency to InSmm flag:

...

Status = InSmm
    ?
    gSmst->SmmHandleProtocol(v15, &EFI_ATA_PASS_THRU_PROTOCOL_GUID, (void **)&EfiAtaPassThruProtocol)
    :
    gEfiBootServices->HandleProtocol(v15, &EFI_ATA_PASS_THRU_PROTOCOL_GUID, (void **)&EfiAtaPassThruProtocol);
				
if ( !Status )
{
    if ( EfiAtaPassThruProtocol )
    {
        Status = gEfiBootServices->LocateHandleBuffer(ByProtocol, &EFI_DISK_INFO_PROTOCOL_GUID, 0, &v46, &v38);
        
...

This vulnerable operation is possible in SMM in case gSmst->SmmHandleProtocol() returns EFI_SUCCESS and EfiAtaPassThruProtocol will be initialized by this call.

Let's go back to the beginning of the SMI handler. In case 2 is passed as the first QWORD inside a Communication Buffer the following code is executed in a loop:

EFI_STATUS SmiHandler(EFI_HANDLE DispatchHandle, const void *Context, void *CommBuffer, UINTN *CommBufferSize)
{
    if ( CommBuffer && CommBufferSize )
    {
        if ( *(_QWORD *)CommBuffer == 2 )
        {
            ...
        }
        if ( *(_QWORD *)CommBuffer == 2 )
        {
            ...
		        
            while ( 1 )
            {
                Status = gSmst->SmmHandleProtocol)(*(_QWORD *)(v8 + 8 * v6), &EFI_STORAGE_SECURITY_COMMAND_PROTOCOL_GUID, &EfiStorageSecurityCommandProtocol);
                if ( !Status )
                {
                    if ( *(_QWORD *)(EfiStorageSecurityCommandProtocol - 0x20) == 'GOTS' && !*(_QWORD *)(EfiStorageSecurityCommandProtocol - 8) )
                    {
                        for ( i = 0; i < v17; v10 = i )
                        {
                            if ( !gEfiBootServices->HandleProtocol)(*(_QWORD *)(v16 + 8 * v10), &EFI_STORAGE_SECURITY_COMMAND_PROTOCOL_GUID, &v19)

...

Here is a similar situation: in case gSmst->SmmHandleProtocol() returns EFI_SUCCESS and valid pointer EfiStorageSecurityCommandProtocol, the EFI_BOOT_SERVICES dereferencing operation has occurred.

Usage of EFI_BOOT_SERVICES and EFI_RUNTIME_SERVICES is unsafe inside a code intended to run in SMM (from SMRAM), especially in SMI handlers, because a possible attacker with a R/W access to system memory could hook pointers in these tables to escalate privileges to SMM (ring -2).

To exploit this vulnerability it is enough to:

1. Find EFI_BOOT_SERVICES table in system memory.
2. Overwrite LocateHandleBuffer() or HandleProtocol() service pointer in it with the shellcode address.
3. Set up the first QWORD of a Communication Buffer with a value of either 1 or 2.
4. Trigger the SW SMI Handler (SW SMI number and pointer to Communication Buffer are specified in UEFI ACPI table) via 0xB2 IO port.

To fix this vulnerability, it is essential that the usage of EFI_BOOT_SERVICES is minimised only to SMM driver's early initialization routine.

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 REsearch Team