[BRLY-2021-028] 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 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-2021-028
• CERT/CC assigned case number: VU#796611
• Insyde PSIRT assigned CVE identifier: CVE-2021-43615
• 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).

Vulnerability description

The vulnerability exists in the SW SMI handler registered with the GUID 9c28be0c-ee32-43d8-a223-e7c1614ef7ca and located at offset 0x23B0 in the driver:

EFI_STATUS SmiHandler(EFI_HANDLE DispatchHandle, const void *Context, void *CommBuffer, UINTN *CommBufferSize)
{
    if ( CommBuffer && CommBufferSize )
    {
        if ( *(_QWORD *)CommBuffer != 1 )
        {
            if ( *(_QWORD *)CommBuffer == 2 )
            {
                ...
            }
            if ( *(_QWORD *)CommBuffer != 3 )
                goto LABEL_43;
        }
        v5 = gEfiBootServices->LocateProtocol)(&UNKNOWN_PROTOCOL_9C28BE0C_GUID, 0, &v16);
...

As we can see, in case 1 or 3 is passed in the first QWORD inside a Communication Buffer, EFI_BOOT_SERVICES pointer is dereferenced to call LocateProtocol().

Usage of the 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 LocateProtocol() service pointer in it with the shellcode address.
3. Set up the first QWORD inside a Communication Buffer with a value of either 1 or 3.
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 efiXplorer team