SMM memory corruption vulnerability in SMM driver on Intel platforms.
BINARLY efiXplorer 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.
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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 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).
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Vulnerability Information
- BINARLY internal vulnerability identifier: BRLY-2021-051
- Intel PSIRT assigned CVE identifier: CVE-2022-23955
- CVSS v3.1: 7.5 High AV:L/AC:H/PR:H/UI:N/S:C/C:H/I:H/A:H
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Affected Intel firmware
| Device/Firmware | File Name | SHA256 (File PE32 section) | File GUID |
|---|---|---|---|
HP EliteBook x360 1040 G8 | 0169 | 4140f347c52cc1a288e6b517a18b6b6434d6085211977dc76a02fc1322db9425 | D80ECBF5-49B2-45AF-80B7-7D3C226E6741 |
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Vulnerability description
The vulnerability exists in child SW SMI handler registered with GUID a7149597-ca0a-4ff5-a12e-5e5deb1051c8 and located at offset 0x1A24 in the binary.
The pseudocode for this handler is shown below:
EFI_STATUS __fastcall SmiHandler(
EFI_HANDLE DispatchHandle,
const void *Context,
void *CommBuffer,
UINTN *CommBufferSize)
{
// [COLLAPSED LOCAL DECLARATIONS. PRESS KEYPAD CTRL-"+" TO EXPAND]
Interface = 0;
if ( CommBuffer && CommBufferSize )
{
Res = gSmst->SmmLocateProtocol(&ProprietaryProtocol_8, 0, &Interface);
if ( !Res )
Res = (*Interface)();
*CommBuffer = Res;
}
return 0;
}
Before SMI handler will be triggered, CommBuffer will be checked for overlap with SMRAM (inside PiSmmCommunicationSmm module):
EFI_STATUS __fastcall SwSmiHandler(
EFI_HANDLE DispatchHandle,
const void *Context,
void *CommBuffer,
UINTN *CommBufferSize)
{
// [COLLAPSED LOCAL DECLARATIONS. PRESS KEYPAD CTRL-"+" TO EXPAND]
Status = 0;
CommunicationHeader = *gCommunicationHeader;
if ( *gCommunicationHeader )
{
if ( SmmIsBufferOutsideSmmValid(*gCommunicationHeader, 24) )
{
CommBufferSizea[0] = CommunicationHeader->MessageLength;
if ( SmmIsBufferOutsideSmmValid(CommunicationHeader->Data, CommBufferSizea[0]) )
Status = gSmst->SmiManage(
&CommunicationHeader->HeaderGuid,
0,
CommunicationHeader->Data,
CommBufferSizea);
}
}
return -(Status != 0) & 0xA000000000000000;
}
If CommunicationHeader->MessageLength is 0 and CommunicationHeader->Data does not overlap with SMRAM, check will pass.
As we can see from the pseudocode, *CommBufferSize is not checked (only the CommBufferSize pointer is compared to NULL).
But at the same time there is a primitive for writing 8 fixed bytes at the address of CommBuffer:
This leads to rewriting pointed area with a predictable data - Status code.
As we can see from header of SMRAM dump, first 8 bytes of SMRAM is signature (SMMS3_64).
$ xxd SMRAM_dump_8b000000_8b7fffff.bin | head
00000000: 534d 4d53 335f 3634 8029 7d8b 0000 0000 SMMS3_64.)}.....
...
Thus, a potential attacker cannot execute arbitrary code, but can carry out a DOS attack.
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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.
| Disclosure Activity | Date |
|---|---|
Intel PSIRT is notified | 2022-01-05 |
HP PSIRT provide patch release | 2022-03-15 |
BINARLY public disclosure date | 2022-05-23 |
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Acknowledgements
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