2 files changed, 73 insertions, 0 deletions
diff --git a/EfiGuardDxe/X64/Cet.asm b/EfiGuardDxe/X64/Cet.asm
new file mode 100644
index 0000000..74433c2
--- /dev/null
+++ b/EfiGuardDxe/X64/Cet.asm
@@ -0,0 +1,37 @@
+MSR_S_CET					EQU 6A2h
+MSR_S_CET_SH_STK_EN			EQU 1
+CR4_CET						EQU (1 SHL 23)
+N_CR4_CET					EQU 23
+
+.code
+
+align 16
+AsmDisableCet PROC
+	mov ecx, MSR_S_CET
+	rdmsr
+	test al, MSR_S_CET_SH_STK_EN
+	jz @F						; if z, shadow stack not enabled
+
+	; Pop pushed data for 'call'
+	mov rax, 1
+	incsspq rax
+
+@@:
+	mov rax, cr4
+	btr eax, N_CR4_CET			; clear CR4_CET
+	mov cr4, rax
+	ret
+AsmDisableCet ENDP
+
+align 16
+AsmEnableCet PROC
+	mov rax, cr4
+	bts eax, N_CR4_CET			; set CR4_CET
+	mov cr4, rax
+
+	; Use jmp to skip check for 'ret'
+	pop rax
+	jmp rax
+AsmEnableCet ENDP
+
+end
diff --git a/EfiGuardDxe/X64/Cet.nasm b/EfiGuardDxe/X64/Cet.nasm
new file mode 100644
index 0000000..b93ca16
--- /dev/null
+++ b/EfiGuardDxe/X64/Cet.nasm
@@ -0,0 +1,36 @@
+%define MSR_S_CET						0x6A2
+%define MSR_S_CET_SH_STK_EN				0x1
+%define CR4_CET							(1 << 23)
+%define N_CR4_CET						23
+
+DEFAULT REL
+SECTION .text
+
+align 16
+global ASM_PFX(AsmDisableCet)
+ASM_PFX(AsmDisableCet):
+	mov ecx, MSR_S_CET
+	rdmsr
+	test al, MSR_S_CET_SH_STK_EN
+	jz .SsDone					; if z, shadow stack not enabled
+
+	; Pop pushed data for 'call'
+	mov rax, 1
+	incsspq rax
+
+.SsDone:
+	mov rax, cr4
+	btr eax, N_CR4_CET			; clear CR4_CET
+	mov cr4, rax
+	ret
+
+align 16
+global ASM_PFX(AsmEnableCet)
+ASM_PFX(AsmEnableCet):
+	mov rax, cr4
+	bts eax, N_CR4_CET			; set CR4_CET
+	mov cr4, rax
+
+	; Use jmp to skip check for 'ret'
+	pop rax
+	jmp rax