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;------------------------------------------------------------------------------
;
; Copyright (c) Microsoft Corporation.
; SPDX-License-Identifier: BSD-2-Clause-Patent
;
; Module Name:
;
; DynamicCookie.nasm
;
; Abstract:
;
; Generates random number through CPU RdRand instruction on 64-bit platform
; to store a random value in the GCC __stack_check_guard stack cookie.
; The first byte is 0'd to prevent string copy functions from clobbering
; the stack cookie.
;
; Notes:
;
; If RdRand fails, the build time static stack cookie value will be used instead.
;
;------------------------------------------------------------------------------
DEFAULT REL
SECTION .text
extern ASM_PFX(__stack_chk_guard)
extern ASM_PFX(_CModuleEntryPoint)
;------------------------------------------------------------------------------
; VOID
; EFIAPI
; _ModuleEntryPoint (
; Parameters are passed through. TODO: Make sure there are only two args on X64
; );
;------------------------------------------------------------------------------
global ASM_PFX(_ModuleEntryPoint)
ASM_PFX(_ModuleEntryPoint):
push rbx
push rcx
push rdx
mov eax, 1 ; Set eax to 1 to get feature information
cpuid ; Call cpuid
test ecx, 0x40000000 ; Test the rdrand bit (bit 30) in ecx
jz c_entry ; If rdrand is not supported, jump to c_entry
rdrand rax ; Call rdrand functionality here, getting a 64 bit value as on
; X64, __stack_chk_guard is a 64 bit value.
; CF=1 if RN generated ok, otherwise CF=0
jnc c_entry ; If the cmd fails, don't, update __stack_chk_guard, we'll have to move forward
; with the static value provided at build time.
lea rbx, [rel ASM_PFX(__stack_chk_guard)] ; load the address of __stack_check_guard into rbx
xor ah, ah ; Zero a byte of the __stack_chk_guard value to protect against string functions
; (such as strcpy like functions) clobbering past the canary
mov [rbx], rax ; Store our random value, with 0'd first byte to __stack_chk_guard
c_entry:
pop rdx
pop rcx
pop rbx
jmp ASM_PFX(_CModuleEntryPoint)
|
