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93 lines
2.3 KiB
C
93 lines
2.3 KiB
C
/*
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* Copyright (C) 2021 Michael Brown <mbrown@fensystems.co.uk>.
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the GNU General Public License as
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* published by the Free Software Foundation; either version 2 of the
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* License, or (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful, but
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* WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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* General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
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* 02110-1301, USA.
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*/
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/**
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* @file
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*
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* Stack cookie
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*
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*/
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#include "wimboot.h"
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/** Stack cookie */
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unsigned long __stack_chk_guard;
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/**
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* Construct stack cookie value
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*
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*/
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static __attribute__ (( noinline )) unsigned long make_cookie ( void ) {
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union {
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struct {
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uint32_t eax;
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uint32_t edx;
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} __attribute__ (( packed ));
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unsigned long tsc;
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} u;
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unsigned long cookie;
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/* We have no viable source of entropy. Use the CPU timestamp
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* counter, which will have at least some minimal randomness
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* in the low bits by the time we are invoked.
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*/
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__asm__ ( "rdtsc" : "=a" ( u.eax ), "=d" ( u.edx ) );
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cookie = u.tsc;
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/* Ensure that the value contains a NUL byte, to act as a
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* runaway string terminator. Construct the NUL using a shift
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* rather than a mask, to avoid losing valuable entropy in the
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* lower-order bits.
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*/
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cookie <<= 8;
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return cookie;
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}
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/**
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* Initialise stack cookie
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*
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* This function must not itself use stack guard
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*/
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void init_cookie ( void ) {
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/* Set stack cookie value
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*
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* This function must not itself use stack protection, since
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* the change in the stack guard value would trigger a false
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* positive.
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*
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* There is unfortunately no way to annotate a function to
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* exclude the use of stack protection. We must therefore
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* rely on correctly anticipating the compiler's decision on
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* the use of stack protection.
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*/
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__stack_chk_guard = make_cookie();
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}
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/**
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* Abort on stack check failure
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*
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*/
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void __stack_chk_fail ( void ) {
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/* Abort program */
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die ( "Stack check failed\n" );
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}
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