cregit-Linux how code gets into the kernel

Release 4.14 arch/x86/include/asm/stackprotector.h

/* SPDX-License-Identifier: GPL-2.0 */
/*
 * GCC stack protector support.
 *
 * Stack protector works by putting predefined pattern at the start of
 * the stack frame and verifying that it hasn't been overwritten when
 * returning from the function.  The pattern is called stack canary
 * and unfortunately gcc requires it to be at a fixed offset from %gs.
 * On x86_64, the offset is 40 bytes and on x86_32 20 bytes.  x86_64
 * and x86_32 use segment registers differently and thus handles this
 * requirement differently.
 *
 * On x86_64, %gs is shared by percpu area and stack canary.  All
 * percpu symbols are zero based and %gs points to the base of percpu
 * area.  The first occupant of the percpu area is always
 * irq_stack_union which contains stack_canary at offset 40.  Userland
 * %gs is always saved and restored on kernel entry and exit using
 * swapgs, so stack protector doesn't add any complexity there.
 *
 * On x86_32, it's slightly more complicated.  As in x86_64, %gs is
 * used for userland TLS.  Unfortunately, some processors are much
 * slower at loading segment registers with different value when
 * entering and leaving the kernel, so the kernel uses %fs for percpu
 * area and manages %gs lazily so that %gs is switched only when
 * necessary, usually during task switch.
 *
 * As gcc requires the stack canary at %gs:20, %gs can't be managed
 * lazily if stack protector is enabled, so the kernel saves and
 * restores userland %gs on kernel entry and exit.  This behavior is
 * controlled by CONFIG_X86_32_LAZY_GS and accessors are defined in
 * system.h to hide the details.
 */

#ifndef _ASM_STACKPROTECTOR_H

#define _ASM_STACKPROTECTOR_H 1

#ifdef CONFIG_CC_STACKPROTECTOR

#include <asm/tsc.h>
#include <asm/processor.h>
#include <asm/percpu.h>
#include <asm/desc.h>

#include <linux/random.h>
#include <linux/sched.h>

/*
 * 24 byte read-only segment initializer for stack canary.  Linker
 * can't handle the address bit shifting.  Address will be set in
 * head_32 for boot CPU and setup_per_cpu_areas() for others.
 */

#define GDT_STACK_CANARY_INIT						\
	[GDT_ENTRY_STACK_CANARY] = GDT_ENTRY_INIT(0x4090, 0, 0x18),

/*
 * Initialize the stackprotector canary value.
 *
 * NOTE: this must only be called from functions that never return,
 * and it must always be inlined.
 */

static __always_inline void boot_init_stack_canary(void) { u64 canary; u64 tsc; #ifdef CONFIG_X86_64 BUILD_BUG_ON(offsetof(union irq_stack_union, stack_canary) != 40); #endif /* * We both use the random pool and the current TSC as a source * of randomness. The TSC only matters for very early init, * there it already has some randomness on most systems. Later * on during the bootup the random pool has true entropy too. */ get_random_bytes(&canary, sizeof(canary)); tsc = rdtsc(); canary += tsc + (tsc << 32UL); canary &= CANARY_MASK; current->stack_canary = canary; #ifdef CONFIG_X86_64 this_cpu_write(irq_stack_union.stack_canary, canary); #else this_cpu_write(stack_canary.canary, canary); #endif }

Contributors

PersonTokensPropCommitsCommitProp
Ingo Molnar5153.68%222.22%
Tejun Heo3132.63%222.22%
Brian Gerst44.21%111.11%
Rik Van Riel44.21%111.11%
Jeremy Fitzhardinge22.11%111.11%
Alex Shi22.11%111.11%
Andrew Lutomirski11.05%111.11%
Total95100.00%9100.00%


static inline void setup_stack_canary_segment(int cpu) { #ifdef CONFIG_X86_32 unsigned long canary = (unsigned long)&per_cpu(stack_canary, cpu); struct desc_struct *gdt_table = get_cpu_gdt_rw(cpu); struct desc_struct desc; desc = gdt_table[GDT_ENTRY_STACK_CANARY]; set_desc_base(&desc, canary); write_gdt_entry(gdt_table, GDT_ENTRY_STACK_CANARY, &desc, DESCTYPE_S); #endif }

Contributors

PersonTokensPropCommitsCommitProp
Tejun Heo6793.06%133.33%
Akinobu Mita45.56%133.33%
Thomas Garnier11.39%133.33%
Total72100.00%3100.00%


static inline void load_stack_canary_segment(void) { #ifdef CONFIG_X86_32 asm("mov %0, %%gs" : : "r" (__KERNEL_STACK_CANARY) : "memory"); #endif }

Contributors

PersonTokensPropCommitsCommitProp
Tejun Heo15100.00%1100.00%
Total15100.00%1100.00%

#else /* CC_STACKPROTECTOR */ #define GDT_STACK_CANARY_INIT /* dummy boot_init_stack_canary() is defined in linux/stackprotector.h */
static inline void setup_stack_canary_segment(int cpu) { }

Contributors

PersonTokensPropCommitsCommitProp
Tejun Heo9100.00%1100.00%
Total9100.00%1100.00%


static inline void load_stack_canary_segment(void) { #ifdef CONFIG_X86_32 asm volatile ("mov %0, %%gs" : : "r" (0)); #endif }

Contributors

PersonTokensPropCommitsCommitProp
Tejun Heo1694.12%150.00%
Ingo Molnar15.88%150.00%
Total17100.00%2100.00%

#endif /* CC_STACKPROTECTOR */ #endif /* _ASM_STACKPROTECTOR_H */

Overall Contributors

PersonTokensPropCommitsCommitProp
Tejun Heo16764.73%317.65%
Ingo Molnar7027.13%529.41%
Akinobu Mita51.94%211.76%
Brian Gerst51.94%15.88%
Rik Van Riel41.55%15.88%
Alex Shi20.78%15.88%
Jeremy Fitzhardinge20.78%15.88%
Thomas Garnier10.39%15.88%
Andrew Lutomirski10.39%15.88%
Greg Kroah-Hartman10.39%15.88%
Total258100.00%17100.00%
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