linux/arch/x86/kernel/ldt.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Copyright (C) 1992 Krishna Balasubramanian and Linus Torvalds
 * Copyright (C) 1999 Ingo Molnar <[email protected]>
 * Copyright (C) 2002 Andi Kleen
 *
 * This handles calls from both 32bit and 64bit mode.
 *
 * Lock order:
 *	context.ldt_usr_sem
 *	  mmap_lock
 *	    context.lock
 */

#include <linux/errno.h>
#include <linux/gfp.h>
#include <linux/sched.h>
#include <linux/string.h>
#include <linux/mm.h>
#include <linux/smp.h>
#include <linux/syscalls.h>
#include <linux/slab.h>
#include <linux/vmalloc.h>
#include <linux/uaccess.h>

#include <asm/ldt.h>
#include <asm/tlb.h>
#include <asm/desc.h>
#include <asm/mmu_context.h>
#include <asm/pgtable_areas.h>

#include <xen/xen.h>

/* This is a multiple of PAGE_SIZE. */
#define LDT_SLOT_STRIDE …

static inline void *ldt_slot_va(int slot)
{ … }

void load_mm_ldt(struct mm_struct *mm)
{ … }

void switch_ldt(struct mm_struct *prev, struct mm_struct *next)
{ … }

static void refresh_ldt_segments(void)
{ … }

/* context.lock is held by the task which issued the smp function call */
static void flush_ldt(void *__mm)
{ … }

/* The caller must call finalize_ldt_struct on the result. LDT starts zeroed. */
static struct ldt_struct *alloc_ldt_struct(unsigned int num_entries)
{ … }

#ifdef CONFIG_MITIGATION_PAGE_TABLE_ISOLATION

static void do_sanity_check(struct mm_struct *mm,
			    bool had_kernel_mapping,
			    bool had_user_mapping)
{ … }

#ifdef CONFIG_X86_PAE

static pmd_t *pgd_to_pmd_walk(pgd_t *pgd, unsigned long va)
{
	p4d_t *p4d;
	pud_t *pud;

	if (pgd->pgd == 0)
		return NULL;

	p4d = p4d_offset(pgd, va);
	if (p4d_none(*p4d))
		return NULL;

	pud = pud_offset(p4d, va);
	if (pud_none(*pud))
		return NULL;

	return pmd_offset(pud, va);
}

static void map_ldt_struct_to_user(struct mm_struct *mm)
{
	pgd_t *k_pgd = pgd_offset(mm, LDT_BASE_ADDR);
	pgd_t *u_pgd = kernel_to_user_pgdp(k_pgd);
	pmd_t *k_pmd, *u_pmd;

	k_pmd = pgd_to_pmd_walk(k_pgd, LDT_BASE_ADDR);
	u_pmd = pgd_to_pmd_walk(u_pgd, LDT_BASE_ADDR);

	if (boot_cpu_has(X86_FEATURE_PTI) && !mm->context.ldt)
		set_pmd(u_pmd, *k_pmd);
}

static void sanity_check_ldt_mapping(struct mm_struct *mm)
{
	pgd_t *k_pgd = pgd_offset(mm, LDT_BASE_ADDR);
	pgd_t *u_pgd = kernel_to_user_pgdp(k_pgd);
	bool had_kernel, had_user;
	pmd_t *k_pmd, *u_pmd;

	k_pmd      = pgd_to_pmd_walk(k_pgd, LDT_BASE_ADDR);
	u_pmd      = pgd_to_pmd_walk(u_pgd, LDT_BASE_ADDR);
	had_kernel = (k_pmd->pmd != 0);
	had_user   = (u_pmd->pmd != 0);

	do_sanity_check(mm, had_kernel, had_user);
}

#else /* !CONFIG_X86_PAE */

static void map_ldt_struct_to_user(struct mm_struct *mm)
{ … }

static void sanity_check_ldt_mapping(struct mm_struct *mm)
{ … }

#endif /* CONFIG_X86_PAE */

/*
 * If PTI is enabled, this maps the LDT into the kernelmode and
 * usermode tables for the given mm.
 */
static int
map_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt, int slot)
{ … }

static void unmap_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt)
{ … }

#else /* !CONFIG_MITIGATION_PAGE_TABLE_ISOLATION */

static int
map_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt, int slot)
{
	return 0;
}

static void unmap_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt)
{
}
#endif /* CONFIG_MITIGATION_PAGE_TABLE_ISOLATION */

static void free_ldt_pgtables(struct mm_struct *mm)
{ … }

/* After calling this, the LDT is immutable. */
static void finalize_ldt_struct(struct ldt_struct *ldt)
{ … }

static void install_ldt(struct mm_struct *mm, struct ldt_struct *ldt)
{ … }

static void free_ldt_struct(struct ldt_struct *ldt)
{ … }

/*
 * Called on fork from arch_dup_mmap(). Just copy the current LDT state,
 * the new task is not running, so nothing can be installed.
 */
int ldt_dup_context(struct mm_struct *old_mm, struct mm_struct *mm)
{ … }

/*
 * No need to lock the MM as we are the last user
 *
 * 64bit: Don't touch the LDT register - we're already in the next thread.
 */
void destroy_context_ldt(struct mm_struct *mm)
{ … }

void ldt_arch_exit_mmap(struct mm_struct *mm)
{ … }

static int read_ldt(void __user *ptr, unsigned long bytecount)
{ … }

static int read_default_ldt(void __user *ptr, unsigned long bytecount)
{ … }

static bool allow_16bit_segments(void)
{ … }

static int write_ldt(void __user *ptr, unsigned long bytecount, int oldmode)
{ … }

SYSCALL_DEFINE3(modify_ldt, int , func , void __user * , ptr ,
		unsigned long , bytecount)
{ … }