arch/x86/kernel/ldt.c - third_party/linux - Git at Google

 // SPDX-License-Identifier: GPL-2.0
 /*
  * Copyright (C) 1992 Krishna Balasubramanian and Linus Torvalds
  * Copyright (C) 1999 Ingo Molnar <mingo@redhat.com>
  * Copyright (C) 2002 Andi Kleen
  *
  * This handles calls from both 32bit and 64bit mode.
  *
  * Lock order:
  *	contex.ldt_usr_sem
  *	  mmap_sem
  *	    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/syscalls.h>

 static void refresh_ldt_segments(void)
 {
 #ifdef CONFIG_X86_64
 	unsigned short sel;

 	/*
 	 * Make sure that the cached DS and ES descriptors match the updated
 	 * LDT.
 	 */
 	savesegment(ds, sel);
 	if ((sel & SEGMENT_TI_MASK) == SEGMENT_LDT)
 		loadsegment(ds, sel);

 	savesegment(es, sel);
 	if ((sel & SEGMENT_TI_MASK) == SEGMENT_LDT)
 		loadsegment(es, sel);
 #endif
 }

 /* context.lock is held by the task which issued the smp function call */
 static void flush_ldt(void *__mm)
 {
 	struct mm_struct *mm = __mm;

 	if (this_cpu_read(cpu_tlbstate.loaded_mm) != mm)
 		return;

 	load_mm_ldt(mm);

 	refresh_ldt_segments();
 }

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

 	if (num_entries > LDT_ENTRIES)
 		return NULL;

 	new_ldt = kmalloc(sizeof(struct ldt_struct), GFP_KERNEL);
 	if (!new_ldt)
 		return NULL;

 	BUILD_BUG_ON(LDT_ENTRY_SIZE != sizeof(struct desc_struct));
 	alloc_size = num_entries * LDT_ENTRY_SIZE;

 	/*
 	 * Xen is very picky: it requires a page-aligned LDT that has no
 	 * trailing nonzero bytes in any page that contains LDT descriptors.
 	 * Keep it simple: zero the whole allocation and never allocate less
 	 * than PAGE_SIZE.
 	 */
 	if (alloc_size > PAGE_SIZE)
 		new_ldt->entries = vzalloc(alloc_size);
 	else
 		new_ldt->entries = (void *)get_zeroed_page(GFP_KERNEL);

 	if (!new_ldt->entries) {
 		kfree(new_ldt);
 		return NULL;
 	}

 	/* The new LDT isn't aliased for PTI yet. */
 	new_ldt->slot = -1;

 	new_ldt->nr_entries = num_entries;
 	return new_ldt;
 }

 /*
  * If PTI is enabled, this maps the LDT into the kernelmode and
  * usermode tables for the given mm.
  *
  * There is no corresponding unmap function.  Even if the LDT is freed, we
  * leave the PTEs around until the slot is reused or the mm is destroyed.
  * This is harmless: the LDT is always in ordinary memory, and no one will
  * access the freed slot.
  *
  * If we wanted to unmap freed LDTs, we'd also need to do a flush to make
  * it useful, and the flush would slow down modify_ldt().
  */
 static int
 map_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt, int slot)
 {
 #ifdef CONFIG_PAGE_TABLE_ISOLATION
 	bool is_vmalloc, had_top_level_entry;
 	unsigned long va;
 	spinlock_t *ptl;
 	pgd_t *pgd;
 	int i;

 	if (!static_cpu_has(X86_FEATURE_PTI))
 		return 0;

 	/*
 	 * Any given ldt_struct should have map_ldt_struct() called at most
 	 * once.
 	 */
 	WARN_ON(ldt->slot != -1);

 	/*
 	 * Did we already have the top level entry allocated?  We can't
 	 * use pgd_none() for this because it doens't do anything on
 	 * 4-level page table kernels.
 	 */
 	pgd = pgd_offset(mm, LDT_BASE_ADDR);
 	had_top_level_entry = (pgd->pgd != 0);

 	is_vmalloc = is_vmalloc_addr(ldt->entries);

 	for (i = 0; i * PAGE_SIZE < ldt->nr_entries * LDT_ENTRY_SIZE; i++) {
 		unsigned long offset = i << PAGE_SHIFT;
 		const void *src = (char *)ldt->entries + offset;
 		unsigned long pfn;
 		pgprot_t pte_prot;
 		pte_t pte, *ptep;

 		va = (unsigned long)ldt_slot_va(slot) + offset;
 		pfn = is_vmalloc ? vmalloc_to_pfn(src) :
 			page_to_pfn(virt_to_page(src));
 		/*
 		 * Treat the PTI LDT range as a *userspace* range.
 		 * get_locked_pte() will allocate all needed pagetables
 		 * and account for them in this mm.
 		 */
 		ptep = get_locked_pte(mm, va, &ptl);
 		if (!ptep)
 			return -ENOMEM;
 		/*
 		 * Map it RO so the easy to find address is not a primary
 		 * target via some kernel interface which misses a
 		 * permission check.
 		 */
 		pte_prot = __pgprot(__PAGE_KERNEL_RO & ~_PAGE_GLOBAL);
 		/* Filter out unsuppored __PAGE_KERNEL* bits: */
 		pgprot_val(pte_prot) &= __supported_pte_mask;
 		pte = pfn_pte(pfn, pte_prot);
 		set_pte_at(mm, va, ptep, pte);
 		pte_unmap_unlock(ptep, ptl);
 	}

 	if (mm->context.ldt) {
 		/*
 		 * We already had an LDT.  The top-level entry should already
 		 * have been allocated and synchronized with the usermode
 		 * tables.
 		 */
 		WARN_ON(!had_top_level_entry);
 		if (static_cpu_has(X86_FEATURE_PTI))
 			WARN_ON(!kernel_to_user_pgdp(pgd)->pgd);
 	} else {
 		/*
 		 * This is the first time we're mapping an LDT for this process.
 		 * Sync the pgd to the usermode tables.
 		 */
 		WARN_ON(had_top_level_entry);
 		if (static_cpu_has(X86_FEATURE_PTI)) {
 			WARN_ON(kernel_to_user_pgdp(pgd)->pgd);
 			set_pgd(kernel_to_user_pgdp(pgd), *pgd);
 		}
 	}

 	va = (unsigned long)ldt_slot_va(slot);
 	flush_tlb_mm_range(mm, va, va + LDT_SLOT_STRIDE, 0);

 	ldt->slot = slot;
 #endif
 	return 0;
 }

 static void free_ldt_pgtables(struct mm_struct *mm)
 {
 #ifdef CONFIG_PAGE_TABLE_ISOLATION
 	struct mmu_gather tlb;
 	unsigned long start = LDT_BASE_ADDR;
 	unsigned long end = start + (1UL << PGDIR_SHIFT);

 	if (!static_cpu_has(X86_FEATURE_PTI))
 		return;

 	tlb_gather_mmu(&tlb, mm, start, end);
 	free_pgd_range(&tlb, start, end, start, end);
 	tlb_finish_mmu(&tlb, start, end);
 #endif
 }

 /* After calling this, the LDT is immutable. */
 static void finalize_ldt_struct(struct ldt_struct *ldt)
 {
 	paravirt_alloc_ldt(ldt->entries, ldt->nr_entries);
 }

 static void install_ldt(struct mm_struct *mm, struct ldt_struct *ldt)
 {
 	mutex_lock(&mm->context.lock);

 	/* Synchronizes with READ_ONCE in load_mm_ldt. */
 	smp_store_release(&mm->context.ldt, ldt);

 	/* Activate the LDT for all CPUs using currents mm. */
 	on_each_cpu_mask(mm_cpumask(mm), flush_ldt, mm, true);

 	mutex_unlock(&mm->context.lock);
 }

 static void free_ldt_struct(struct ldt_struct *ldt)
 {
 	if (likely(!ldt))
 		return;

 	paravirt_free_ldt(ldt->entries, ldt->nr_entries);
 	if (ldt->nr_entries * LDT_ENTRY_SIZE > PAGE_SIZE)
 		vfree_atomic(ldt->entries);
 	else
 		free_page((unsigned long)ldt->entries);
 	kfree(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)
 {
 	struct ldt_struct *new_ldt;
 	int retval = 0;

 	if (!old_mm)
 		return 0;

 	mutex_lock(&old_mm->context.lock);
 	if (!old_mm->context.ldt)
 		goto out_unlock;

 	new_ldt = alloc_ldt_struct(old_mm->context.ldt->nr_entries);
 	if (!new_ldt) {
 		retval = -ENOMEM;
 		goto out_unlock;
 	}

 	memcpy(new_ldt->entries, old_mm->context.ldt->entries,
 	       new_ldt->nr_entries * LDT_ENTRY_SIZE);
 	finalize_ldt_struct(new_ldt);

 	retval = map_ldt_struct(mm, new_ldt, 0);
 	if (retval) {
 		free_ldt_pgtables(mm);
 		free_ldt_struct(new_ldt);
 		goto out_unlock;
 	}
 	mm->context.ldt = new_ldt;

 out_unlock:
 	mutex_unlock(&old_mm->context.lock);
 	return retval;
 }

 /*
  * 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)
 {
 	free_ldt_struct(mm->context.ldt);
 	mm->context.ldt = NULL;
 }

 void ldt_arch_exit_mmap(struct mm_struct *mm)
 {
 	free_ldt_pgtables(mm);
 }

 static int read_ldt(void __user *ptr, unsigned long bytecount)
 {
 	struct mm_struct *mm = current->mm;
 	unsigned long entries_size;
 	int retval;

 	down_read(&mm->context.ldt_usr_sem);

 	if (!mm->context.ldt) {
 		retval = 0;
 		goto out_unlock;
 	}

 	if (bytecount > LDT_ENTRY_SIZE * LDT_ENTRIES)
 		bytecount = LDT_ENTRY_SIZE * LDT_ENTRIES;

 	entries_size = mm->context.ldt->nr_entries * LDT_ENTRY_SIZE;
 	if (entries_size > bytecount)
 		entries_size = bytecount;

 	if (copy_to_user(ptr, mm->context.ldt->entries, entries_size)) {
 		retval = -EFAULT;
 		goto out_unlock;
 	}

 	if (entries_size != bytecount) {
 		/* Zero-fill the rest and pretend we read bytecount bytes. */
 		if (clear_user(ptr + entries_size, bytecount - entries_size)) {
 			retval = -EFAULT;
 			goto out_unlock;
 		}
 	}
 	retval = bytecount;

 out_unlock:
 	up_read(&mm->context.ldt_usr_sem);
 	return retval;
 }

 static int read_default_ldt(void __user *ptr, unsigned long bytecount)
 {
 	/* CHECKME: Can we use _one_ random number ? */
 #ifdef CONFIG_X86_32
 	unsigned long size = 5 * sizeof(struct desc_struct);
 #else
 	unsigned long size = 128;
 #endif
 	if (bytecount > size)
 		bytecount = size;
 	if (clear_user(ptr, bytecount))
 		return -EFAULT;
 	return bytecount;
 }

 static int write_ldt(void __user *ptr, unsigned long bytecount, int oldmode)
 {
 	struct mm_struct *mm = current->mm;
 	struct ldt_struct *new_ldt, *old_ldt;
 	unsigned int old_nr_entries, new_nr_entries;
 	struct user_desc ldt_info;
 	struct desc_struct ldt;
 	int error;

 	error = -EINVAL;
 	if (bytecount != sizeof(ldt_info))
 		goto out;
 	error = -EFAULT;
 	if (copy_from_user(&ldt_info, ptr, sizeof(ldt_info)))
 		goto out;

 	error = -EINVAL;
 	if (ldt_info.entry_number >= LDT_ENTRIES)
 		goto out;
 	if (ldt_info.contents == 3) {
 		if (oldmode)
 			goto out;
 		if (ldt_info.seg_not_present == 0)
 			goto out;
 	}

 	if ((oldmode && !ldt_info.base_addr && !ldt_info.limit) ||
 	    LDT_empty(&ldt_info)) {
 		/* The user wants to clear the entry. */
 		memset(&ldt, 0, sizeof(ldt));
 	} else {
 		if (!IS_ENABLED(CONFIG_X86_16BIT) && !ldt_info.seg_32bit) {
 			error = -EINVAL;
 			goto out;
 		}

 		fill_ldt(&ldt, &ldt_info);
 		if (oldmode)
 			ldt.avl = 0;
 	}

 	if (down_write_killable(&mm->context.ldt_usr_sem))
 		return -EINTR;

 	old_ldt       = mm->context.ldt;
 	old_nr_entries = old_ldt ? old_ldt->nr_entries : 0;
 	new_nr_entries = max(ldt_info.entry_number + 1, old_nr_entries);

 	error = -ENOMEM;
 	new_ldt = alloc_ldt_struct(new_nr_entries);
 	if (!new_ldt)
 		goto out_unlock;

 	if (old_ldt)
 		memcpy(new_ldt->entries, old_ldt->entries, old_nr_entries * LDT_ENTRY_SIZE);

 	new_ldt->entries[ldt_info.entry_number] = ldt;
 	finalize_ldt_struct(new_ldt);

 	/*
 	 * If we are using PTI, map the new LDT into the userspace pagetables.
 	 * If there is already an LDT, use the other slot so that other CPUs
 	 * will continue to use the old LDT until install_ldt() switches
 	 * them over to the new LDT.
 	 */
 	error = map_ldt_struct(mm, new_ldt, old_ldt ? !old_ldt->slot : 0);
 	if (error) {
 		/*
 		 * This only can fail for the first LDT setup. If an LDT is
 		 * already installed then the PTE page is already
 		 * populated. Mop up a half populated page table.
 		 */
 		if (!WARN_ON_ONCE(old_ldt))
 			free_ldt_pgtables(mm);
 		free_ldt_struct(new_ldt);
 		goto out_unlock;
 	}

 	install_ldt(mm, new_ldt);
 	free_ldt_struct(old_ldt);
 	error = 0;

 out_unlock:
 	up_write(&mm->context.ldt_usr_sem);
 out:
 	return error;
 }

 SYSCALL_DEFINE3(modify_ldt, int , func , void __user * , ptr ,
 		unsigned long , bytecount)
 {
 	int ret = -ENOSYS;

 	switch (func) {
 	case 0:
 		ret = read_ldt(ptr, bytecount);
 		break;
 	case 1:
 		ret = write_ldt(ptr, bytecount, 1);
 		break;
 	case 2:
 		ret = read_default_ldt(ptr, bytecount);
 		break;
 	case 0x11:
 		ret = write_ldt(ptr, bytecount, 0);
 		break;
 	}
 	/*
 	 * The SYSCALL_DEFINE() macros give us an 'unsigned long'
 	 * return type, but tht ABI for sys_modify_ldt() expects
 	 * 'int'.  This cast gives us an int-sized value in %rax
 	 * for the return code.  The 'unsigned' is necessary so
 	 * the compiler does not try to sign-extend the negative
 	 * return codes into the high half of the register when
 	 * taking the value from int->long.
 	 */
 	return (unsigned int)ret;
 }
	// SPDX-License-Identifier: GPL-2.0
	/*
	* Copyright (C) 1992 Krishna Balasubramanian and Linus Torvalds
	* Copyright (C) 1999 Ingo Molnar <mingo@redhat.com>
	* Copyright (C) 2002 Andi Kleen
	*
	* This handles calls from both 32bit and 64bit mode.
	*
	* Lock order:
	* contex.ldt_usr_sem
	* mmap_sem
	* 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/syscalls.h>

	static void refresh_ldt_segments(void)
	{
	#ifdef CONFIG_X86_64
	unsigned short sel;

	/*
	* Make sure that the cached DS and ES descriptors match the updated
	* LDT.
	*/
	savesegment(ds, sel);
	if ((sel & SEGMENT_TI_MASK) == SEGMENT_LDT)
	loadsegment(ds, sel);

	savesegment(es, sel);
	if ((sel & SEGMENT_TI_MASK) == SEGMENT_LDT)
	loadsegment(es, sel);
	#endif
	}

	/* context.lock is held by the task which issued the smp function call */
	static void flush_ldt(void *__mm)
	{
	struct mm_struct *mm = __mm;

	if (this_cpu_read(cpu_tlbstate.loaded_mm) != mm)
	return;

	load_mm_ldt(mm);

	refresh_ldt_segments();
	}

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

	if (num_entries > LDT_ENTRIES)
	return NULL;

	new_ldt = kmalloc(sizeof(struct ldt_struct), GFP_KERNEL);
	if (!new_ldt)
	return NULL;

	BUILD_BUG_ON(LDT_ENTRY_SIZE != sizeof(struct desc_struct));
	alloc_size = num_entries * LDT_ENTRY_SIZE;

	/*
	* Xen is very picky: it requires a page-aligned LDT that has no
	* trailing nonzero bytes in any page that contains LDT descriptors.
	* Keep it simple: zero the whole allocation and never allocate less
	* than PAGE_SIZE.
	*/
	if (alloc_size > PAGE_SIZE)
	new_ldt->entries = vzalloc(alloc_size);
	else
	new_ldt->entries = (void *)get_zeroed_page(GFP_KERNEL);

	if (!new_ldt->entries) {
	kfree(new_ldt);
	return NULL;
	}

	/* The new LDT isn't aliased for PTI yet. */
	new_ldt->slot = -1;

	new_ldt->nr_entries = num_entries;
	return new_ldt;
	}

	/*
	* If PTI is enabled, this maps the LDT into the kernelmode and
	* usermode tables for the given mm.
	*
	* There is no corresponding unmap function. Even if the LDT is freed, we
	* leave the PTEs around until the slot is reused or the mm is destroyed.
	* This is harmless: the LDT is always in ordinary memory, and no one will
	* access the freed slot.
	*
	* If we wanted to unmap freed LDTs, we'd also need to do a flush to make
	* it useful, and the flush would slow down modify_ldt().
	*/
	static int
	map_ldt_struct(struct mm_struct mm, struct ldt_struct ldt, int slot)
	{
	#ifdef CONFIG_PAGE_TABLE_ISOLATION
	bool is_vmalloc, had_top_level_entry;
	unsigned long va;
	spinlock_t *ptl;
	pgd_t *pgd;
	int i;

	if (!static_cpu_has(X86_FEATURE_PTI))
	return 0;

	/*
	* Any given ldt_struct should have map_ldt_struct() called at most
	* once.
	*/
	WARN_ON(ldt->slot != -1);

	/*
	* Did we already have the top level entry allocated? We can't
	* use pgd_none() for this because it doens't do anything on
	* 4-level page table kernels.
	*/
	pgd = pgd_offset(mm, LDT_BASE_ADDR);
	had_top_level_entry = (pgd->pgd != 0);

	is_vmalloc = is_vmalloc_addr(ldt->entries);

	for (i = 0; i * PAGE_SIZE < ldt->nr_entries * LDT_ENTRY_SIZE; i++) {
	unsigned long offset = i << PAGE_SHIFT;
	const void src = (char )ldt->entries + offset;
	unsigned long pfn;
	pgprot_t pte_prot;
	pte_t pte, *ptep;

	va = (unsigned long)ldt_slot_va(slot) + offset;
	pfn = is_vmalloc ? vmalloc_to_pfn(src) :
	page_to_pfn(virt_to_page(src));
	/*
	* Treat the PTI LDT range as a userspace range.
	* get_locked_pte() will allocate all needed pagetables
	* and account for them in this mm.
	*/
	ptep = get_locked_pte(mm, va, &ptl);
	if (!ptep)
	return -ENOMEM;
	/*
	* Map it RO so the easy to find address is not a primary
	* target via some kernel interface which misses a
	* permission check.
	*/
	pte_prot = __pgprot(__PAGE_KERNEL_RO & ~_PAGE_GLOBAL);
	/* Filter out unsuppored __PAGE_KERNEL* bits: */
	pgprot_val(pte_prot) &= __supported_pte_mask;
	pte = pfn_pte(pfn, pte_prot);
	set_pte_at(mm, va, ptep, pte);
	pte_unmap_unlock(ptep, ptl);
	}

	if (mm->context.ldt) {
	/*
	* We already had an LDT. The top-level entry should already
	* have been allocated and synchronized with the usermode
	* tables.
	*/
	WARN_ON(!had_top_level_entry);
	if (static_cpu_has(X86_FEATURE_PTI))
	WARN_ON(!kernel_to_user_pgdp(pgd)->pgd);
	} else {
	/*
	* This is the first time we're mapping an LDT for this process.
	* Sync the pgd to the usermode tables.
	*/
	WARN_ON(had_top_level_entry);
	if (static_cpu_has(X86_FEATURE_PTI)) {
	WARN_ON(kernel_to_user_pgdp(pgd)->pgd);
	set_pgd(kernel_to_user_pgdp(pgd), *pgd);
	}
	}

	va = (unsigned long)ldt_slot_va(slot);
	flush_tlb_mm_range(mm, va, va + LDT_SLOT_STRIDE, 0);

	ldt->slot = slot;
	#endif
	return 0;
	}

	static void free_ldt_pgtables(struct mm_struct *mm)
	{
	#ifdef CONFIG_PAGE_TABLE_ISOLATION
	struct mmu_gather tlb;
	unsigned long start = LDT_BASE_ADDR;
	unsigned long end = start + (1UL << PGDIR_SHIFT);

	if (!static_cpu_has(X86_FEATURE_PTI))
	return;

	tlb_gather_mmu(&tlb, mm, start, end);
	free_pgd_range(&tlb, start, end, start, end);
	tlb_finish_mmu(&tlb, start, end);
	#endif
	}

	/* After calling this, the LDT is immutable. */
	static void finalize_ldt_struct(struct ldt_struct *ldt)
	{
	paravirt_alloc_ldt(ldt->entries, ldt->nr_entries);
	}

	static void install_ldt(struct mm_struct mm, struct ldt_struct ldt)
	{
	mutex_lock(&mm->context.lock);

	/* Synchronizes with READ_ONCE in load_mm_ldt. */
	smp_store_release(&mm->context.ldt, ldt);

	/* Activate the LDT for all CPUs using currents mm. */
	on_each_cpu_mask(mm_cpumask(mm), flush_ldt, mm, true);

	mutex_unlock(&mm->context.lock);
	}

	static void free_ldt_struct(struct ldt_struct *ldt)
	{
	if (likely(!ldt))
	return;

	paravirt_free_ldt(ldt->entries, ldt->nr_entries);
	if (ldt->nr_entries * LDT_ENTRY_SIZE > PAGE_SIZE)
	vfree_atomic(ldt->entries);
	else
	free_page((unsigned long)ldt->entries);
	kfree(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)
	{
	struct ldt_struct *new_ldt;
	int retval = 0;

	if (!old_mm)
	return 0;

	mutex_lock(&old_mm->context.lock);
	if (!old_mm->context.ldt)
	goto out_unlock;

	new_ldt = alloc_ldt_struct(old_mm->context.ldt->nr_entries);
	if (!new_ldt) {
	retval = -ENOMEM;
	goto out_unlock;
	}

	memcpy(new_ldt->entries, old_mm->context.ldt->entries,
	new_ldt->nr_entries * LDT_ENTRY_SIZE);
	finalize_ldt_struct(new_ldt);

	retval = map_ldt_struct(mm, new_ldt, 0);
	if (retval) {
	free_ldt_pgtables(mm);
	free_ldt_struct(new_ldt);
	goto out_unlock;
	}
	mm->context.ldt = new_ldt;

	out_unlock:
	mutex_unlock(&old_mm->context.lock);
	return retval;
	}

	/*
	* 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)
	{
	free_ldt_struct(mm->context.ldt);
	mm->context.ldt = NULL;
	}

	void ldt_arch_exit_mmap(struct mm_struct *mm)
	{
	free_ldt_pgtables(mm);
	}

	static int read_ldt(void __user *ptr, unsigned long bytecount)
	{
	struct mm_struct *mm = current->mm;
	unsigned long entries_size;
	int retval;

	down_read(&mm->context.ldt_usr_sem);

	if (!mm->context.ldt) {
	retval = 0;
	goto out_unlock;
	}

	if (bytecount > LDT_ENTRY_SIZE * LDT_ENTRIES)
	bytecount = LDT_ENTRY_SIZE * LDT_ENTRIES;

	entries_size = mm->context.ldt->nr_entries * LDT_ENTRY_SIZE;
	if (entries_size > bytecount)
	entries_size = bytecount;

	if (copy_to_user(ptr, mm->context.ldt->entries, entries_size)) {
	retval = -EFAULT;
	goto out_unlock;
	}

	if (entries_size != bytecount) {
	/* Zero-fill the rest and pretend we read bytecount bytes. */
	if (clear_user(ptr + entries_size, bytecount - entries_size)) {
	retval = -EFAULT;
	goto out_unlock;
	}
	}
	retval = bytecount;

	out_unlock:
	up_read(&mm->context.ldt_usr_sem);
	return retval;
	}

	static int read_default_ldt(void __user *ptr, unsigned long bytecount)
	{
	/* CHECKME: Can we use _one_ random number ? */
	#ifdef CONFIG_X86_32
	unsigned long size = 5 * sizeof(struct desc_struct);
	#else
	unsigned long size = 128;
	#endif
	if (bytecount > size)
	bytecount = size;
	if (clear_user(ptr, bytecount))
	return -EFAULT;
	return bytecount;
	}

	static int write_ldt(void __user *ptr, unsigned long bytecount, int oldmode)
	{
	struct mm_struct *mm = current->mm;
	struct ldt_struct new_ldt, old_ldt;
	unsigned int old_nr_entries, new_nr_entries;
	struct user_desc ldt_info;
	struct desc_struct ldt;
	int error;

	error = -EINVAL;
	if (bytecount != sizeof(ldt_info))
	goto out;
	error = -EFAULT;
	if (copy_from_user(&ldt_info, ptr, sizeof(ldt_info)))
	goto out;

	error = -EINVAL;
	if (ldt_info.entry_number >= LDT_ENTRIES)
	goto out;
	if (ldt_info.contents == 3) {
	if (oldmode)
	goto out;
	if (ldt_info.seg_not_present == 0)
	goto out;
	}

	if ((oldmode && !ldt_info.base_addr && !ldt_info.limit) \|\|
	LDT_empty(&ldt_info)) {
	/* The user wants to clear the entry. */
	memset(&ldt, 0, sizeof(ldt));
	} else {
	if (!IS_ENABLED(CONFIG_X86_16BIT) && !ldt_info.seg_32bit) {
	error = -EINVAL;
	goto out;
	}

	fill_ldt(&ldt, &ldt_info);
	if (oldmode)
	ldt.avl = 0;
	}

	if (down_write_killable(&mm->context.ldt_usr_sem))
	return -EINTR;

	old_ldt = mm->context.ldt;
	old_nr_entries = old_ldt ? old_ldt->nr_entries : 0;
	new_nr_entries = max(ldt_info.entry_number + 1, old_nr_entries);

	error = -ENOMEM;
	new_ldt = alloc_ldt_struct(new_nr_entries);
	if (!new_ldt)
	goto out_unlock;

	if (old_ldt)
	memcpy(new_ldt->entries, old_ldt->entries, old_nr_entries * LDT_ENTRY_SIZE);

	new_ldt->entries[ldt_info.entry_number] = ldt;
	finalize_ldt_struct(new_ldt);

	/*
	* If we are using PTI, map the new LDT into the userspace pagetables.
	* If there is already an LDT, use the other slot so that other CPUs
	* will continue to use the old LDT until install_ldt() switches
	* them over to the new LDT.
	*/
	error = map_ldt_struct(mm, new_ldt, old_ldt ? !old_ldt->slot : 0);
	if (error) {
	/*
	* This only can fail for the first LDT setup. If an LDT is
	* already installed then the PTE page is already
	* populated. Mop up a half populated page table.
	*/
	if (!WARN_ON_ONCE(old_ldt))
	free_ldt_pgtables(mm);
	free_ldt_struct(new_ldt);
	goto out_unlock;
	}

	install_ldt(mm, new_ldt);
	free_ldt_struct(old_ldt);
	error = 0;

	out_unlock:
	up_write(&mm->context.ldt_usr_sem);
	out:
	return error;
	}

	SYSCALL_DEFINE3(modify_ldt, int , func , void __user * , ptr ,
	unsigned long , bytecount)
	{
	int ret = -ENOSYS;

	switch (func) {
	case 0:
	ret = read_ldt(ptr, bytecount);
	break;
	case 1:
	ret = write_ldt(ptr, bytecount, 1);
	break;
	case 2:
	ret = read_default_ldt(ptr, bytecount);
	break;
	case 0x11:
	ret = write_ldt(ptr, bytecount, 0);
	break;
	}
	/*
	* The SYSCALL_DEFINE() macros give us an 'unsigned long'
	* return type, but tht ABI for sys_modify_ldt() expects
	* 'int'. This cast gives us an int-sized value in %rax
	* for the return code. The 'unsigned' is necessary so
	* the compiler does not try to sign-extend the negative
	* return codes into the high half of the register when
	* taking the value from int->long.
	*/
	return (unsigned int)ret;
	}