1 files changed, 557 insertions, 128 deletions

diff --git a/arch/x86/kernel/ldt.c b/arch/x86/kernel/ldt.c
index ebc987398923..0f19ef355f5f 100644
--- a/arch/x86/kernel/ldt.c
+++ b/arch/x86/kernel/ldt.c
@@ -1,9 +1,15 @@
+// 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:
+ *	context.ldt_usr_sem
+ *	  mmap_lock
+ *	    context.lock
  */
 
 #include <linux/errno.h>
@@ -12,109 +18,468 @@
 #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>
+#include <asm/pgtable_areas.h>
+
+#include <xen/xen.h>
+
+/* This is a multiple of PAGE_SIZE. */
+#define LDT_SLOT_STRIDE (LDT_ENTRIES * LDT_ENTRY_SIZE)
 
-#ifdef CONFIG_SMP
-static void flush_ldt(void *current_mm)
+static inline void *ldt_slot_va(int slot)
 {
-	if (current->active_mm == current_mm)
-		load_LDT(&current->active_mm->context);
+	return (void *)(LDT_BASE_ADDR + LDT_SLOT_STRIDE * slot);
 }
-#endif
 
-static int alloc_ldt(mm_context_t *pc, int mincount, int reload)
+void load_mm_ldt(struct mm_struct *mm)
 {
-	void *oldldt, *newldt;
-	int oldsize;
+	struct ldt_struct *ldt;
 
-	if (mincount <= pc->size)
-		return 0;
-	oldsize = pc->size;
-	mincount = (mincount + (PAGE_SIZE / LDT_ENTRY_SIZE - 1)) &
-			(~(PAGE_SIZE / LDT_ENTRY_SIZE - 1));
-	if (mincount * LDT_ENTRY_SIZE > PAGE_SIZE)
-		newldt = vmalloc(mincount * LDT_ENTRY_SIZE);
-	else
-		newldt = (void *)__get_free_page(GFP_KERNEL);
+	/* READ_ONCE synchronizes with smp_store_release */
+	ldt = READ_ONCE(mm->context.ldt);
 
-	if (!newldt)
-		return -ENOMEM;
+	/*
+	 * Any change to mm->context.ldt is followed by an IPI to all
+	 * CPUs with the mm active.  The LDT will not be freed until
+	 * after the IPI is handled by all such CPUs.  This means that
+	 * if the ldt_struct changes before we return, the values we see
+	 * will be safe, and the new values will be loaded before we run
+	 * any user code.
+	 *
+	 * NB: don't try to convert this to use RCU without extreme care.
+	 * We would still need IRQs off, because we don't want to change
+	 * the local LDT after an IPI loaded a newer value than the one
+	 * that we can see.
+	 */
 
-	if (oldsize)
-		memcpy(newldt, pc->ldt, oldsize * LDT_ENTRY_SIZE);
-	oldldt = pc->ldt;
-	memset(newldt + oldsize * LDT_ENTRY_SIZE, 0,
-	       (mincount - oldsize) * LDT_ENTRY_SIZE);
+	if (unlikely(ldt)) {
+		if (static_cpu_has(X86_FEATURE_PTI)) {
+			if (WARN_ON_ONCE((unsigned long)ldt->slot > 1)) {
+				/*
+				 * Whoops -- either the new LDT isn't mapped
+				 * (if slot == -1) or is mapped into a bogus
+				 * slot (if slot > 1).
+				 */
+				clear_LDT();
+				return;
+			}
 
-	paravirt_alloc_ldt(newldt, mincount);
+			/*
+			 * If page table isolation is enabled, ldt->entries
+			 * will not be mapped in the userspace pagetables.
+			 * Tell the CPU to access the LDT through the alias
+			 * at ldt_slot_va(ldt->slot).
+			 */
+			set_ldt(ldt_slot_va(ldt->slot), ldt->nr_entries);
+		} else {
+			set_ldt(ldt->entries, ldt->nr_entries);
+		}
+	} else {
+		clear_LDT();
+	}
+}
+
+void switch_ldt(struct mm_struct *prev, struct mm_struct *next)
+{
+	/*
+	 * Load the LDT if either the old or new mm had an LDT.
+	 *
+	 * An mm will never go from having an LDT to not having an LDT.  Two
+	 * mms never share an LDT, so we don't gain anything by checking to
+	 * see whether the LDT changed.  There's also no guarantee that
+	 * prev->context.ldt actually matches LDTR, but, if LDTR is non-NULL,
+	 * then prev->context.ldt will also be non-NULL.
+	 *
+	 * If we really cared, we could optimize the case where prev == next
+	 * and we're exiting lazy mode.  Most of the time, if this happens,
+	 * we don't actually need to reload LDTR, but modify_ldt() is mostly
+	 * used by legacy code and emulators where we don't need this level of
+	 * performance.
+	 *
+	 * This uses | instead of || because it generates better code.
+	 */
+	if (unlikely((unsigned long)prev->context.ldt |
+		     (unsigned long)next->context.ldt))
+		load_mm_ldt(next);
 
+	DEBUG_LOCKS_WARN_ON(preemptible());
+}
+
+static void refresh_ldt_segments(void)
+{
 #ifdef CONFIG_X86_64
-	/* CHECKME: Do we really need this ? */
-	wmb();
-#endif
-	pc->ldt = newldt;
-	wmb();
-	pc->size = mincount;
-	wmb();
-
-	if (reload) {
-#ifdef CONFIG_SMP
-		preempt_disable();
-		load_LDT(pc);
-		if (!cpumask_equal(mm_cpumask(current->mm),
-				   cpumask_of(smp_processor_id())))
-			smp_call_function(flush_ldt, current->mm, 1);
-		preempt_enable();
-#else
-		load_LDT(pc);
+	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_ACCOUNT);
+	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 = __vmalloc(alloc_size, GFP_KERNEL_ACCOUNT | __GFP_ZERO);
+	else
+		new_ldt->entries = (void *)get_zeroed_page(GFP_KERNEL_ACCOUNT);
+
+	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;
+}
+
+#ifdef CONFIG_MITIGATION_PAGE_TABLE_ISOLATION
+
+static void do_sanity_check(struct mm_struct *mm,
+			    bool had_kernel_mapping,
+			    bool had_user_mapping)
+{
+	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_kernel_mapping);
+		if (boot_cpu_has(X86_FEATURE_PTI))
+			WARN_ON(!had_user_mapping);
+	} else {
+		/*
+		 * This is the first time we're mapping an LDT for this process.
+		 * Sync the pgd to the usermode tables.
+		 */
+		WARN_ON(had_kernel_mapping);
+		if (boot_cpu_has(X86_FEATURE_PTI))
+			WARN_ON(had_user_mapping);
 	}
-	if (oldsize) {
-		paravirt_free_ldt(oldldt, oldsize);
-		if (oldsize * LDT_ENTRY_SIZE > PAGE_SIZE)
-			vfree(oldldt);
-		else
-			put_page(virt_to_page(oldldt));
+}
+
+#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)
+{
+	pgd_t *pgd = pgd_offset(mm, LDT_BASE_ADDR);
+
+	if (boot_cpu_has(X86_FEATURE_PTI) && !mm->context.ldt)
+		set_pgd(kernel_to_user_pgdp(pgd), *pgd);
+}
+
+static void sanity_check_ldt_mapping(struct mm_struct *mm)
+{
+	pgd_t *pgd = pgd_offset(mm, LDT_BASE_ADDR);
+	bool had_kernel = (pgd->pgd != 0);
+	bool had_user   = (kernel_to_user_pgdp(pgd)->pgd != 0);
+
+	do_sanity_check(mm, had_kernel, had_user);
+}
+
+#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)
+{
+	unsigned long va;
+	bool is_vmalloc;
+	spinlock_t *ptl;
+	int i, nr_pages;
+
+	if (!boot_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);
+
+	/* Check if the current mappings are sane */
+	sanity_check_ldt_mapping(mm);
+
+	is_vmalloc = is_vmalloc_addr(ldt->entries);
+
+	nr_pages = DIV_ROUND_UP(ldt->nr_entries * LDT_ENTRY_SIZE, PAGE_SIZE);
+
+	for (i = 0; i < nr_pages; 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);
 	}
+
+	/* Propagate LDT mapping to the user page-table */
+	map_ldt_struct_to_user(mm);
+
+	ldt->slot = slot;
 	return 0;
 }
 
-static inline int copy_ldt(mm_context_t *new, mm_context_t *old)
+static void unmap_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt)
 {
-	int err = alloc_ldt(new, old->size, 0);
-	int i;
+	unsigned long va;
+	int i, nr_pages;
+
+	if (!ldt)
+		return;
 
-	if (err < 0)
-		return err;
+	/* LDT map/unmap is only required for PTI */
+	if (!boot_cpu_has(X86_FEATURE_PTI))
+		return;
 
-	for (i = 0; i < old->size; i++)
-		write_ldt_entry(new->ldt, i, old->ldt + i * LDT_ENTRY_SIZE);
+	nr_pages = DIV_ROUND_UP(ldt->nr_entries * LDT_ENTRY_SIZE, PAGE_SIZE);
+
+	for (i = 0; i < nr_pages; i++) {
+		unsigned long offset = i << PAGE_SHIFT;
+		spinlock_t *ptl;
+		pte_t *ptep;
+
+		va = (unsigned long)ldt_slot_va(ldt->slot) + offset;
+		ptep = get_locked_pte(mm, va, &ptl);
+		if (!WARN_ON_ONCE(!ptep)) {
+			pte_clear(mm, va, ptep);
+			pte_unmap_unlock(ptep, ptl);
+		}
+	}
+
+	va = (unsigned long)ldt_slot_va(ldt->slot);
+	flush_tlb_mm_range(mm, va, va + nr_pages * PAGE_SIZE, PAGE_SHIFT, false);
+}
+
+#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)
+{
+#ifdef CONFIG_MITIGATION_PAGE_TABLE_ISOLATION
+	struct mmu_gather tlb;
+	unsigned long start = LDT_BASE_ADDR;
+	unsigned long end = LDT_END_ADDR;
+
+	if (!boot_cpu_has(X86_FEATURE_PTI))
+		return;
+
+	/*
+	 * Although free_pgd_range() is intended for freeing user
+	 * page-tables, it also works out for kernel mappings on x86.
+	 * We use tlb_gather_mmu_fullmm() to avoid confusing the
+	 * range-tracking logic in __tlb_adjust_range().
+	 */
+	tlb_gather_mmu_fullmm(&tlb, mm);
+	free_pgd_range(&tlb, start, end, start, end);
+	tlb_finish_mmu(&tlb);
+#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);
+}
+
 /*
- * we do not have to muck with descriptors here, that is
- * done in switch_mm() as needed.
+ * 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 init_new_context(struct task_struct *tsk, struct mm_struct *mm)
+int ldt_dup_context(struct mm_struct *old_mm, struct mm_struct *mm)
 {
-	struct mm_struct *old_mm;
+	struct ldt_struct *new_ldt;
 	int retval = 0;
 
-	mutex_init(&mm->context.lock);
-	mm->context.size = 0;
-	old_mm = current->mm;
-	if (old_mm && old_mm->context.size > 0) {
-		mutex_lock(&old_mm->context.lock);
-		retval = copy_ldt(&mm->context, &old_mm->context);
-		mutex_unlock(&old_mm->context.lock);
+	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;
 }
 
@@ -123,55 +488,54 @@ int init_new_context(struct task_struct *tsk, struct mm_struct *mm)
  *
  * 64bit: Don't touch the LDT register - we're already in the next thread.
  */
-void destroy_context(struct mm_struct *mm)
+void destroy_context_ldt(struct mm_struct *mm)
 {
-	if (mm->context.size) {
-#ifdef CONFIG_X86_32
-		/* CHECKME: Can this ever happen ? */
-		if (mm == current->active_mm)
-			clear_LDT();
-#endif
-		paravirt_free_ldt(mm->context.ldt, mm->context.size);
-		if (mm->context.size * LDT_ENTRY_SIZE > PAGE_SIZE)
-			vfree(mm->context.ldt);
-		else
-			put_page(virt_to_page(mm->context.ldt));
-		mm->context.size = 0;
-	}
+	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)
 {
-	int err;
-	unsigned long size;
 	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 (!mm->context.size)
-		return 0;
 	if (bytecount > LDT_ENTRY_SIZE * LDT_ENTRIES)
 		bytecount = LDT_ENTRY_SIZE * LDT_ENTRIES;
 
-	mutex_lock(&mm->context.lock);
-	size = mm->context.size * LDT_ENTRY_SIZE;
-	if (size > bytecount)
-		size = bytecount;
+	entries_size = mm->context.ldt->nr_entries * LDT_ENTRY_SIZE;
+	if (entries_size > bytecount)
+		entries_size = bytecount;
 
-	err = 0;
-	if (copy_to_user(ptr, mm->context.ldt, size))
-		err = -EFAULT;
-	mutex_unlock(&mm->context.lock);
-	if (err < 0)
-		goto error_return;
-	if (size != bytecount) {
-		/* zero-fill the rest */
-		if (clear_user(ptr + size, bytecount - size) != 0) {
-			err = -EFAULT;
-			goto error_return;
+	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;
 		}
 	}
-	return bytecount;
-error_return:
-	return err;
+	retval = bytecount;
+
+out_unlock:
+	up_read(&mm->context.ldt_usr_sem);
+	return retval;
 }
 
 static int read_default_ldt(void __user *ptr, unsigned long bytecount)
@@ -189,12 +553,36 @@ static int read_default_ldt(void __user *ptr, unsigned long bytecount)
 	return bytecount;
 }
 
+static bool allow_16bit_segments(void)
+{
+	if (!IS_ENABLED(CONFIG_X86_16BIT))
+		return false;
+
+#ifdef CONFIG_XEN_PV
+	/*
+	 * Xen PV does not implement ESPFIX64, which means that 16-bit
+	 * segments will not work correctly.  Until either Xen PV implements
+	 * ESPFIX64 and can signal this fact to the guest or unless someone
+	 * provides compelling evidence that allowing broken 16-bit segments
+	 * is worthwhile, disallow 16-bit segments under Xen PV.
+	 */
+	if (xen_pv_domain()) {
+		pr_info_once("Warning: 16-bit segments do not work correctly in a Xen PV guest\n");
+		return false;
+	}
+#endif
+
+	return true;
+}
+
 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;
-	struct user_desc ldt_info;
 
 	error = -EINVAL;
 	if (bytecount != sizeof(ldt_info))
@@ -213,39 +601,71 @@ static int write_ldt(void __user *ptr, unsigned long bytecount, int oldmode)
 			goto out;
 	}
 
-	mutex_lock(&mm->context.lock);
-	if (ldt_info.entry_number >= mm->context.size) {
-		error = alloc_ldt(&current->mm->context,
-				  ldt_info.entry_number + 1, 1);
-		if (error < 0)
-			goto out_unlock;
-	}
-
-	/* Allow LDTs to be cleared by the user. */
-	if (ldt_info.base_addr == 0 && ldt_info.limit == 0) {
-		if (oldmode || LDT_empty(&ldt_info)) {
-			memset(&ldt, 0, sizeof(ldt));
-			goto install;
+	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 (!ldt_info.seg_32bit && !allow_16bit_segments()) {
+			error = -EINVAL;
+			goto out;
 		}
+
+		fill_ldt(&ldt, &ldt_info);
+		if (oldmode)
+			ldt.avl = 0;
 	}
 
-	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;
 
-	/* Install the new entry ...  */
-install:
-	write_ldt_entry(mm->context.ldt, ldt_info.entry_number, &ldt);
+	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);
+	unmap_ldt_struct(mm, old_ldt);
+	free_ldt_struct(old_ldt);
 	error = 0;
 
 out_unlock:
-	mutex_unlock(&mm->context.lock);
+	up_write(&mm->context.ldt_usr_sem);
 out:
 	return error;
 }
 
-asmlinkage int sys_modify_ldt(int func, void __user *ptr,
-			      unsigned long bytecount)
+SYSCALL_DEFINE3(modify_ldt, int , func , void __user * , ptr ,
+		unsigned long , bytecount)
 {
 	int ret = -ENOSYS;
 
@@ -263,5 +683,14 @@ asmlinkage int sys_modify_ldt(int func, void __user *ptr,
 		ret = write_ldt(ptr, bytecount, 0);
 		break;
 	}
-	return ret;
+	/*
+	 * The SYSCALL_DEFINE() macros give us an 'unsigned long'
+	 * return type, but the 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;
 }