xref: /openbmc/linux/arch/x86/kernel/ldt.c (revision f5c27da4)
1  // SPDX-License-Identifier: GPL-2.0
2  /*
3   * Copyright (C) 1992 Krishna Balasubramanian and Linus Torvalds
4   * Copyright (C) 1999 Ingo Molnar <mingo@redhat.com>
5   * Copyright (C) 2002 Andi Kleen
6   *
7   * This handles calls from both 32bit and 64bit mode.
8   *
9   * Lock order:
10   *	contex.ldt_usr_sem
11   *	  mmap_lock
12   *	    context.lock
13   */
14  
15  #include <linux/errno.h>
16  #include <linux/gfp.h>
17  #include <linux/sched.h>
18  #include <linux/string.h>
19  #include <linux/mm.h>
20  #include <linux/smp.h>
21  #include <linux/syscalls.h>
22  #include <linux/slab.h>
23  #include <linux/vmalloc.h>
24  #include <linux/uaccess.h>
25  
26  #include <asm/ldt.h>
27  #include <asm/tlb.h>
28  #include <asm/desc.h>
29  #include <asm/mmu_context.h>
30  #include <asm/pgtable_areas.h>
31  
32  #include <xen/xen.h>
33  
34  /* This is a multiple of PAGE_SIZE. */
35  #define LDT_SLOT_STRIDE (LDT_ENTRIES * LDT_ENTRY_SIZE)
36  
37  static inline void *ldt_slot_va(int slot)
38  {
39  	return (void *)(LDT_BASE_ADDR + LDT_SLOT_STRIDE * slot);
40  }
41  
42  void load_mm_ldt(struct mm_struct *mm)
43  {
44  	struct ldt_struct *ldt;
45  
46  	/* READ_ONCE synchronizes with smp_store_release */
47  	ldt = READ_ONCE(mm->context.ldt);
48  
49  	/*
50  	 * Any change to mm->context.ldt is followed by an IPI to all
51  	 * CPUs with the mm active.  The LDT will not be freed until
52  	 * after the IPI is handled by all such CPUs.  This means that,
53  	 * if the ldt_struct changes before we return, the values we see
54  	 * will be safe, and the new values will be loaded before we run
55  	 * any user code.
56  	 *
57  	 * NB: don't try to convert this to use RCU without extreme care.
58  	 * We would still need IRQs off, because we don't want to change
59  	 * the local LDT after an IPI loaded a newer value than the one
60  	 * that we can see.
61  	 */
62  
63  	if (unlikely(ldt)) {
64  		if (static_cpu_has(X86_FEATURE_PTI)) {
65  			if (WARN_ON_ONCE((unsigned long)ldt->slot > 1)) {
66  				/*
67  				 * Whoops -- either the new LDT isn't mapped
68  				 * (if slot == -1) or is mapped into a bogus
69  				 * slot (if slot > 1).
70  				 */
71  				clear_LDT();
72  				return;
73  			}
74  
75  			/*
76  			 * If page table isolation is enabled, ldt->entries
77  			 * will not be mapped in the userspace pagetables.
78  			 * Tell the CPU to access the LDT through the alias
79  			 * at ldt_slot_va(ldt->slot).
80  			 */
81  			set_ldt(ldt_slot_va(ldt->slot), ldt->nr_entries);
82  		} else {
83  			set_ldt(ldt->entries, ldt->nr_entries);
84  		}
85  	} else {
86  		clear_LDT();
87  	}
88  }
89  
90  void switch_ldt(struct mm_struct *prev, struct mm_struct *next)
91  {
92  	/*
93  	 * Load the LDT if either the old or new mm had an LDT.
94  	 *
95  	 * An mm will never go from having an LDT to not having an LDT.  Two
96  	 * mms never share an LDT, so we don't gain anything by checking to
97  	 * see whether the LDT changed.  There's also no guarantee that
98  	 * prev->context.ldt actually matches LDTR, but, if LDTR is non-NULL,
99  	 * then prev->context.ldt will also be non-NULL.
100  	 *
101  	 * If we really cared, we could optimize the case where prev == next
102  	 * and we're exiting lazy mode.  Most of the time, if this happens,
103  	 * we don't actually need to reload LDTR, but modify_ldt() is mostly
104  	 * used by legacy code and emulators where we don't need this level of
105  	 * performance.
106  	 *
107  	 * This uses | instead of || because it generates better code.
108  	 */
109  	if (unlikely((unsigned long)prev->context.ldt |
110  		     (unsigned long)next->context.ldt))
111  		load_mm_ldt(next);
112  
113  	DEBUG_LOCKS_WARN_ON(preemptible());
114  }
115  
116  static void refresh_ldt_segments(void)
117  {
118  #ifdef CONFIG_X86_64
119  	unsigned short sel;
120  
121  	/*
122  	 * Make sure that the cached DS and ES descriptors match the updated
123  	 * LDT.
124  	 */
125  	savesegment(ds, sel);
126  	if ((sel & SEGMENT_TI_MASK) == SEGMENT_LDT)
127  		loadsegment(ds, sel);
128  
129  	savesegment(es, sel);
130  	if ((sel & SEGMENT_TI_MASK) == SEGMENT_LDT)
131  		loadsegment(es, sel);
132  #endif
133  }
134  
135  /* context.lock is held by the task which issued the smp function call */
136  static void flush_ldt(void *__mm)
137  {
138  	struct mm_struct *mm = __mm;
139  
140  	if (this_cpu_read(cpu_tlbstate.loaded_mm) != mm)
141  		return;
142  
143  	load_mm_ldt(mm);
144  
145  	refresh_ldt_segments();
146  }
147  
148  /* The caller must call finalize_ldt_struct on the result. LDT starts zeroed. */
149  static struct ldt_struct *alloc_ldt_struct(unsigned int num_entries)
150  {
151  	struct ldt_struct *new_ldt;
152  	unsigned int alloc_size;
153  
154  	if (num_entries > LDT_ENTRIES)
155  		return NULL;
156  
157  	new_ldt = kmalloc(sizeof(struct ldt_struct), GFP_KERNEL_ACCOUNT);
158  	if (!new_ldt)
159  		return NULL;
160  
161  	BUILD_BUG_ON(LDT_ENTRY_SIZE != sizeof(struct desc_struct));
162  	alloc_size = num_entries * LDT_ENTRY_SIZE;
163  
164  	/*
165  	 * Xen is very picky: it requires a page-aligned LDT that has no
166  	 * trailing nonzero bytes in any page that contains LDT descriptors.
167  	 * Keep it simple: zero the whole allocation and never allocate less
168  	 * than PAGE_SIZE.
169  	 */
170  	if (alloc_size > PAGE_SIZE)
171  		new_ldt->entries = __vmalloc(alloc_size, GFP_KERNEL_ACCOUNT | __GFP_ZERO);
172  	else
173  		new_ldt->entries = (void *)get_zeroed_page(GFP_KERNEL_ACCOUNT);
174  
175  	if (!new_ldt->entries) {
176  		kfree(new_ldt);
177  		return NULL;
178  	}
179  
180  	/* The new LDT isn't aliased for PTI yet. */
181  	new_ldt->slot = -1;
182  
183  	new_ldt->nr_entries = num_entries;
184  	return new_ldt;
185  }
186  
187  #ifdef CONFIG_PAGE_TABLE_ISOLATION
188  
189  static void do_sanity_check(struct mm_struct *mm,
190  			    bool had_kernel_mapping,
191  			    bool had_user_mapping)
192  {
193  	if (mm->context.ldt) {
194  		/*
195  		 * We already had an LDT.  The top-level entry should already
196  		 * have been allocated and synchronized with the usermode
197  		 * tables.
198  		 */
199  		WARN_ON(!had_kernel_mapping);
200  		if (boot_cpu_has(X86_FEATURE_PTI))
201  			WARN_ON(!had_user_mapping);
202  	} else {
203  		/*
204  		 * This is the first time we're mapping an LDT for this process.
205  		 * Sync the pgd to the usermode tables.
206  		 */
207  		WARN_ON(had_kernel_mapping);
208  		if (boot_cpu_has(X86_FEATURE_PTI))
209  			WARN_ON(had_user_mapping);
210  	}
211  }
212  
213  #ifdef CONFIG_X86_PAE
214  
215  static pmd_t *pgd_to_pmd_walk(pgd_t *pgd, unsigned long va)
216  {
217  	p4d_t *p4d;
218  	pud_t *pud;
219  
220  	if (pgd->pgd == 0)
221  		return NULL;
222  
223  	p4d = p4d_offset(pgd, va);
224  	if (p4d_none(*p4d))
225  		return NULL;
226  
227  	pud = pud_offset(p4d, va);
228  	if (pud_none(*pud))
229  		return NULL;
230  
231  	return pmd_offset(pud, va);
232  }
233  
234  static void map_ldt_struct_to_user(struct mm_struct *mm)
235  {
236  	pgd_t *k_pgd = pgd_offset(mm, LDT_BASE_ADDR);
237  	pgd_t *u_pgd = kernel_to_user_pgdp(k_pgd);
238  	pmd_t *k_pmd, *u_pmd;
239  
240  	k_pmd = pgd_to_pmd_walk(k_pgd, LDT_BASE_ADDR);
241  	u_pmd = pgd_to_pmd_walk(u_pgd, LDT_BASE_ADDR);
242  
243  	if (boot_cpu_has(X86_FEATURE_PTI) && !mm->context.ldt)
244  		set_pmd(u_pmd, *k_pmd);
245  }
246  
247  static void sanity_check_ldt_mapping(struct mm_struct *mm)
248  {
249  	pgd_t *k_pgd = pgd_offset(mm, LDT_BASE_ADDR);
250  	pgd_t *u_pgd = kernel_to_user_pgdp(k_pgd);
251  	bool had_kernel, had_user;
252  	pmd_t *k_pmd, *u_pmd;
253  
254  	k_pmd      = pgd_to_pmd_walk(k_pgd, LDT_BASE_ADDR);
255  	u_pmd      = pgd_to_pmd_walk(u_pgd, LDT_BASE_ADDR);
256  	had_kernel = (k_pmd->pmd != 0);
257  	had_user   = (u_pmd->pmd != 0);
258  
259  	do_sanity_check(mm, had_kernel, had_user);
260  }
261  
262  #else /* !CONFIG_X86_PAE */
263  
264  static void map_ldt_struct_to_user(struct mm_struct *mm)
265  {
266  	pgd_t *pgd = pgd_offset(mm, LDT_BASE_ADDR);
267  
268  	if (boot_cpu_has(X86_FEATURE_PTI) && !mm->context.ldt)
269  		set_pgd(kernel_to_user_pgdp(pgd), *pgd);
270  }
271  
272  static void sanity_check_ldt_mapping(struct mm_struct *mm)
273  {
274  	pgd_t *pgd = pgd_offset(mm, LDT_BASE_ADDR);
275  	bool had_kernel = (pgd->pgd != 0);
276  	bool had_user   = (kernel_to_user_pgdp(pgd)->pgd != 0);
277  
278  	do_sanity_check(mm, had_kernel, had_user);
279  }
280  
281  #endif /* CONFIG_X86_PAE */
282  
283  /*
284   * If PTI is enabled, this maps the LDT into the kernelmode and
285   * usermode tables for the given mm.
286   */
287  static int
288  map_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt, int slot)
289  {
290  	unsigned long va;
291  	bool is_vmalloc;
292  	spinlock_t *ptl;
293  	int i, nr_pages;
294  
295  	if (!boot_cpu_has(X86_FEATURE_PTI))
296  		return 0;
297  
298  	/*
299  	 * Any given ldt_struct should have map_ldt_struct() called at most
300  	 * once.
301  	 */
302  	WARN_ON(ldt->slot != -1);
303  
304  	/* Check if the current mappings are sane */
305  	sanity_check_ldt_mapping(mm);
306  
307  	is_vmalloc = is_vmalloc_addr(ldt->entries);
308  
309  	nr_pages = DIV_ROUND_UP(ldt->nr_entries * LDT_ENTRY_SIZE, PAGE_SIZE);
310  
311  	for (i = 0; i < nr_pages; i++) {
312  		unsigned long offset = i << PAGE_SHIFT;
313  		const void *src = (char *)ldt->entries + offset;
314  		unsigned long pfn;
315  		pgprot_t pte_prot;
316  		pte_t pte, *ptep;
317  
318  		va = (unsigned long)ldt_slot_va(slot) + offset;
319  		pfn = is_vmalloc ? vmalloc_to_pfn(src) :
320  			page_to_pfn(virt_to_page(src));
321  		/*
322  		 * Treat the PTI LDT range as a *userspace* range.
323  		 * get_locked_pte() will allocate all needed pagetables
324  		 * and account for them in this mm.
325  		 */
326  		ptep = get_locked_pte(mm, va, &ptl);
327  		if (!ptep)
328  			return -ENOMEM;
329  		/*
330  		 * Map it RO so the easy to find address is not a primary
331  		 * target via some kernel interface which misses a
332  		 * permission check.
333  		 */
334  		pte_prot = __pgprot(__PAGE_KERNEL_RO & ~_PAGE_GLOBAL);
335  		/* Filter out unsuppored __PAGE_KERNEL* bits: */
336  		pgprot_val(pte_prot) &= __supported_pte_mask;
337  		pte = pfn_pte(pfn, pte_prot);
338  		set_pte_at(mm, va, ptep, pte);
339  		pte_unmap_unlock(ptep, ptl);
340  	}
341  
342  	/* Propagate LDT mapping to the user page-table */
343  	map_ldt_struct_to_user(mm);
344  
345  	ldt->slot = slot;
346  	return 0;
347  }
348  
349  static void unmap_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt)
350  {
351  	unsigned long va;
352  	int i, nr_pages;
353  
354  	if (!ldt)
355  		return;
356  
357  	/* LDT map/unmap is only required for PTI */
358  	if (!boot_cpu_has(X86_FEATURE_PTI))
359  		return;
360  
361  	nr_pages = DIV_ROUND_UP(ldt->nr_entries * LDT_ENTRY_SIZE, PAGE_SIZE);
362  
363  	for (i = 0; i < nr_pages; i++) {
364  		unsigned long offset = i << PAGE_SHIFT;
365  		spinlock_t *ptl;
366  		pte_t *ptep;
367  
368  		va = (unsigned long)ldt_slot_va(ldt->slot) + offset;
369  		ptep = get_locked_pte(mm, va, &ptl);
370  		pte_clear(mm, va, ptep);
371  		pte_unmap_unlock(ptep, ptl);
372  	}
373  
374  	va = (unsigned long)ldt_slot_va(ldt->slot);
375  	flush_tlb_mm_range(mm, va, va + nr_pages * PAGE_SIZE, PAGE_SHIFT, false);
376  }
377  
378  #else /* !CONFIG_PAGE_TABLE_ISOLATION */
379  
380  static int
381  map_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt, int slot)
382  {
383  	return 0;
384  }
385  
386  static void unmap_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt)
387  {
388  }
389  #endif /* CONFIG_PAGE_TABLE_ISOLATION */
390  
391  static void free_ldt_pgtables(struct mm_struct *mm)
392  {
393  #ifdef CONFIG_PAGE_TABLE_ISOLATION
394  	struct mmu_gather tlb;
395  	unsigned long start = LDT_BASE_ADDR;
396  	unsigned long end = LDT_END_ADDR;
397  
398  	if (!boot_cpu_has(X86_FEATURE_PTI))
399  		return;
400  
401  	/*
402  	 * Although free_pgd_range() is intended for freeing user
403  	 * page-tables, it also works out for kernel mappings on x86.
404  	 * We use tlb_gather_mmu_fullmm() to avoid confusing the
405  	 * range-tracking logic in __tlb_adjust_range().
406  	 */
407  	tlb_gather_mmu_fullmm(&tlb, mm);
408  	free_pgd_range(&tlb, start, end, start, end);
409  	tlb_finish_mmu(&tlb);
410  #endif
411  }
412  
413  /* After calling this, the LDT is immutable. */
414  static void finalize_ldt_struct(struct ldt_struct *ldt)
415  {
416  	paravirt_alloc_ldt(ldt->entries, ldt->nr_entries);
417  }
418  
419  static void install_ldt(struct mm_struct *mm, struct ldt_struct *ldt)
420  {
421  	mutex_lock(&mm->context.lock);
422  
423  	/* Synchronizes with READ_ONCE in load_mm_ldt. */
424  	smp_store_release(&mm->context.ldt, ldt);
425  
426  	/* Activate the LDT for all CPUs using currents mm. */
427  	on_each_cpu_mask(mm_cpumask(mm), flush_ldt, mm, true);
428  
429  	mutex_unlock(&mm->context.lock);
430  }
431  
432  static void free_ldt_struct(struct ldt_struct *ldt)
433  {
434  	if (likely(!ldt))
435  		return;
436  
437  	paravirt_free_ldt(ldt->entries, ldt->nr_entries);
438  	if (ldt->nr_entries * LDT_ENTRY_SIZE > PAGE_SIZE)
439  		vfree_atomic(ldt->entries);
440  	else
441  		free_page((unsigned long)ldt->entries);
442  	kfree(ldt);
443  }
444  
445  /*
446   * Called on fork from arch_dup_mmap(). Just copy the current LDT state,
447   * the new task is not running, so nothing can be installed.
448   */
449  int ldt_dup_context(struct mm_struct *old_mm, struct mm_struct *mm)
450  {
451  	struct ldt_struct *new_ldt;
452  	int retval = 0;
453  
454  	if (!old_mm)
455  		return 0;
456  
457  	mutex_lock(&old_mm->context.lock);
458  	if (!old_mm->context.ldt)
459  		goto out_unlock;
460  
461  	new_ldt = alloc_ldt_struct(old_mm->context.ldt->nr_entries);
462  	if (!new_ldt) {
463  		retval = -ENOMEM;
464  		goto out_unlock;
465  	}
466  
467  	memcpy(new_ldt->entries, old_mm->context.ldt->entries,
468  	       new_ldt->nr_entries * LDT_ENTRY_SIZE);
469  	finalize_ldt_struct(new_ldt);
470  
471  	retval = map_ldt_struct(mm, new_ldt, 0);
472  	if (retval) {
473  		free_ldt_pgtables(mm);
474  		free_ldt_struct(new_ldt);
475  		goto out_unlock;
476  	}
477  	mm->context.ldt = new_ldt;
478  
479  out_unlock:
480  	mutex_unlock(&old_mm->context.lock);
481  	return retval;
482  }
483  
484  /*
485   * No need to lock the MM as we are the last user
486   *
487   * 64bit: Don't touch the LDT register - we're already in the next thread.
488   */
489  void destroy_context_ldt(struct mm_struct *mm)
490  {
491  	free_ldt_struct(mm->context.ldt);
492  	mm->context.ldt = NULL;
493  }
494  
495  void ldt_arch_exit_mmap(struct mm_struct *mm)
496  {
497  	free_ldt_pgtables(mm);
498  }
499  
500  static int read_ldt(void __user *ptr, unsigned long bytecount)
501  {
502  	struct mm_struct *mm = current->mm;
503  	unsigned long entries_size;
504  	int retval;
505  
506  	down_read(&mm->context.ldt_usr_sem);
507  
508  	if (!mm->context.ldt) {
509  		retval = 0;
510  		goto out_unlock;
511  	}
512  
513  	if (bytecount > LDT_ENTRY_SIZE * LDT_ENTRIES)
514  		bytecount = LDT_ENTRY_SIZE * LDT_ENTRIES;
515  
516  	entries_size = mm->context.ldt->nr_entries * LDT_ENTRY_SIZE;
517  	if (entries_size > bytecount)
518  		entries_size = bytecount;
519  
520  	if (copy_to_user(ptr, mm->context.ldt->entries, entries_size)) {
521  		retval = -EFAULT;
522  		goto out_unlock;
523  	}
524  
525  	if (entries_size != bytecount) {
526  		/* Zero-fill the rest and pretend we read bytecount bytes. */
527  		if (clear_user(ptr + entries_size, bytecount - entries_size)) {
528  			retval = -EFAULT;
529  			goto out_unlock;
530  		}
531  	}
532  	retval = bytecount;
533  
534  out_unlock:
535  	up_read(&mm->context.ldt_usr_sem);
536  	return retval;
537  }
538  
539  static int read_default_ldt(void __user *ptr, unsigned long bytecount)
540  {
541  	/* CHECKME: Can we use _one_ random number ? */
542  #ifdef CONFIG_X86_32
543  	unsigned long size = 5 * sizeof(struct desc_struct);
544  #else
545  	unsigned long size = 128;
546  #endif
547  	if (bytecount > size)
548  		bytecount = size;
549  	if (clear_user(ptr, bytecount))
550  		return -EFAULT;
551  	return bytecount;
552  }
553  
554  static bool allow_16bit_segments(void)
555  {
556  	if (!IS_ENABLED(CONFIG_X86_16BIT))
557  		return false;
558  
559  #ifdef CONFIG_XEN_PV
560  	/*
561  	 * Xen PV does not implement ESPFIX64, which means that 16-bit
562  	 * segments will not work correctly.  Until either Xen PV implements
563  	 * ESPFIX64 and can signal this fact to the guest or unless someone
564  	 * provides compelling evidence that allowing broken 16-bit segments
565  	 * is worthwhile, disallow 16-bit segments under Xen PV.
566  	 */
567  	if (xen_pv_domain()) {
568  		pr_info_once("Warning: 16-bit segments do not work correctly in a Xen PV guest\n");
569  		return false;
570  	}
571  #endif
572  
573  	return true;
574  }
575  
576  static int write_ldt(void __user *ptr, unsigned long bytecount, int oldmode)
577  {
578  	struct mm_struct *mm = current->mm;
579  	struct ldt_struct *new_ldt, *old_ldt;
580  	unsigned int old_nr_entries, new_nr_entries;
581  	struct user_desc ldt_info;
582  	struct desc_struct ldt;
583  	int error;
584  
585  	error = -EINVAL;
586  	if (bytecount != sizeof(ldt_info))
587  		goto out;
588  	error = -EFAULT;
589  	if (copy_from_user(&ldt_info, ptr, sizeof(ldt_info)))
590  		goto out;
591  
592  	error = -EINVAL;
593  	if (ldt_info.entry_number >= LDT_ENTRIES)
594  		goto out;
595  	if (ldt_info.contents == 3) {
596  		if (oldmode)
597  			goto out;
598  		if (ldt_info.seg_not_present == 0)
599  			goto out;
600  	}
601  
602  	if ((oldmode && !ldt_info.base_addr && !ldt_info.limit) ||
603  	    LDT_empty(&ldt_info)) {
604  		/* The user wants to clear the entry. */
605  		memset(&ldt, 0, sizeof(ldt));
606  	} else {
607  		if (!ldt_info.seg_32bit && !allow_16bit_segments()) {
608  			error = -EINVAL;
609  			goto out;
610  		}
611  
612  		fill_ldt(&ldt, &ldt_info);
613  		if (oldmode)
614  			ldt.avl = 0;
615  	}
616  
617  	if (down_write_killable(&mm->context.ldt_usr_sem))
618  		return -EINTR;
619  
620  	old_ldt       = mm->context.ldt;
621  	old_nr_entries = old_ldt ? old_ldt->nr_entries : 0;
622  	new_nr_entries = max(ldt_info.entry_number + 1, old_nr_entries);
623  
624  	error = -ENOMEM;
625  	new_ldt = alloc_ldt_struct(new_nr_entries);
626  	if (!new_ldt)
627  		goto out_unlock;
628  
629  	if (old_ldt)
630  		memcpy(new_ldt->entries, old_ldt->entries, old_nr_entries * LDT_ENTRY_SIZE);
631  
632  	new_ldt->entries[ldt_info.entry_number] = ldt;
633  	finalize_ldt_struct(new_ldt);
634  
635  	/*
636  	 * If we are using PTI, map the new LDT into the userspace pagetables.
637  	 * If there is already an LDT, use the other slot so that other CPUs
638  	 * will continue to use the old LDT until install_ldt() switches
639  	 * them over to the new LDT.
640  	 */
641  	error = map_ldt_struct(mm, new_ldt, old_ldt ? !old_ldt->slot : 0);
642  	if (error) {
643  		/*
644  		 * This only can fail for the first LDT setup. If an LDT is
645  		 * already installed then the PTE page is already
646  		 * populated. Mop up a half populated page table.
647  		 */
648  		if (!WARN_ON_ONCE(old_ldt))
649  			free_ldt_pgtables(mm);
650  		free_ldt_struct(new_ldt);
651  		goto out_unlock;
652  	}
653  
654  	install_ldt(mm, new_ldt);
655  	unmap_ldt_struct(mm, old_ldt);
656  	free_ldt_struct(old_ldt);
657  	error = 0;
658  
659  out_unlock:
660  	up_write(&mm->context.ldt_usr_sem);
661  out:
662  	return error;
663  }
664  
665  SYSCALL_DEFINE3(modify_ldt, int , func , void __user * , ptr ,
666  		unsigned long , bytecount)
667  {
668  	int ret = -ENOSYS;
669  
670  	switch (func) {
671  	case 0:
672  		ret = read_ldt(ptr, bytecount);
673  		break;
674  	case 1:
675  		ret = write_ldt(ptr, bytecount, 1);
676  		break;
677  	case 2:
678  		ret = read_default_ldt(ptr, bytecount);
679  		break;
680  	case 0x11:
681  		ret = write_ldt(ptr, bytecount, 0);
682  		break;
683  	}
684  	/*
685  	 * The SYSCALL_DEFINE() macros give us an 'unsigned long'
686  	 * return type, but tht ABI for sys_modify_ldt() expects
687  	 * 'int'.  This cast gives us an int-sized value in %rax
688  	 * for the return code.  The 'unsigned' is necessary so
689  	 * the compiler does not try to sign-extend the negative
690  	 * return codes into the high half of the register when
691  	 * taking the value from int->long.
692  	 */
693  	return (unsigned int)ret;
694  }
695