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Linux/arch/x86/kernel/ldt.c

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  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_sem
 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/syscalls.h>
 31 
 32 static void refresh_ldt_segments(void)
 33 {
 34 #ifdef CONFIG_X86_64
 35         unsigned short sel;
 36 
 37         /*
 38          * Make sure that the cached DS and ES descriptors match the updated
 39          * LDT.
 40          */
 41         savesegment(ds, sel);
 42         if ((sel & SEGMENT_TI_MASK) == SEGMENT_LDT)
 43                 loadsegment(ds, sel);
 44 
 45         savesegment(es, sel);
 46         if ((sel & SEGMENT_TI_MASK) == SEGMENT_LDT)
 47                 loadsegment(es, sel);
 48 #endif
 49 }
 50 
 51 /* context.lock is held by the task which issued the smp function call */
 52 static void flush_ldt(void *__mm)
 53 {
 54         struct mm_struct *mm = __mm;
 55 
 56         if (this_cpu_read(cpu_tlbstate.loaded_mm) != mm)
 57                 return;
 58 
 59         load_mm_ldt(mm);
 60 
 61         refresh_ldt_segments();
 62 }
 63 
 64 /* The caller must call finalize_ldt_struct on the result. LDT starts zeroed. */
 65 static struct ldt_struct *alloc_ldt_struct(unsigned int num_entries)
 66 {
 67         struct ldt_struct *new_ldt;
 68         unsigned int alloc_size;
 69 
 70         if (num_entries > LDT_ENTRIES)
 71                 return NULL;
 72 
 73         new_ldt = kmalloc(sizeof(struct ldt_struct), GFP_KERNEL);
 74         if (!new_ldt)
 75                 return NULL;
 76 
 77         BUILD_BUG_ON(LDT_ENTRY_SIZE != sizeof(struct desc_struct));
 78         alloc_size = num_entries * LDT_ENTRY_SIZE;
 79 
 80         /*
 81          * Xen is very picky: it requires a page-aligned LDT that has no
 82          * trailing nonzero bytes in any page that contains LDT descriptors.
 83          * Keep it simple: zero the whole allocation and never allocate less
 84          * than PAGE_SIZE.
 85          */
 86         if (alloc_size > PAGE_SIZE)
 87                 new_ldt->entries = vzalloc(alloc_size);
 88         else
 89                 new_ldt->entries = (void *)get_zeroed_page(GFP_KERNEL);
 90 
 91         if (!new_ldt->entries) {
 92                 kfree(new_ldt);
 93                 return NULL;
 94         }
 95 
 96         /* The new LDT isn't aliased for PTI yet. */
 97         new_ldt->slot = -1;
 98 
 99         new_ldt->nr_entries = num_entries;
100         return new_ldt;
101 }
102 
103 #ifdef CONFIG_PAGE_TABLE_ISOLATION
104 
105 static void do_sanity_check(struct mm_struct *mm,
106                             bool had_kernel_mapping,
107                             bool had_user_mapping)
108 {
109         if (mm->context.ldt) {
110                 /*
111                  * We already had an LDT.  The top-level entry should already
112                  * have been allocated and synchronized with the usermode
113                  * tables.
114                  */
115                 WARN_ON(!had_kernel_mapping);
116                 if (static_cpu_has(X86_FEATURE_PTI))
117                         WARN_ON(!had_user_mapping);
118         } else {
119                 /*
120                  * This is the first time we're mapping an LDT for this process.
121                  * Sync the pgd to the usermode tables.
122                  */
123                 WARN_ON(had_kernel_mapping);
124                 if (static_cpu_has(X86_FEATURE_PTI))
125                         WARN_ON(had_user_mapping);
126         }
127 }
128 
129 #ifdef CONFIG_X86_PAE
130 
131 static pmd_t *pgd_to_pmd_walk(pgd_t *pgd, unsigned long va)
132 {
133         p4d_t *p4d;
134         pud_t *pud;
135 
136         if (pgd->pgd == 0)
137                 return NULL;
138 
139         p4d = p4d_offset(pgd, va);
140         if (p4d_none(*p4d))
141                 return NULL;
142 
143         pud = pud_offset(p4d, va);
144         if (pud_none(*pud))
145                 return NULL;
146 
147         return pmd_offset(pud, va);
148 }
149 
150 static void map_ldt_struct_to_user(struct mm_struct *mm)
151 {
152         pgd_t *k_pgd = pgd_offset(mm, LDT_BASE_ADDR);
153         pgd_t *u_pgd = kernel_to_user_pgdp(k_pgd);
154         pmd_t *k_pmd, *u_pmd;
155 
156         k_pmd = pgd_to_pmd_walk(k_pgd, LDT_BASE_ADDR);
157         u_pmd = pgd_to_pmd_walk(u_pgd, LDT_BASE_ADDR);
158 
159         if (static_cpu_has(X86_FEATURE_PTI) && !mm->context.ldt)
160                 set_pmd(u_pmd, *k_pmd);
161 }
162 
163 static void sanity_check_ldt_mapping(struct mm_struct *mm)
164 {
165         pgd_t *k_pgd = pgd_offset(mm, LDT_BASE_ADDR);
166         pgd_t *u_pgd = kernel_to_user_pgdp(k_pgd);
167         bool had_kernel, had_user;
168         pmd_t *k_pmd, *u_pmd;
169 
170         k_pmd      = pgd_to_pmd_walk(k_pgd, LDT_BASE_ADDR);
171         u_pmd      = pgd_to_pmd_walk(u_pgd, LDT_BASE_ADDR);
172         had_kernel = (k_pmd->pmd != 0);
173         had_user   = (u_pmd->pmd != 0);
174 
175         do_sanity_check(mm, had_kernel, had_user);
176 }
177 
178 #else /* !CONFIG_X86_PAE */
179 
180 static void map_ldt_struct_to_user(struct mm_struct *mm)
181 {
182         pgd_t *pgd = pgd_offset(mm, LDT_BASE_ADDR);
183 
184         if (static_cpu_has(X86_FEATURE_PTI) && !mm->context.ldt)
185                 set_pgd(kernel_to_user_pgdp(pgd), *pgd);
186 }
187 
188 static void sanity_check_ldt_mapping(struct mm_struct *mm)
189 {
190         pgd_t *pgd = pgd_offset(mm, LDT_BASE_ADDR);
191         bool had_kernel = (pgd->pgd != 0);
192         bool had_user   = (kernel_to_user_pgdp(pgd)->pgd != 0);
193 
194         do_sanity_check(mm, had_kernel, had_user);
195 }
196 
197 #endif /* CONFIG_X86_PAE */
198 
199 /*
200  * If PTI is enabled, this maps the LDT into the kernelmode and
201  * usermode tables for the given mm.
202  *
203  * There is no corresponding unmap function.  Even if the LDT is freed, we
204  * leave the PTEs around until the slot is reused or the mm is destroyed.
205  * This is harmless: the LDT is always in ordinary memory, and no one will
206  * access the freed slot.
207  *
208  * If we wanted to unmap freed LDTs, we'd also need to do a flush to make
209  * it useful, and the flush would slow down modify_ldt().
210  */
211 static int
212 map_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt, int slot)
213 {
214         unsigned long va;
215         bool is_vmalloc;
216         spinlock_t *ptl;
217         pgd_t *pgd;
218         int i;
219 
220         if (!static_cpu_has(X86_FEATURE_PTI))
221                 return 0;
222 
223         /*
224          * Any given ldt_struct should have map_ldt_struct() called at most
225          * once.
226          */
227         WARN_ON(ldt->slot != -1);
228 
229         /* Check if the current mappings are sane */
230         sanity_check_ldt_mapping(mm);
231 
232         /*
233          * Did we already have the top level entry allocated?  We can't
234          * use pgd_none() for this because it doens't do anything on
235          * 4-level page table kernels.
236          */
237         pgd = pgd_offset(mm, LDT_BASE_ADDR);
238 
239         is_vmalloc = is_vmalloc_addr(ldt->entries);
240 
241         for (i = 0; i * PAGE_SIZE < ldt->nr_entries * LDT_ENTRY_SIZE; i++) {
242                 unsigned long offset = i << PAGE_SHIFT;
243                 const void *src = (char *)ldt->entries + offset;
244                 unsigned long pfn;
245                 pgprot_t pte_prot;
246                 pte_t pte, *ptep;
247 
248                 va = (unsigned long)ldt_slot_va(slot) + offset;
249                 pfn = is_vmalloc ? vmalloc_to_pfn(src) :
250                         page_to_pfn(virt_to_page(src));
251                 /*
252                  * Treat the PTI LDT range as a *userspace* range.
253                  * get_locked_pte() will allocate all needed pagetables
254                  * and account for them in this mm.
255                  */
256                 ptep = get_locked_pte(mm, va, &ptl);
257                 if (!ptep)
258                         return -ENOMEM;
259                 /*
260                  * Map it RO so the easy to find address is not a primary
261                  * target via some kernel interface which misses a
262                  * permission check.
263                  */
264                 pte_prot = __pgprot(__PAGE_KERNEL_RO & ~_PAGE_GLOBAL);
265                 /* Filter out unsuppored __PAGE_KERNEL* bits: */
266                 pgprot_val(pte_prot) &= __supported_pte_mask;
267                 pte = pfn_pte(pfn, pte_prot);
268                 set_pte_at(mm, va, ptep, pte);
269                 pte_unmap_unlock(ptep, ptl);
270         }
271 
272         /* Propagate LDT mapping to the user page-table */
273         map_ldt_struct_to_user(mm);
274 
275         va = (unsigned long)ldt_slot_va(slot);
276         flush_tlb_mm_range(mm, va, va + LDT_SLOT_STRIDE, 0);
277 
278         ldt->slot = slot;
279         return 0;
280 }
281 
282 #else /* !CONFIG_PAGE_TABLE_ISOLATION */
283 
284 static int
285 map_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt, int slot)
286 {
287         return 0;
288 }
289 #endif /* CONFIG_PAGE_TABLE_ISOLATION */
290 
291 static void free_ldt_pgtables(struct mm_struct *mm)
292 {
293 #ifdef CONFIG_PAGE_TABLE_ISOLATION
294         struct mmu_gather tlb;
295         unsigned long start = LDT_BASE_ADDR;
296         unsigned long end = LDT_END_ADDR;
297 
298         if (!static_cpu_has(X86_FEATURE_PTI))
299                 return;
300 
301         tlb_gather_mmu(&tlb, mm, start, end);
302         free_pgd_range(&tlb, start, end, start, end);
303         tlb_finish_mmu(&tlb, start, end);
304 #endif
305 }
306 
307 /* After calling this, the LDT is immutable. */
308 static void finalize_ldt_struct(struct ldt_struct *ldt)
309 {
310         paravirt_alloc_ldt(ldt->entries, ldt->nr_entries);
311 }
312 
313 static void install_ldt(struct mm_struct *mm, struct ldt_struct *ldt)
314 {
315         mutex_lock(&mm->context.lock);
316 
317         /* Synchronizes with READ_ONCE in load_mm_ldt. */
318         smp_store_release(&mm->context.ldt, ldt);
319 
320         /* Activate the LDT for all CPUs using currents mm. */
321         on_each_cpu_mask(mm_cpumask(mm), flush_ldt, mm, true);
322 
323         mutex_unlock(&mm->context.lock);
324 }
325 
326 static void free_ldt_struct(struct ldt_struct *ldt)
327 {
328         if (likely(!ldt))
329                 return;
330 
331         paravirt_free_ldt(ldt->entries, ldt->nr_entries);
332         if (ldt->nr_entries * LDT_ENTRY_SIZE > PAGE_SIZE)
333                 vfree_atomic(ldt->entries);
334         else
335                 free_page((unsigned long)ldt->entries);
336         kfree(ldt);
337 }
338 
339 /*
340  * Called on fork from arch_dup_mmap(). Just copy the current LDT state,
341  * the new task is not running, so nothing can be installed.
342  */
343 int ldt_dup_context(struct mm_struct *old_mm, struct mm_struct *mm)
344 {
345         struct ldt_struct *new_ldt;
346         int retval = 0;
347 
348         if (!old_mm)
349                 return 0;
350 
351         mutex_lock(&old_mm->context.lock);
352         if (!old_mm->context.ldt)
353                 goto out_unlock;
354 
355         new_ldt = alloc_ldt_struct(old_mm->context.ldt->nr_entries);
356         if (!new_ldt) {
357                 retval = -ENOMEM;
358                 goto out_unlock;
359         }
360 
361         memcpy(new_ldt->entries, old_mm->context.ldt->entries,
362                new_ldt->nr_entries * LDT_ENTRY_SIZE);
363         finalize_ldt_struct(new_ldt);
364 
365         retval = map_ldt_struct(mm, new_ldt, 0);
366         if (retval) {
367                 free_ldt_pgtables(mm);
368                 free_ldt_struct(new_ldt);
369                 goto out_unlock;
370         }
371         mm->context.ldt = new_ldt;
372 
373 out_unlock:
374         mutex_unlock(&old_mm->context.lock);
375         return retval;
376 }
377 
378 /*
379  * No need to lock the MM as we are the last user
380  *
381  * 64bit: Don't touch the LDT register - we're already in the next thread.
382  */
383 void destroy_context_ldt(struct mm_struct *mm)
384 {
385         free_ldt_struct(mm->context.ldt);
386         mm->context.ldt = NULL;
387 }
388 
389 void ldt_arch_exit_mmap(struct mm_struct *mm)
390 {
391         free_ldt_pgtables(mm);
392 }
393 
394 static int read_ldt(void __user *ptr, unsigned long bytecount)
395 {
396         struct mm_struct *mm = current->mm;
397         unsigned long entries_size;
398         int retval;
399 
400         down_read(&mm->context.ldt_usr_sem);
401 
402         if (!mm->context.ldt) {
403                 retval = 0;
404                 goto out_unlock;
405         }
406 
407         if (bytecount > LDT_ENTRY_SIZE * LDT_ENTRIES)
408                 bytecount = LDT_ENTRY_SIZE * LDT_ENTRIES;
409 
410         entries_size = mm->context.ldt->nr_entries * LDT_ENTRY_SIZE;
411         if (entries_size > bytecount)
412                 entries_size = bytecount;
413 
414         if (copy_to_user(ptr, mm->context.ldt->entries, entries_size)) {
415                 retval = -EFAULT;
416                 goto out_unlock;
417         }
418 
419         if (entries_size != bytecount) {
420                 /* Zero-fill the rest and pretend we read bytecount bytes. */
421                 if (clear_user(ptr + entries_size, bytecount - entries_size)) {
422                         retval = -EFAULT;
423                         goto out_unlock;
424                 }
425         }
426         retval = bytecount;
427 
428 out_unlock:
429         up_read(&mm->context.ldt_usr_sem);
430         return retval;
431 }
432 
433 static int read_default_ldt(void __user *ptr, unsigned long bytecount)
434 {
435         /* CHECKME: Can we use _one_ random number ? */
436 #ifdef CONFIG_X86_32
437         unsigned long size = 5 * sizeof(struct desc_struct);
438 #else
439         unsigned long size = 128;
440 #endif
441         if (bytecount > size)
442                 bytecount = size;
443         if (clear_user(ptr, bytecount))
444                 return -EFAULT;
445         return bytecount;
446 }
447 
448 static int write_ldt(void __user *ptr, unsigned long bytecount, int oldmode)
449 {
450         struct mm_struct *mm = current->mm;
451         struct ldt_struct *new_ldt, *old_ldt;
452         unsigned int old_nr_entries, new_nr_entries;
453         struct user_desc ldt_info;
454         struct desc_struct ldt;
455         int error;
456 
457         error = -EINVAL;
458         if (bytecount != sizeof(ldt_info))
459                 goto out;
460         error = -EFAULT;
461         if (copy_from_user(&ldt_info, ptr, sizeof(ldt_info)))
462                 goto out;
463 
464         error = -EINVAL;
465         if (ldt_info.entry_number >= LDT_ENTRIES)
466                 goto out;
467         if (ldt_info.contents == 3) {
468                 if (oldmode)
469                         goto out;
470                 if (ldt_info.seg_not_present == 0)
471                         goto out;
472         }
473 
474         if ((oldmode && !ldt_info.base_addr && !ldt_info.limit) ||
475             LDT_empty(&ldt_info)) {
476                 /* The user wants to clear the entry. */
477                 memset(&ldt, 0, sizeof(ldt));
478         } else {
479                 if (!IS_ENABLED(CONFIG_X86_16BIT) && !ldt_info.seg_32bit) {
480                         error = -EINVAL;
481                         goto out;
482                 }
483 
484                 fill_ldt(&ldt, &ldt_info);
485                 if (oldmode)
486                         ldt.avl = 0;
487         }
488 
489         if (down_write_killable(&mm->context.ldt_usr_sem))
490                 return -EINTR;
491 
492         old_ldt       = mm->context.ldt;
493         old_nr_entries = old_ldt ? old_ldt->nr_entries : 0;
494         new_nr_entries = max(ldt_info.entry_number + 1, old_nr_entries);
495 
496         error = -ENOMEM;
497         new_ldt = alloc_ldt_struct(new_nr_entries);
498         if (!new_ldt)
499                 goto out_unlock;
500 
501         if (old_ldt)
502                 memcpy(new_ldt->entries, old_ldt->entries, old_nr_entries * LDT_ENTRY_SIZE);
503 
504         new_ldt->entries[ldt_info.entry_number] = ldt;
505         finalize_ldt_struct(new_ldt);
506 
507         /*
508          * If we are using PTI, map the new LDT into the userspace pagetables.
509          * If there is already an LDT, use the other slot so that other CPUs
510          * will continue to use the old LDT until install_ldt() switches
511          * them over to the new LDT.
512          */
513         error = map_ldt_struct(mm, new_ldt, old_ldt ? !old_ldt->slot : 0);
514         if (error) {
515                 /*
516                  * This only can fail for the first LDT setup. If an LDT is
517                  * already installed then the PTE page is already
518                  * populated. Mop up a half populated page table.
519                  */
520                 if (!WARN_ON_ONCE(old_ldt))
521                         free_ldt_pgtables(mm);
522                 free_ldt_struct(new_ldt);
523                 goto out_unlock;
524         }
525 
526         install_ldt(mm, new_ldt);
527         free_ldt_struct(old_ldt);
528         error = 0;
529 
530 out_unlock:
531         up_write(&mm->context.ldt_usr_sem);
532 out:
533         return error;
534 }
535 
536 SYSCALL_DEFINE3(modify_ldt, int , func , void __user * , ptr ,
537                 unsigned long , bytecount)
538 {
539         int ret = -ENOSYS;
540 
541         switch (func) {
542         case 0:
543                 ret = read_ldt(ptr, bytecount);
544                 break;
545         case 1:
546                 ret = write_ldt(ptr, bytecount, 1);
547                 break;
548         case 2:
549                 ret = read_default_ldt(ptr, bytecount);
550                 break;
551         case 0x11:
552                 ret = write_ldt(ptr, bytecount, 0);
553                 break;
554         }
555         /*
556          * The SYSCALL_DEFINE() macros give us an 'unsigned long'
557          * return type, but tht ABI for sys_modify_ldt() expects
558          * 'int'.  This cast gives us an int-sized value in %rax
559          * for the return code.  The 'unsigned' is necessary so
560          * the compiler does not try to sign-extend the negative
561          * return codes into the high half of the register when
562          * taking the value from int->long.
563          */
564         return (unsigned int)ret;
565 }
566 

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