arch/x86/kernel/ldt.c

   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 /*
 104  * If PTI is enabled, this maps the LDT into the kernelmode and
 105  * usermode tables for the given mm.
 106  *
 107  * There is no corresponding unmap function.  Even if the LDT is freed, we
 108  * leave the PTEs around until the slot is reused or the mm is destroyed.
 109  * This is harmless: the LDT is always in ordinary memory, and no one will
 110  * access the freed slot.
 111  *
 112  * If we wanted to unmap freed LDTs, we'd also need to do a flush to make
 113  * it useful, and the flush would slow down modify_ldt().
 114  */
 115 static int
 116 map_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt, int slot)
 117 {
 118 #ifdef CONFIG_PAGE_TABLE_ISOLATION
 119         bool is_vmalloc, had_top_level_entry;
 120         unsigned long va;
 121         spinlock_t *ptl;
 122         pgd_t *pgd;
 123         int i;
 124
 125         if (!static_cpu_has(X86_FEATURE_PTI))
 126                 return 0;
 127
 128         /*
 129          * Any given ldt_struct should have map_ldt_struct() called at most
 130          * once.
 131          */
 132         WARN_ON(ldt->slot != -1);
 133
 134         /*
 135          * Did we already have the top level entry allocated?  We can't
 136          * use pgd_none() for this because it doens't do anything on
 137          * 4-level page table kernels.
 138          */
 139         pgd = pgd_offset(mm, LDT_BASE_ADDR);
 140         had_top_level_entry = (pgd->pgd != 0);
 141
 142         is_vmalloc = is_vmalloc_addr(ldt->entries);
 143
 144         for (i = 0; i * PAGE_SIZE < ldt->nr_entries * LDT_ENTRY_SIZE; i++) {
 145                 unsigned long offset = i << PAGE_SHIFT;
 146                 const void *src = (char *)ldt->entries + offset;
 147                 unsigned long pfn;
 148                 pgprot_t pte_prot;
 149                 pte_t pte, *ptep;
 150
 151                 va = (unsigned long)ldt_slot_va(slot) + offset;
 152                 pfn = is_vmalloc ? vmalloc_to_pfn(src) :
 153                         page_to_pfn(virt_to_page(src));
 154                 /*
 155                  * Treat the PTI LDT range as a *userspace* range.
 156                  * get_locked_pte() will allocate all needed pagetables
 157                  * and account for them in this mm.
 158                  */
 159                 ptep = get_locked_pte(mm, va, &ptl);
 160                 if (!ptep)
 161                         return -ENOMEM;
 162                 /*
 163                  * Map it RO so the easy to find address is not a primary
 164                  * target via some kernel interface which misses a
 165                  * permission check.
 166                  */
 167                 pte_prot = __pgprot(__PAGE_KERNEL_RO & ~_PAGE_GLOBAL);
 168                 /* Filter out unsuppored __PAGE_KERNEL* bits: */
 169                 pgprot_val(pte_prot) &= __supported_pte_mask;
 170                 pte = pfn_pte(pfn, pte_prot);
 171                 set_pte_at(mm, va, ptep, pte);
 172                 pte_unmap_unlock(ptep, ptl);
 173         }
 174
 175         if (mm->context.ldt) {
 176                 /*
 177                  * We already had an LDT.  The top-level entry should already
 178                  * have been allocated and synchronized with the usermode
 179                  * tables.
 180                  */
 181                 WARN_ON(!had_top_level_entry);
 182                 if (static_cpu_has(X86_FEATURE_PTI))
 183                         WARN_ON(!kernel_to_user_pgdp(pgd)->pgd);
 184         } else {
 185                 /*
 186                  * This is the first time we're mapping an LDT for this process.
 187                  * Sync the pgd to the usermode tables.
 188                  */
 189                 WARN_ON(had_top_level_entry);
 190                 if (static_cpu_has(X86_FEATURE_PTI)) {
 191                         WARN_ON(kernel_to_user_pgdp(pgd)->pgd);
 192                         set_pgd(kernel_to_user_pgdp(pgd), *pgd);
 193                 }
 194         }
 195
 196         va = (unsigned long)ldt_slot_va(slot);
 197         flush_tlb_mm_range(mm, va, va + LDT_SLOT_STRIDE, 0);
 198
 199         ldt->slot = slot;
 200 #endif
 201         return 0;
 202 }
 203
 204 static void free_ldt_pgtables(struct mm_struct *mm)
 205 {
 206 #ifdef CONFIG_PAGE_TABLE_ISOLATION
 207         struct mmu_gather tlb;
 208         unsigned long start = LDT_BASE_ADDR;
 209         unsigned long end = start + (1UL << PGDIR_SHIFT);
 210
 211         if (!static_cpu_has(X86_FEATURE_PTI))
 212                 return;
 213
 214         tlb_gather_mmu(&tlb, mm, start, end);
 215         free_pgd_range(&tlb, start, end, start, end);
 216         tlb_finish_mmu(&tlb, start, end);
 217 #endif
 218 }
 219
 220 /* After calling this, the LDT is immutable. */
 221 static void finalize_ldt_struct(struct ldt_struct *ldt)
 222 {
 223         paravirt_alloc_ldt(ldt->entries, ldt->nr_entries);
 224 }
 225
 226 static void install_ldt(struct mm_struct *mm, struct ldt_struct *ldt)
 227 {
 228         mutex_lock(&mm->context.lock);
 229
 230         /* Synchronizes with READ_ONCE in load_mm_ldt. */
 231         smp_store_release(&mm->context.ldt, ldt);
 232
 233         /* Activate the LDT for all CPUs using currents mm. */
 234         on_each_cpu_mask(mm_cpumask(mm), flush_ldt, mm, true);
 235
 236         mutex_unlock(&mm->context.lock);
 237 }
 238
 239 static void free_ldt_struct(struct ldt_struct *ldt)
 240 {
 241         if (likely(!ldt))
 242                 return;
 243
 244         paravirt_free_ldt(ldt->entries, ldt->nr_entries);
 245         if (ldt->nr_entries * LDT_ENTRY_SIZE > PAGE_SIZE)
 246                 vfree_atomic(ldt->entries);
 247         else
 248                 free_page((unsigned long)ldt->entries);
 249         kfree(ldt);
 250 }
 251
 252 /*
 253  * Called on fork from arch_dup_mmap(). Just copy the current LDT state,
 254  * the new task is not running, so nothing can be installed.
 255  */
 256 int ldt_dup_context(struct mm_struct *old_mm, struct mm_struct *mm)
 257 {
 258         struct ldt_struct *new_ldt;
 259         int retval = 0;
 260
 261         if (!old_mm)
 262                 return 0;
 263
 264         mutex_lock(&old_mm->context.lock);
 265         if (!old_mm->context.ldt)
 266                 goto out_unlock;
 267
 268         new_ldt = alloc_ldt_struct(old_mm->context.ldt->nr_entries);
 269         if (!new_ldt) {
 270                 retval = -ENOMEM;
 271                 goto out_unlock;
 272         }
 273
 274         memcpy(new_ldt->entries, old_mm->context.ldt->entries,
 275                new_ldt->nr_entries * LDT_ENTRY_SIZE);
 276         finalize_ldt_struct(new_ldt);
 277
 278         retval = map_ldt_struct(mm, new_ldt, 0);
 279         if (retval) {
 280                 free_ldt_pgtables(mm);
 281                 free_ldt_struct(new_ldt);
 282                 goto out_unlock;
 283         }
 284         mm->context.ldt = new_ldt;
 285
 286 out_unlock:
 287         mutex_unlock(&old_mm->context.lock);
 288         return retval;
 289 }
 290
 291 /*
 292  * No need to lock the MM as we are the last user
 293  *
 294  * 64bit: Don't touch the LDT register - we're already in the next thread.
 295  */
 296 void destroy_context_ldt(struct mm_struct *mm)
 297 {
 298         free_ldt_struct(mm->context.ldt);
 299         mm->context.ldt = NULL;
 300 }
 301
 302 void ldt_arch_exit_mmap(struct mm_struct *mm)
 303 {
 304         free_ldt_pgtables(mm);
 305 }
 306
 307 static int read_ldt(void __user *ptr, unsigned long bytecount)
 308 {
 309         struct mm_struct *mm = current->mm;
 310         unsigned long entries_size;
 311         int retval;
 312
 313         down_read(&mm->context.ldt_usr_sem);
 314
 315         if (!mm->context.ldt) {
 316                 retval = 0;
 317                 goto out_unlock;
 318         }
 319
 320         if (bytecount > LDT_ENTRY_SIZE * LDT_ENTRIES)
 321                 bytecount = LDT_ENTRY_SIZE * LDT_ENTRIES;
 322
 323         entries_size = mm->context.ldt->nr_entries * LDT_ENTRY_SIZE;
 324         if (entries_size > bytecount)
 325                 entries_size = bytecount;
 326
 327         if (copy_to_user(ptr, mm->context.ldt->entries, entries_size)) {
 328                 retval = -EFAULT;
 329                 goto out_unlock;
 330         }
 331
 332         if (entries_size != bytecount) {
 333                 /* Zero-fill the rest and pretend we read bytecount bytes. */
 334                 if (clear_user(ptr + entries_size, bytecount - entries_size)) {
 335                         retval = -EFAULT;
 336                         goto out_unlock;
 337                 }
 338         }
 339         retval = bytecount;
 340
 341 out_unlock:
 342         up_read(&mm->context.ldt_usr_sem);
 343         return retval;
 344 }
 345
 346 static int read_default_ldt(void __user *ptr, unsigned long bytecount)
 347 {
 348         /* CHECKME: Can we use _one_ random number ? */
 349 #ifdef CONFIG_X86_32
 350         unsigned long size = 5 * sizeof(struct desc_struct);
 351 #else
 352         unsigned long size = 128;
 353 #endif
 354         if (bytecount > size)
 355                 bytecount = size;
 356         if (clear_user(ptr, bytecount))
 357                 return -EFAULT;
 358         return bytecount;
 359 }
 360
 361 static int write_ldt(void __user *ptr, unsigned long bytecount, int oldmode)
 362 {
 363         struct mm_struct *mm = current->mm;
 364         struct ldt_struct *new_ldt, *old_ldt;
 365         unsigned int old_nr_entries, new_nr_entries;
 366         struct user_desc ldt_info;
 367         struct desc_struct ldt;
 368         int error;
 369
 370         error = -EINVAL;
 371         if (bytecount != sizeof(ldt_info))
 372                 goto out;
 373         error = -EFAULT;
 374         if (copy_from_user(&ldt_info, ptr, sizeof(ldt_info)))
 375                 goto out;
 376
 377         error = -EINVAL;
 378         if (ldt_info.entry_number >= LDT_ENTRIES)
 379                 goto out;
 380         if (ldt_info.contents == 3) {
 381                 if (oldmode)
 382                         goto out;
 383                 if (ldt_info.seg_not_present == 0)
 384                         goto out;
 385         }
 386
 387         if ((oldmode && !ldt_info.base_addr && !ldt_info.limit) ||
 388             LDT_empty(&ldt_info)) {
 389                 /* The user wants to clear the entry. */
 390                 memset(&ldt, 0, sizeof(ldt));
 391         } else {
 392                 if (!IS_ENABLED(CONFIG_X86_16BIT) && !ldt_info.seg_32bit) {
 393                         error = -EINVAL;
 394                         goto out;
 395                 }
 396
 397                 fill_ldt(&ldt, &ldt_info);
 398                 if (oldmode)
 399                         ldt.avl = 0;
 400         }
 401
 402         if (down_write_killable(&mm->context.ldt_usr_sem))
 403                 return -EINTR;
 404
 405         old_ldt       = mm->context.ldt;
 406         old_nr_entries = old_ldt ? old_ldt->nr_entries : 0;
 407         new_nr_entries = max(ldt_info.entry_number + 1, old_nr_entries);
 408
 409         error = -ENOMEM;
 410         new_ldt = alloc_ldt_struct(new_nr_entries);
 411         if (!new_ldt)
 412                 goto out_unlock;
 413
 414         if (old_ldt)
 415                 memcpy(new_ldt->entries, old_ldt->entries, old_nr_entries * LDT_ENTRY_SIZE);
 416
 417         new_ldt->entries[ldt_info.entry_number] = ldt;
 418         finalize_ldt_struct(new_ldt);
 419
 420         /*
 421          * If we are using PTI, map the new LDT into the userspace pagetables.
 422          * If there is already an LDT, use the other slot so that other CPUs
 423          * will continue to use the old LDT until install_ldt() switches
 424          * them over to the new LDT.
 425          */
 426         error = map_ldt_struct(mm, new_ldt, old_ldt ? !old_ldt->slot : 0);
 427         if (error) {
 428                 /*
 429                  * This only can fail for the first LDT setup. If an LDT is
 430                  * already installed then the PTE page is already
 431                  * populated. Mop up a half populated page table.
 432                  */
 433                 if (!WARN_ON_ONCE(old_ldt))
 434                         free_ldt_pgtables(mm);
 435                 free_ldt_struct(new_ldt);
 436                 goto out_unlock;
 437         }
 438
 439         install_ldt(mm, new_ldt);
 440         free_ldt_struct(old_ldt);
 441         error = 0;
 442
 443 out_unlock:
 444         up_write(&mm->context.ldt_usr_sem);
 445 out:
 446         return error;
 447 }
 448
 449 SYSCALL_DEFINE3(modify_ldt, int , func , void __user * , ptr ,
 450                 unsigned long , bytecount)
 451 {
 452         int ret = -ENOSYS;
 453
 454         switch (func) {
 455         case 0:
 456                 ret = read_ldt(ptr, bytecount);
 457                 break;
 458         case 1:
 459                 ret = write_ldt(ptr, bytecount, 1);
 460                 break;
 461         case 2:
 462                 ret = read_default_ldt(ptr, bytecount);
 463                 break;
 464         case 0x11:
 465                 ret = write_ldt(ptr, bytecount, 0);
 466                 break;
 467         }
 468         /*
 469          * The SYSCALL_DEFINE() macros give us an 'unsigned long'
 470          * return type, but tht ABI for sys_modify_ldt() expects
 471          * 'int'.  This cast gives us an int-sized value in %rax
 472          * for the return code.  The 'unsigned' is necessary so
 473          * the compiler does not try to sign-extend the negative
 474          * return codes into the high half of the register when
 475          * taking the value from int->long.
 476          */
 477         return (unsigned int)ret;
 478 }