mm/mremap.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  *      mm/mremap.c
   4  *
   5  *      (C) Copyright 1996 Linus Torvalds
   6  *
   7  *      Address space accounting code   <alan@lxorguk.ukuu.org.uk>
   8  *      (C) Copyright 2002 Red Hat Inc, All Rights Reserved
   9  */
  10
  11 #include <linux/mm.h>
  12 #include <linux/hugetlb.h>
  13 #include <linux/shm.h>
  14 #include <linux/ksm.h>
  15 #include <linux/mman.h>
  16 #include <linux/swap.h>
  17 #include <linux/capability.h>
  18 #include <linux/fs.h>
  19 #include <linux/swapops.h>
  20 #include <linux/highmem.h>
  21 #include <linux/security.h>
  22 #include <linux/syscalls.h>
  23 #include <linux/mmu_notifier.h>
  24 #include <linux/uaccess.h>
  25 #include <linux/mm-arch-hooks.h>
  26 #include <linux/userfaultfd_k.h>
  27
  28 #include <asm/cacheflush.h>
  29 #include <asm/tlbflush.h>
  30
  31 #include "internal.h"
  32
  33 static pmd_t *get_old_pmd(struct mm_struct *mm, unsigned long addr)
  34 {
  35         pgd_t *pgd;
  36         p4d_t *p4d;
  37         pud_t *pud;
  38         pmd_t *pmd;
  39
  40         pgd = pgd_offset(mm, addr);
  41         if (pgd_none_or_clear_bad(pgd))
  42                 return NULL;
  43
  44         p4d = p4d_offset(pgd, addr);
  45         if (p4d_none_or_clear_bad(p4d))
  46                 return NULL;
  47
  48         pud = pud_offset(p4d, addr);
  49         if (pud_none_or_clear_bad(pud))
  50                 return NULL;
  51
  52         pmd = pmd_offset(pud, addr);
  53         if (pmd_none(*pmd))
  54                 return NULL;
  55
  56         return pmd;
  57 }
  58
  59 static pmd_t *alloc_new_pmd(struct mm_struct *mm, struct vm_area_struct *vma,
  60                             unsigned long addr)
  61 {
  62         pgd_t *pgd;
  63         p4d_t *p4d;
  64         pud_t *pud;
  65         pmd_t *pmd;
  66
  67         pgd = pgd_offset(mm, addr);
  68         p4d = p4d_alloc(mm, pgd, addr);
  69         if (!p4d)
  70                 return NULL;
  71         pud = pud_alloc(mm, p4d, addr);
  72         if (!pud)
  73                 return NULL;
  74
  75         pmd = pmd_alloc(mm, pud, addr);
  76         if (!pmd)
  77                 return NULL;
  78
  79         VM_BUG_ON(pmd_trans_huge(*pmd));
  80
  81         return pmd;
  82 }
  83
  84 static void take_rmap_locks(struct vm_area_struct *vma)
  85 {
  86         if (vma->vm_file)
  87                 i_mmap_lock_write(vma->vm_file->f_mapping);
  88         if (vma->anon_vma)
  89                 anon_vma_lock_write(vma->anon_vma);
  90 }
  91
  92 static void drop_rmap_locks(struct vm_area_struct *vma)
  93 {
  94         if (vma->anon_vma)
  95                 anon_vma_unlock_write(vma->anon_vma);
  96         if (vma->vm_file)
  97                 i_mmap_unlock_write(vma->vm_file->f_mapping);
  98 }
  99
 100 static pte_t move_soft_dirty_pte(pte_t pte)
 101 {
 102         /*
 103          * Set soft dirty bit so we can notice
 104          * in userspace the ptes were moved.
 105          */
 106 #ifdef CONFIG_MEM_SOFT_DIRTY
 107         if (pte_present(pte))
 108                 pte = pte_mksoft_dirty(pte);
 109         else if (is_swap_pte(pte))
 110                 pte = pte_swp_mksoft_dirty(pte);
 111 #endif
 112         return pte;
 113 }
 114
 115 static void move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd,
 116                 unsigned long old_addr, unsigned long old_end,
 117                 struct vm_area_struct *new_vma, pmd_t *new_pmd,
 118                 unsigned long new_addr, bool need_rmap_locks)
 119 {
 120         struct mm_struct *mm = vma->vm_mm;
 121         pte_t *old_pte, *new_pte, pte;
 122         spinlock_t *old_ptl, *new_ptl;
 123         bool force_flush = false;
 124         unsigned long len = old_end - old_addr;
 125
 126         /*
 127          * When need_rmap_locks is true, we take the i_mmap_rwsem and anon_vma
 128          * locks to ensure that rmap will always observe either the old or the
 129          * new ptes. This is the easiest way to avoid races with
 130          * truncate_pagecache(), page migration, etc...
 131          *
 132          * When need_rmap_locks is false, we use other ways to avoid
 133          * such races:
 134          *
 135          * - During exec() shift_arg_pages(), we use a specially tagged vma
 136          *   which rmap call sites look for using is_vma_temporary_stack().
 137          *
 138          * - During mremap(), new_vma is often known to be placed after vma
 139          *   in rmap traversal order. This ensures rmap will always observe
 140          *   either the old pte, or the new pte, or both (the page table locks
 141          *   serialize access to individual ptes, but only rmap traversal
 142          *   order guarantees that we won't miss both the old and new ptes).
 143          */
 144         if (need_rmap_locks)
 145                 take_rmap_locks(vma);
 146
 147         /*
 148          * We don't have to worry about the ordering of src and dst
 149          * pte locks because exclusive mmap_sem prevents deadlock.
 150          */
 151         old_pte = pte_offset_map_lock(mm, old_pmd, old_addr, &old_ptl);
 152         new_pte = pte_offset_map(new_pmd, new_addr);
 153         new_ptl = pte_lockptr(mm, new_pmd);
 154         if (new_ptl != old_ptl)
 155                 spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
 156         flush_tlb_batched_pending(vma->vm_mm);
 157         arch_enter_lazy_mmu_mode();
 158
 159         for (; old_addr < old_end; old_pte++, old_addr += PAGE_SIZE,
 160                                    new_pte++, new_addr += PAGE_SIZE) {
 161                 if (pte_none(*old_pte))
 162                         continue;
 163
 164                 pte = ptep_get_and_clear(mm, old_addr, old_pte);
 165                 /*
 166                  * If we are remapping a valid PTE, make sure
 167                  * to flush TLB before we drop the PTL for the
 168                  * PTE.
 169                  *
 170                  * NOTE! Both old and new PTL matter: the old one
 171                  * for racing with page_mkclean(), the new one to
 172                  * make sure the physical page stays valid until
 173                  * the TLB entry for the old mapping has been
 174                  * flushed.
 175                  */
 176                 if (pte_present(pte))
 177                         force_flush = true;
 178                 pte = move_pte(pte, new_vma->vm_page_prot, old_addr, new_addr);
 179                 pte = move_soft_dirty_pte(pte);
 180                 set_pte_at(mm, new_addr, new_pte, pte);
 181         }
 182
 183         arch_leave_lazy_mmu_mode();
 184         if (force_flush)
 185                 flush_tlb_range(vma, old_end - len, old_end);
 186         if (new_ptl != old_ptl)
 187                 spin_unlock(new_ptl);
 188         pte_unmap(new_pte - 1);
 189         pte_unmap_unlock(old_pte - 1, old_ptl);
 190         if (need_rmap_locks)
 191                 drop_rmap_locks(vma);
 192 }
 193
 194 #ifdef CONFIG_HAVE_MOVE_PMD
 195 static bool move_normal_pmd(struct vm_area_struct *vma, unsigned long old_addr,
 196                   unsigned long new_addr, unsigned long old_end,
 197                   pmd_t *old_pmd, pmd_t *new_pmd)
 198 {
 199         spinlock_t *old_ptl, *new_ptl;
 200         struct mm_struct *mm = vma->vm_mm;
 201         pmd_t pmd;
 202
 203         if ((old_addr & ~PMD_MASK) || (new_addr & ~PMD_MASK)
 204             || old_end - old_addr < PMD_SIZE)
 205                 return false;
 206
 207         /*
 208          * The destination pmd shouldn't be established, free_pgtables()
 209          * should have release it.
 210          */
 211         if (WARN_ON(!pmd_none(*new_pmd)))
 212                 return false;
 213
 214         /*
 215          * We don't have to worry about the ordering of src and dst
 216          * ptlocks because exclusive mmap_sem prevents deadlock.
 217          */
 218         old_ptl = pmd_lock(vma->vm_mm, old_pmd);
 219         new_ptl = pmd_lockptr(mm, new_pmd);
 220         if (new_ptl != old_ptl)
 221                 spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
 222
 223         /* Clear the pmd */
 224         pmd = *old_pmd;
 225         pmd_clear(old_pmd);
 226
 227         VM_BUG_ON(!pmd_none(*new_pmd));
 228
 229         /* Set the new pmd */
 230         set_pmd_at(mm, new_addr, new_pmd, pmd);
 231         flush_tlb_range(vma, old_addr, old_addr + PMD_SIZE);
 232         if (new_ptl != old_ptl)
 233                 spin_unlock(new_ptl);
 234         spin_unlock(old_ptl);
 235
 236         return true;
 237 }
 238 #endif
 239
 240 unsigned long move_page_tables(struct vm_area_struct *vma,
 241                 unsigned long old_addr, struct vm_area_struct *new_vma,
 242                 unsigned long new_addr, unsigned long len,
 243                 bool need_rmap_locks)
 244 {
 245         unsigned long extent, next, old_end;
 246         struct mmu_notifier_range range;
 247         pmd_t *old_pmd, *new_pmd;
 248
 249         if (!len)
 250                 return 0;
 251
 252         old_end = old_addr + len;
 253         flush_cache_range(vma, old_addr, old_end);
 254
 255         mmu_notifier_range_init(&range, MMU_NOTIFY_UNMAP, 0, vma, vma->vm_mm,
 256                                 old_addr, old_end);
 257         mmu_notifier_invalidate_range_start(&range);
 258
 259         for (; old_addr < old_end; old_addr += extent, new_addr += extent) {
 260                 cond_resched();
 261                 next = (old_addr + PMD_SIZE) & PMD_MASK;
 262                 /* even if next overflowed, extent below will be ok */
 263                 extent = next - old_addr;
 264                 if (extent > old_end - old_addr)
 265                         extent = old_end - old_addr;
 266                 old_pmd = get_old_pmd(vma->vm_mm, old_addr);
 267                 if (!old_pmd)
 268                         continue;
 269                 new_pmd = alloc_new_pmd(vma->vm_mm, vma, new_addr);
 270                 if (!new_pmd)
 271                         break;
 272                 if (is_swap_pmd(*old_pmd) || pmd_trans_huge(*old_pmd) || pmd_devmap(*old_pmd)) {
 273                         if (extent == HPAGE_PMD_SIZE) {
 274                                 bool moved;
 275                                 /* See comment in move_ptes() */
 276                                 if (need_rmap_locks)
 277                                         take_rmap_locks(vma);
 278                                 moved = move_huge_pmd(vma, old_addr, new_addr,
 279                                                     old_end, old_pmd, new_pmd);
 280                                 if (need_rmap_locks)
 281                                         drop_rmap_locks(vma);
 282                                 if (moved)
 283                                         continue;
 284                         }
 285                         split_huge_pmd(vma, old_pmd, old_addr);
 286                         if (pmd_trans_unstable(old_pmd))
 287                                 continue;
 288                 } else if (extent == PMD_SIZE) {
 289 #ifdef CONFIG_HAVE_MOVE_PMD
 290                         /*
 291                          * If the extent is PMD-sized, try to speed the move by
 292                          * moving at the PMD level if possible.
 293                          */
 294                         bool moved;
 295
 296                         take_rmap_locks(vma);
 297                         moved = move_normal_pmd(vma, old_addr, new_addr,
 298                                         old_end, old_pmd, new_pmd);
 299                         drop_rmap_locks(vma);
 300                         if (moved)
 301                                 continue;
 302 #endif
 303                 }
 304
 305                 if (pte_alloc(new_vma->vm_mm, new_pmd))
 306                         break;
 307                 next = (new_addr + PMD_SIZE) & PMD_MASK;
 308                 if (extent > next - new_addr)
 309                         extent = next - new_addr;
 310                 move_ptes(vma, old_pmd, old_addr, old_addr + extent, new_vma,
 311                           new_pmd, new_addr, need_rmap_locks);
 312         }
 313
 314         mmu_notifier_invalidate_range_end(&range);
 315
 316         return len + old_addr - old_end;        /* how much done */
 317 }
 318
 319 static unsigned long move_vma(struct vm_area_struct *vma,
 320                 unsigned long old_addr, unsigned long old_len,
 321                 unsigned long new_len, unsigned long new_addr,
 322                 bool *locked, struct vm_userfaultfd_ctx *uf,
 323                 struct list_head *uf_unmap)
 324 {
 325         struct mm_struct *mm = vma->vm_mm;
 326         struct vm_area_struct *new_vma;
 327         unsigned long vm_flags = vma->vm_flags;
 328         unsigned long new_pgoff;
 329         unsigned long moved_len;
 330         unsigned long excess = 0;
 331         unsigned long hiwater_vm;
 332         int split = 0;
 333         int err;
 334         bool need_rmap_locks;
 335
 336         /*
 337          * We'd prefer to avoid failure later on in do_munmap:
 338          * which may split one vma into three before unmapping.
 339          */
 340         if (mm->map_count >= sysctl_max_map_count - 3)
 341                 return -ENOMEM;
 342
 343         /*
 344          * Advise KSM to break any KSM pages in the area to be moved:
 345          * it would be confusing if they were to turn up at the new
 346          * location, where they happen to coincide with different KSM
 347          * pages recently unmapped.  But leave vma->vm_flags as it was,
 348          * so KSM can come around to merge on vma and new_vma afterwards.
 349          */
 350         err = ksm_madvise(vma, old_addr, old_addr + old_len,
 351                                                 MADV_UNMERGEABLE, &vm_flags);
 352         if (err)
 353                 return err;
 354
 355         new_pgoff = vma->vm_pgoff + ((old_addr - vma->vm_start) >> PAGE_SHIFT);
 356         new_vma = copy_vma(&vma, new_addr, new_len, new_pgoff,
 357                            &need_rmap_locks);
 358         if (!new_vma)
 359                 return -ENOMEM;
 360
 361         moved_len = move_page_tables(vma, old_addr, new_vma, new_addr, old_len,
 362                                      need_rmap_locks);
 363         if (moved_len < old_len) {
 364                 err = -ENOMEM;
 365         } else if (vma->vm_ops && vma->vm_ops->mremap) {
 366                 err = vma->vm_ops->mremap(new_vma);
 367         }
 368
 369         if (unlikely(err)) {
 370                 /*
 371                  * On error, move entries back from new area to old,
 372                  * which will succeed since page tables still there,
 373                  * and then proceed to unmap new area instead of old.
 374                  */
 375                 move_page_tables(new_vma, new_addr, vma, old_addr, moved_len,
 376                                  true);
 377                 vma = new_vma;
 378                 old_len = new_len;
 379                 old_addr = new_addr;
 380                 new_addr = err;
 381         } else {
 382                 mremap_userfaultfd_prep(new_vma, uf);
 383                 arch_remap(mm, old_addr, old_addr + old_len,
 384                            new_addr, new_addr + new_len);
 385         }
 386
 387         /* Conceal VM_ACCOUNT so old reservation is not undone */
 388         if (vm_flags & VM_ACCOUNT) {
 389                 vma->vm_flags &= ~VM_ACCOUNT;
 390                 excess = vma->vm_end - vma->vm_start - old_len;
 391                 if (old_addr > vma->vm_start &&
 392                     old_addr + old_len < vma->vm_end)
 393                         split = 1;
 394         }
 395
 396         /*
 397          * If we failed to move page tables we still do total_vm increment
 398          * since do_munmap() will decrement it by old_len == new_len.
 399          *
 400          * Since total_vm is about to be raised artificially high for a
 401          * moment, we need to restore high watermark afterwards: if stats
 402          * are taken meanwhile, total_vm and hiwater_vm appear too high.
 403          * If this were a serious issue, we'd add a flag to do_munmap().
 404          */
 405         hiwater_vm = mm->hiwater_vm;
 406         vm_stat_account(mm, vma->vm_flags, new_len >> PAGE_SHIFT);
 407
 408         /* Tell pfnmap has moved from this vma */
 409         if (unlikely(vma->vm_flags & VM_PFNMAP))
 410                 untrack_pfn_moved(vma);
 411
 412         if (do_munmap(mm, old_addr, old_len, uf_unmap) < 0) {
 413                 /* OOM: unable to split vma, just get accounts right */
 414                 vm_unacct_memory(excess >> PAGE_SHIFT);
 415                 excess = 0;
 416         }
 417         mm->hiwater_vm = hiwater_vm;
 418
 419         /* Restore VM_ACCOUNT if one or two pieces of vma left */
 420         if (excess) {
 421                 vma->vm_flags |= VM_ACCOUNT;
 422                 if (split)
 423                         vma->vm_next->vm_flags |= VM_ACCOUNT;
 424         }
 425
 426         if (vm_flags & VM_LOCKED) {
 427                 mm->locked_vm += new_len >> PAGE_SHIFT;
 428                 *locked = true;
 429         }
 430
 431         return new_addr;
 432 }
 433
 434 static struct vm_area_struct *vma_to_resize(unsigned long addr,
 435         unsigned long old_len, unsigned long new_len, unsigned long *p)
 436 {
 437         struct mm_struct *mm = current->mm;
 438         struct vm_area_struct *vma = find_vma(mm, addr);
 439         unsigned long pgoff;
 440
 441         if (!vma || vma->vm_start > addr)
 442                 return ERR_PTR(-EFAULT);
 443
 444         /*
 445          * !old_len is a special case where an attempt is made to 'duplicate'
 446          * a mapping.  This makes no sense for private mappings as it will
 447          * instead create a fresh/new mapping unrelated to the original.  This
 448          * is contrary to the basic idea of mremap which creates new mappings
 449          * based on the original.  There are no known use cases for this
 450          * behavior.  As a result, fail such attempts.
 451          */
 452         if (!old_len && !(vma->vm_flags & (VM_SHARED | VM_MAYSHARE))) {
 453                 pr_warn_once("%s (%d): attempted to duplicate a private mapping with mremap.  This is not supported.\n", current->comm, current->pid);
 454                 return ERR_PTR(-EINVAL);
 455         }
 456
 457         if (is_vm_hugetlb_page(vma))
 458                 return ERR_PTR(-EINVAL);
 459
 460         /* We can't remap across vm area boundaries */
 461         if (old_len > vma->vm_end - addr)
 462                 return ERR_PTR(-EFAULT);
 463
 464         if (new_len == old_len)
 465                 return vma;
 466
 467         /* Need to be careful about a growing mapping */
 468         pgoff = (addr - vma->vm_start) >> PAGE_SHIFT;
 469         pgoff += vma->vm_pgoff;
 470         if (pgoff + (new_len >> PAGE_SHIFT) < pgoff)
 471                 return ERR_PTR(-EINVAL);
 472
 473         if (vma->vm_flags & (VM_DONTEXPAND | VM_PFNMAP))
 474                 return ERR_PTR(-EFAULT);
 475
 476         if (vma->vm_flags & VM_LOCKED) {
 477                 unsigned long locked, lock_limit;
 478                 locked = mm->locked_vm << PAGE_SHIFT;
 479                 lock_limit = rlimit(RLIMIT_MEMLOCK);
 480                 locked += new_len - old_len;
 481                 if (locked > lock_limit && !capable(CAP_IPC_LOCK))
 482                         return ERR_PTR(-EAGAIN);
 483         }
 484
 485         if (!may_expand_vm(mm, vma->vm_flags,
 486                                 (new_len - old_len) >> PAGE_SHIFT))
 487                 return ERR_PTR(-ENOMEM);
 488
 489         if (vma->vm_flags & VM_ACCOUNT) {
 490                 unsigned long charged = (new_len - old_len) >> PAGE_SHIFT;
 491                 if (security_vm_enough_memory_mm(mm, charged))
 492                         return ERR_PTR(-ENOMEM);
 493                 *p = charged;
 494         }
 495
 496         return vma;
 497 }
 498
 499 static unsigned long mremap_to(unsigned long addr, unsigned long old_len,
 500                 unsigned long new_addr, unsigned long new_len, bool *locked,
 501                 struct vm_userfaultfd_ctx *uf,
 502                 struct list_head *uf_unmap_early,
 503                 struct list_head *uf_unmap)
 504 {
 505         struct mm_struct *mm = current->mm;
 506         struct vm_area_struct *vma;
 507         unsigned long ret = -EINVAL;
 508         unsigned long charged = 0;
 509         unsigned long map_flags;
 510
 511         if (offset_in_page(new_addr))
 512                 goto out;
 513
 514         if (new_len > TASK_SIZE || new_addr > TASK_SIZE - new_len)
 515                 goto out;
 516
 517         /* Ensure the old/new locations do not overlap */
 518         if (addr + old_len > new_addr && new_addr + new_len > addr)
 519                 goto out;
 520
 521         /*
 522          * move_vma() need us to stay 4 maps below the threshold, otherwise
 523          * it will bail out at the very beginning.
 524          * That is a problem if we have already unmaped the regions here
 525          * (new_addr, and old_addr), because userspace will not know the
 526          * state of the vma's after it gets -ENOMEM.
 527          * So, to avoid such scenario we can pre-compute if the whole
 528          * operation has high chances to success map-wise.
 529          * Worst-scenario case is when both vma's (new_addr and old_addr) get
 530          * split in 3 before unmaping it.
 531          * That means 2 more maps (1 for each) to the ones we already hold.
 532          * Check whether current map count plus 2 still leads us to 4 maps below
 533          * the threshold, otherwise return -ENOMEM here to be more safe.
 534          */
 535         if ((mm->map_count + 2) >= sysctl_max_map_count - 3)
 536                 return -ENOMEM;
 537
 538         ret = do_munmap(mm, new_addr, new_len, uf_unmap_early);
 539         if (ret)
 540                 goto out;
 541
 542         if (old_len >= new_len) {
 543                 ret = do_munmap(mm, addr+new_len, old_len - new_len, uf_unmap);
 544                 if (ret && old_len != new_len)
 545                         goto out;
 546                 old_len = new_len;
 547         }
 548
 549         vma = vma_to_resize(addr, old_len, new_len, &charged);
 550         if (IS_ERR(vma)) {
 551                 ret = PTR_ERR(vma);
 552                 goto out;
 553         }
 554
 555         map_flags = MAP_FIXED;
 556         if (vma->vm_flags & VM_MAYSHARE)
 557                 map_flags |= MAP_SHARED;
 558
 559         ret = get_unmapped_area(vma->vm_file, new_addr, new_len, vma->vm_pgoff +
 560                                 ((addr - vma->vm_start) >> PAGE_SHIFT),
 561                                 map_flags);
 562         if (offset_in_page(ret))
 563                 goto out1;
 564
 565         ret = move_vma(vma, addr, old_len, new_len, new_addr, locked, uf,
 566                        uf_unmap);
 567         if (!(offset_in_page(ret)))
 568                 goto out;
 569 out1:
 570         vm_unacct_memory(charged);
 571
 572 out:
 573         return ret;
 574 }
 575
 576 static int vma_expandable(struct vm_area_struct *vma, unsigned long delta)
 577 {
 578         unsigned long end = vma->vm_end + delta;
 579         if (end < vma->vm_end) /* overflow */
 580                 return 0;
 581         if (vma->vm_next && vma->vm_next->vm_start < end) /* intersection */
 582                 return 0;
 583         if (get_unmapped_area(NULL, vma->vm_start, end - vma->vm_start,
 584                               0, MAP_FIXED) & ~PAGE_MASK)
 585                 return 0;
 586         return 1;
 587 }
 588
 589 /*
 590  * Expand (or shrink) an existing mapping, potentially moving it at the
 591  * same time (controlled by the MREMAP_MAYMOVE flag and available VM space)
 592  *
 593  * MREMAP_FIXED option added 5-Dec-1999 by Benjamin LaHaise
 594  * This option implies MREMAP_MAYMOVE.
 595  */
 596 SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
 597                 unsigned long, new_len, unsigned long, flags,
 598                 unsigned long, new_addr)
 599 {
 600         struct mm_struct *mm = current->mm;
 601         struct vm_area_struct *vma;
 602         unsigned long ret = -EINVAL;
 603         unsigned long charged = 0;
 604         bool locked = false;
 605         bool downgraded = false;
 606         struct vm_userfaultfd_ctx uf = NULL_VM_UFFD_CTX;
 607         LIST_HEAD(uf_unmap_early);
 608         LIST_HEAD(uf_unmap);
 609
 610         /*
 611          * There is a deliberate asymmetry here: we strip the pointer tag
 612          * from the old address but leave the new address alone. This is
 613          * for consistency with mmap(), where we prevent the creation of
 614          * aliasing mappings in userspace by leaving the tag bits of the
 615          * mapping address intact. A non-zero tag will cause the subsequent
 616          * range checks to reject the address as invalid.
 617          *
 618          * See Documentation/arm64/tagged-address-abi.rst for more information.
 619          */
 620         addr = untagged_addr(addr);
 621
 622         if (flags & ~(MREMAP_FIXED | MREMAP_MAYMOVE))
 623                 return ret;
 624
 625         if (flags & MREMAP_FIXED && !(flags & MREMAP_MAYMOVE))
 626                 return ret;
 627
 628         if (offset_in_page(addr))
 629                 return ret;
 630
 631         old_len = PAGE_ALIGN(old_len);
 632         new_len = PAGE_ALIGN(new_len);
 633
 634         /*
 635          * We allow a zero old-len as a special case
 636          * for DOS-emu "duplicate shm area" thing. But
 637          * a zero new-len is nonsensical.
 638          */
 639         if (!new_len)
 640                 return ret;
 641
 642         if (down_write_killable(&current->mm->mmap_sem))
 643                 return -EINTR;
 644
 645         if (flags & MREMAP_FIXED) {
 646                 ret = mremap_to(addr, old_len, new_addr, new_len,
 647                                 &locked, &uf, &uf_unmap_early, &uf_unmap);
 648                 goto out;
 649         }
 650
 651         /*
 652          * Always allow a shrinking remap: that just unmaps
 653          * the unnecessary pages..
 654          * __do_munmap does all the needed commit accounting, and
 655          * downgrades mmap_sem to read if so directed.
 656          */
 657         if (old_len >= new_len) {
 658                 int retval;
 659
 660                 retval = __do_munmap(mm, addr+new_len, old_len - new_len,
 661                                   &uf_unmap, true);
 662                 if (retval < 0 && old_len != new_len) {
 663                         ret = retval;
 664                         goto out;
 665                 /* Returning 1 indicates mmap_sem is downgraded to read. */
 666                 } else if (retval == 1)
 667                         downgraded = true;
 668                 ret = addr;
 669                 goto out;
 670         }
 671
 672         /*
 673          * Ok, we need to grow..
 674          */
 675         vma = vma_to_resize(addr, old_len, new_len, &charged);
 676         if (IS_ERR(vma)) {
 677                 ret = PTR_ERR(vma);
 678                 goto out;
 679         }
 680
 681         /* old_len exactly to the end of the area..
 682          */
 683         if (old_len == vma->vm_end - addr) {
 684                 /* can we just expand the current mapping? */
 685                 if (vma_expandable(vma, new_len - old_len)) {
 686                         int pages = (new_len - old_len) >> PAGE_SHIFT;
 687
 688                         if (vma_adjust(vma, vma->vm_start, addr + new_len,
 689                                        vma->vm_pgoff, NULL)) {
 690                                 ret = -ENOMEM;
 691                                 goto out;
 692                         }
 693
 694                         vm_stat_account(mm, vma->vm_flags, pages);
 695                         if (vma->vm_flags & VM_LOCKED) {
 696                                 mm->locked_vm += pages;
 697                                 locked = true;
 698                                 new_addr = addr;
 699                         }
 700                         ret = addr;
 701                         goto out;
 702                 }
 703         }
 704
 705         /*
 706          * We weren't able to just expand or shrink the area,
 707          * we need to create a new one and move it..
 708          */
 709         ret = -ENOMEM;
 710         if (flags & MREMAP_MAYMOVE) {
 711                 unsigned long map_flags = 0;
 712                 if (vma->vm_flags & VM_MAYSHARE)
 713                         map_flags |= MAP_SHARED;
 714
 715                 new_addr = get_unmapped_area(vma->vm_file, 0, new_len,
 716                                         vma->vm_pgoff +
 717                                         ((addr - vma->vm_start) >> PAGE_SHIFT),
 718                                         map_flags);
 719                 if (offset_in_page(new_addr)) {
 720                         ret = new_addr;
 721                         goto out;
 722                 }
 723
 724                 ret = move_vma(vma, addr, old_len, new_len, new_addr,
 725                                &locked, &uf, &uf_unmap);
 726         }
 727 out:
 728         if (offset_in_page(ret)) {
 729                 vm_unacct_memory(charged);
 730                 locked = 0;
 731         }
 732         if (downgraded)
 733                 up_read(&current->mm->mmap_sem);
 734         else
 735                 up_write(&current->mm->mmap_sem);
 736         if (locked && new_len > old_len)
 737                 mm_populate(new_addr + old_len, new_len - old_len);
 738         userfaultfd_unmap_complete(mm, &uf_unmap_early);
 739         mremap_userfaultfd_complete(&uf, addr, new_addr, old_len);
 740         userfaultfd_unmap_complete(mm, &uf_unmap);
 741         return ret;
 742 }