mm/userfaultfd.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /*
   3  *  mm/userfaultfd.c
   4  *
   5  *  Copyright (C) 2015  Red Hat, Inc.
   6  */
   7
   8 #include <linux/mm.h>
   9 #include <linux/sched/signal.h>
  10 #include <linux/pagemap.h>
  11 #include <linux/rmap.h>
  12 #include <linux/swap.h>
  13 #include <linux/swapops.h>
  14 #include <linux/userfaultfd_k.h>
  15 #include <linux/mmu_notifier.h>
  16 #include <linux/hugetlb.h>
  17 #include <linux/shmem_fs.h>
  18 #include <asm/tlbflush.h>
  19 #include "internal.h"
  20
  21 static __always_inline
  22 struct vm_area_struct *find_dst_vma(struct mm_struct *dst_mm,
  23                                     unsigned long dst_start,
  24                                     unsigned long len)
  25 {
  26         /*
  27          * Make sure that the dst range is both valid and fully within a
  28          * single existing vma.
  29          */
  30         struct vm_area_struct *dst_vma;
  31
  32         dst_vma = find_vma(dst_mm, dst_start);
  33         if (!dst_vma)
  34                 return NULL;
  35
  36         if (dst_start < dst_vma->vm_start ||
  37             dst_start + len > dst_vma->vm_end)
  38                 return NULL;
  39
  40         /*
  41          * Check the vma is registered in uffd, this is required to
  42          * enforce the VM_MAYWRITE check done at uffd registration
  43          * time.
  44          */
  45         if (!dst_vma->vm_userfaultfd_ctx.ctx)
  46                 return NULL;
  47
  48         return dst_vma;
  49 }
  50
  51 static int mcopy_atomic_pte(struct mm_struct *dst_mm,
  52                             pmd_t *dst_pmd,
  53                             struct vm_area_struct *dst_vma,
  54                             unsigned long dst_addr,
  55                             unsigned long src_addr,
  56                             struct page **pagep,
  57                             bool wp_copy)
  58 {
  59         pte_t _dst_pte, *dst_pte;
  60         spinlock_t *ptl;
  61         void *page_kaddr;
  62         int ret;
  63         struct page *page;
  64         pgoff_t offset, max_off;
  65         struct inode *inode;
  66
  67         if (!*pagep) {
  68                 ret = -ENOMEM;
  69                 page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, dst_vma, dst_addr);
  70                 if (!page)
  71                         goto out;
  72
  73                 page_kaddr = kmap_atomic(page);
  74                 ret = copy_from_user(page_kaddr,
  75                                      (const void __user *) src_addr,
  76                                      PAGE_SIZE);
  77                 kunmap_atomic(page_kaddr);
  78
  79                 /* fallback to copy_from_user outside mmap_lock */
  80                 if (unlikely(ret)) {
  81                         ret = -ENOENT;
  82                         *pagep = page;
  83                         /* don't free the page */
  84                         goto out;
  85                 }
  86         } else {
  87                 page = *pagep;
  88                 *pagep = NULL;
  89         }
  90
  91         /*
  92          * The memory barrier inside __SetPageUptodate makes sure that
  93          * preceding stores to the page contents become visible before
  94          * the set_pte_at() write.
  95          */
  96         __SetPageUptodate(page);
  97
  98         ret = -ENOMEM;
  99         if (mem_cgroup_charge(page, dst_mm, GFP_KERNEL))
 100                 goto out_release;
 101
 102         _dst_pte = pte_mkdirty(mk_pte(page, dst_vma->vm_page_prot));
 103         if (dst_vma->vm_flags & VM_WRITE) {
 104                 if (wp_copy)
 105                         _dst_pte = pte_mkuffd_wp(_dst_pte);
 106                 else
 107                         _dst_pte = pte_mkwrite(_dst_pte);
 108         }
 109
 110         dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
 111         if (dst_vma->vm_file) {
 112                 /* the shmem MAP_PRIVATE case requires checking the i_size */
 113                 inode = dst_vma->vm_file->f_inode;
 114                 offset = linear_page_index(dst_vma, dst_addr);
 115                 max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
 116                 ret = -EFAULT;
 117                 if (unlikely(offset >= max_off))
 118                         goto out_release_uncharge_unlock;
 119         }
 120         ret = -EEXIST;
 121         if (!pte_none(*dst_pte))
 122                 goto out_release_uncharge_unlock;
 123
 124         inc_mm_counter(dst_mm, MM_ANONPAGES);
 125         page_add_new_anon_rmap(page, dst_vma, dst_addr, false);
 126         lru_cache_add_active_or_unevictable(page, dst_vma);
 127
 128         set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);
 129
 130         /* No need to invalidate - it was non-present before */
 131         update_mmu_cache(dst_vma, dst_addr, dst_pte);
 132
 133         pte_unmap_unlock(dst_pte, ptl);
 134         ret = 0;
 135 out:
 136         return ret;
 137 out_release_uncharge_unlock:
 138         pte_unmap_unlock(dst_pte, ptl);
 139 out_release:
 140         put_page(page);
 141         goto out;
 142 }
 143
 144 static int mfill_zeropage_pte(struct mm_struct *dst_mm,
 145                               pmd_t *dst_pmd,
 146                               struct vm_area_struct *dst_vma,
 147                               unsigned long dst_addr)
 148 {
 149         pte_t _dst_pte, *dst_pte;
 150         spinlock_t *ptl;
 151         int ret;
 152         pgoff_t offset, max_off;
 153         struct inode *inode;
 154
 155         _dst_pte = pte_mkspecial(pfn_pte(my_zero_pfn(dst_addr),
 156                                          dst_vma->vm_page_prot));
 157         dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
 158         if (dst_vma->vm_file) {
 159                 /* the shmem MAP_PRIVATE case requires checking the i_size */
 160                 inode = dst_vma->vm_file->f_inode;
 161                 offset = linear_page_index(dst_vma, dst_addr);
 162                 max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
 163                 ret = -EFAULT;
 164                 if (unlikely(offset >= max_off))
 165                         goto out_unlock;
 166         }
 167         ret = -EEXIST;
 168         if (!pte_none(*dst_pte))
 169                 goto out_unlock;
 170         set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);
 171         /* No need to invalidate - it was non-present before */
 172         update_mmu_cache(dst_vma, dst_addr, dst_pte);
 173         ret = 0;
 174 out_unlock:
 175         pte_unmap_unlock(dst_pte, ptl);
 176         return ret;
 177 }
 178
 179 static pmd_t *mm_alloc_pmd(struct mm_struct *mm, unsigned long address)
 180 {
 181         pgd_t *pgd;
 182         p4d_t *p4d;
 183         pud_t *pud;
 184
 185         pgd = pgd_offset(mm, address);
 186         p4d = p4d_alloc(mm, pgd, address);
 187         if (!p4d)
 188                 return NULL;
 189         pud = pud_alloc(mm, p4d, address);
 190         if (!pud)
 191                 return NULL;
 192         /*
 193          * Note that we didn't run this because the pmd was
 194          * missing, the *pmd may be already established and in
 195          * turn it may also be a trans_huge_pmd.
 196          */
 197         return pmd_alloc(mm, pud, address);
 198 }
 199
 200 #ifdef CONFIG_HUGETLB_PAGE
 201 /*
 202  * __mcopy_atomic processing for HUGETLB vmas.  Note that this routine is
 203  * called with mmap_lock held, it will release mmap_lock before returning.
 204  */
 205 static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
 206                                               struct vm_area_struct *dst_vma,
 207                                               unsigned long dst_start,
 208                                               unsigned long src_start,
 209                                               unsigned long len,
 210                                               bool zeropage)
 211 {
 212         int vm_alloc_shared = dst_vma->vm_flags & VM_SHARED;
 213         int vm_shared = dst_vma->vm_flags & VM_SHARED;
 214         ssize_t err;
 215         pte_t *dst_pte;
 216         unsigned long src_addr, dst_addr;
 217         long copied;
 218         struct page *page;
 219         unsigned long vma_hpagesize;
 220         pgoff_t idx;
 221         u32 hash;
 222         struct address_space *mapping;
 223
 224         /*
 225          * There is no default zero huge page for all huge page sizes as
 226          * supported by hugetlb.  A PMD_SIZE huge pages may exist as used
 227          * by THP.  Since we can not reliably insert a zero page, this
 228          * feature is not supported.
 229          */
 230         if (zeropage) {
 231                 mmap_read_unlock(dst_mm);
 232                 return -EINVAL;
 233         }
 234
 235         src_addr = src_start;
 236         dst_addr = dst_start;
 237         copied = 0;
 238         page = NULL;
 239         vma_hpagesize = vma_kernel_pagesize(dst_vma);
 240
 241         /*
 242          * Validate alignment based on huge page size
 243          */
 244         err = -EINVAL;
 245         if (dst_start & (vma_hpagesize - 1) || len & (vma_hpagesize - 1))
 246                 goto out_unlock;
 247
 248 retry:
 249         /*
 250          * On routine entry dst_vma is set.  If we had to drop mmap_lock and
 251          * retry, dst_vma will be set to NULL and we must lookup again.
 252          */
 253         if (!dst_vma) {
 254                 err = -ENOENT;
 255                 dst_vma = find_dst_vma(dst_mm, dst_start, len);
 256                 if (!dst_vma || !is_vm_hugetlb_page(dst_vma))
 257                         goto out_unlock;
 258
 259                 err = -EINVAL;
 260                 if (vma_hpagesize != vma_kernel_pagesize(dst_vma))
 261                         goto out_unlock;
 262
 263                 vm_shared = dst_vma->vm_flags & VM_SHARED;
 264         }
 265
 266         /*
 267          * If not shared, ensure the dst_vma has a anon_vma.
 268          */
 269         err = -ENOMEM;
 270         if (!vm_shared) {
 271                 if (unlikely(anon_vma_prepare(dst_vma)))
 272                         goto out_unlock;
 273         }
 274
 275         while (src_addr < src_start + len) {
 276                 pte_t dst_pteval;
 277
 278                 BUG_ON(dst_addr >= dst_start + len);
 279
 280                 /*
 281                  * Serialize via i_mmap_rwsem and hugetlb_fault_mutex.
 282                  * i_mmap_rwsem ensures the dst_pte remains valid even
 283                  * in the case of shared pmds.  fault mutex prevents
 284                  * races with other faulting threads.
 285                  */
 286                 mapping = dst_vma->vm_file->f_mapping;
 287                 i_mmap_lock_read(mapping);
 288                 idx = linear_page_index(dst_vma, dst_addr);
 289                 hash = hugetlb_fault_mutex_hash(mapping, idx);
 290                 mutex_lock(&hugetlb_fault_mutex_table[hash]);
 291
 292                 err = -ENOMEM;
 293                 dst_pte = huge_pte_alloc(dst_mm, dst_addr, vma_hpagesize);
 294                 if (!dst_pte) {
 295                         mutex_unlock(&hugetlb_fault_mutex_table[hash]);
 296                         i_mmap_unlock_read(mapping);
 297                         goto out_unlock;
 298                 }
 299
 300                 err = -EEXIST;
 301                 dst_pteval = huge_ptep_get(dst_pte);
 302                 if (!huge_pte_none(dst_pteval)) {
 303                         mutex_unlock(&hugetlb_fault_mutex_table[hash]);
 304                         i_mmap_unlock_read(mapping);
 305                         goto out_unlock;
 306                 }
 307
 308                 err = hugetlb_mcopy_atomic_pte(dst_mm, dst_pte, dst_vma,
 309                                                 dst_addr, src_addr, &page);
 310
 311                 mutex_unlock(&hugetlb_fault_mutex_table[hash]);
 312                 i_mmap_unlock_read(mapping);
 313                 vm_alloc_shared = vm_shared;
 314
 315                 cond_resched();
 316
 317                 if (unlikely(err == -ENOENT)) {
 318                         mmap_read_unlock(dst_mm);
 319                         BUG_ON(!page);
 320
 321                         err = copy_huge_page_from_user(page,
 322                                                 (const void __user *)src_addr,
 323                                                 vma_hpagesize / PAGE_SIZE,
 324                                                 true);
 325                         if (unlikely(err)) {
 326                                 err = -EFAULT;
 327                                 goto out;
 328                         }
 329                         mmap_read_lock(dst_mm);
 330
 331                         dst_vma = NULL;
 332                         goto retry;
 333                 } else
 334                         BUG_ON(page);
 335
 336                 if (!err) {
 337                         dst_addr += vma_hpagesize;
 338                         src_addr += vma_hpagesize;
 339                         copied += vma_hpagesize;
 340
 341                         if (fatal_signal_pending(current))
 342                                 err = -EINTR;
 343                 }
 344                 if (err)
 345                         break;
 346         }
 347
 348 out_unlock:
 349         mmap_read_unlock(dst_mm);
 350 out:
 351         if (page) {
 352                 /*
 353                  * We encountered an error and are about to free a newly
 354                  * allocated huge page.
 355                  *
 356                  * Reservation handling is very subtle, and is different for
 357                  * private and shared mappings.  See the routine
 358                  * restore_reserve_on_error for details.  Unfortunately, we
 359                  * can not call restore_reserve_on_error now as it would
 360                  * require holding mmap_lock.
 361                  *
 362                  * If a reservation for the page existed in the reservation
 363                  * map of a private mapping, the map was modified to indicate
 364                  * the reservation was consumed when the page was allocated.
 365                  * We clear the PagePrivate flag now so that the global
 366                  * reserve count will not be incremented in free_huge_page.
 367                  * The reservation map will still indicate the reservation
 368                  * was consumed and possibly prevent later page allocation.
 369                  * This is better than leaking a global reservation.  If no
 370                  * reservation existed, it is still safe to clear PagePrivate
 371                  * as no adjustments to reservation counts were made during
 372                  * allocation.
 373                  *
 374                  * The reservation map for shared mappings indicates which
 375                  * pages have reservations.  When a huge page is allocated
 376                  * for an address with a reservation, no change is made to
 377                  * the reserve map.  In this case PagePrivate will be set
 378                  * to indicate that the global reservation count should be
 379                  * incremented when the page is freed.  This is the desired
 380                  * behavior.  However, when a huge page is allocated for an
 381                  * address without a reservation a reservation entry is added
 382                  * to the reservation map, and PagePrivate will not be set.
 383                  * When the page is freed, the global reserve count will NOT
 384                  * be incremented and it will appear as though we have leaked
 385                  * reserved page.  In this case, set PagePrivate so that the
 386                  * global reserve count will be incremented to match the
 387                  * reservation map entry which was created.
 388                  *
 389                  * Note that vm_alloc_shared is based on the flags of the vma
 390                  * for which the page was originally allocated.  dst_vma could
 391                  * be different or NULL on error.
 392                  */
 393                 if (vm_alloc_shared)
 394                         SetPagePrivate(page);
 395                 else
 396                         ClearPagePrivate(page);
 397                 put_page(page);
 398         }
 399         BUG_ON(copied < 0);
 400         BUG_ON(err > 0);
 401         BUG_ON(!copied && !err);
 402         return copied ? copied : err;
 403 }
 404 #else /* !CONFIG_HUGETLB_PAGE */
 405 /* fail at build time if gcc attempts to use this */
 406 extern ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
 407                                       struct vm_area_struct *dst_vma,
 408                                       unsigned long dst_start,
 409                                       unsigned long src_start,
 410                                       unsigned long len,
 411                                       bool zeropage);
 412 #endif /* CONFIG_HUGETLB_PAGE */
 413
 414 static __always_inline ssize_t mfill_atomic_pte(struct mm_struct *dst_mm,
 415                                                 pmd_t *dst_pmd,
 416                                                 struct vm_area_struct *dst_vma,
 417                                                 unsigned long dst_addr,
 418                                                 unsigned long src_addr,
 419                                                 struct page **page,
 420                                                 bool zeropage,
 421                                                 bool wp_copy)
 422 {
 423         ssize_t err;
 424
 425         /*
 426          * The normal page fault path for a shmem will invoke the
 427          * fault, fill the hole in the file and COW it right away. The
 428          * result generates plain anonymous memory. So when we are
 429          * asked to fill an hole in a MAP_PRIVATE shmem mapping, we'll
 430          * generate anonymous memory directly without actually filling
 431          * the hole. For the MAP_PRIVATE case the robustness check
 432          * only happens in the pagetable (to verify it's still none)
 433          * and not in the radix tree.
 434          */
 435         if (!(dst_vma->vm_flags & VM_SHARED)) {
 436                 if (!zeropage)
 437                         err = mcopy_atomic_pte(dst_mm, dst_pmd, dst_vma,
 438                                                dst_addr, src_addr, page,
 439                                                wp_copy);
 440                 else
 441                         err = mfill_zeropage_pte(dst_mm, dst_pmd,
 442                                                  dst_vma, dst_addr);
 443         } else {
 444                 VM_WARN_ON_ONCE(wp_copy);
 445                 if (!zeropage)
 446                         err = shmem_mcopy_atomic_pte(dst_mm, dst_pmd,
 447                                                      dst_vma, dst_addr,
 448                                                      src_addr, page);
 449                 else
 450                         err = shmem_mfill_zeropage_pte(dst_mm, dst_pmd,
 451                                                        dst_vma, dst_addr);
 452         }
 453
 454         return err;
 455 }
 456
 457 static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm,
 458                                               unsigned long dst_start,
 459                                               unsigned long src_start,
 460                                               unsigned long len,
 461                                               bool zeropage,
 462                                               bool *mmap_changing,
 463                                               __u64 mode)
 464 {
 465         struct vm_area_struct *dst_vma;
 466         ssize_t err;
 467         pmd_t *dst_pmd;
 468         unsigned long src_addr, dst_addr;
 469         long copied;
 470         struct page *page;
 471         bool wp_copy;
 472
 473         /*
 474          * Sanitize the command parameters:
 475          */
 476         BUG_ON(dst_start & ~PAGE_MASK);
 477         BUG_ON(len & ~PAGE_MASK);
 478
 479         /* Does the address range wrap, or is the span zero-sized? */
 480         BUG_ON(src_start + len <= src_start);
 481         BUG_ON(dst_start + len <= dst_start);
 482
 483         src_addr = src_start;
 484         dst_addr = dst_start;
 485         copied = 0;
 486         page = NULL;
 487 retry:
 488         mmap_read_lock(dst_mm);
 489
 490         /*
 491          * If memory mappings are changing because of non-cooperative
 492          * operation (e.g. mremap) running in parallel, bail out and
 493          * request the user to retry later
 494          */
 495         err = -EAGAIN;
 496         if (mmap_changing && READ_ONCE(*mmap_changing))
 497                 goto out_unlock;
 498
 499         /*
 500          * Make sure the vma is not shared, that the dst range is
 501          * both valid and fully within a single existing vma.
 502          */
 503         err = -ENOENT;
 504         dst_vma = find_dst_vma(dst_mm, dst_start, len);
 505         if (!dst_vma)
 506                 goto out_unlock;
 507
 508         err = -EINVAL;
 509         /*
 510          * shmem_zero_setup is invoked in mmap for MAP_ANONYMOUS|MAP_SHARED but
 511          * it will overwrite vm_ops, so vma_is_anonymous must return false.
 512          */
 513         if (WARN_ON_ONCE(vma_is_anonymous(dst_vma) &&
 514             dst_vma->vm_flags & VM_SHARED))
 515                 goto out_unlock;
 516
 517         /*
 518          * validate 'mode' now that we know the dst_vma: don't allow
 519          * a wrprotect copy if the userfaultfd didn't register as WP.
 520          */
 521         wp_copy = mode & UFFDIO_COPY_MODE_WP;
 522         if (wp_copy && !(dst_vma->vm_flags & VM_UFFD_WP))
 523                 goto out_unlock;
 524
 525         /*
 526          * If this is a HUGETLB vma, pass off to appropriate routine
 527          */
 528         if (is_vm_hugetlb_page(dst_vma))
 529                 return  __mcopy_atomic_hugetlb(dst_mm, dst_vma, dst_start,
 530                                                 src_start, len, zeropage);
 531
 532         if (!vma_is_anonymous(dst_vma) && !vma_is_shmem(dst_vma))
 533                 goto out_unlock;
 534
 535         /*
 536          * Ensure the dst_vma has a anon_vma or this page
 537          * would get a NULL anon_vma when moved in the
 538          * dst_vma.
 539          */
 540         err = -ENOMEM;
 541         if (!(dst_vma->vm_flags & VM_SHARED) &&
 542             unlikely(anon_vma_prepare(dst_vma)))
 543                 goto out_unlock;
 544
 545         while (src_addr < src_start + len) {
 546                 pmd_t dst_pmdval;
 547
 548                 BUG_ON(dst_addr >= dst_start + len);
 549
 550                 dst_pmd = mm_alloc_pmd(dst_mm, dst_addr);
 551                 if (unlikely(!dst_pmd)) {
 552                         err = -ENOMEM;
 553                         break;
 554                 }
 555
 556                 dst_pmdval = pmd_read_atomic(dst_pmd);
 557                 /*
 558                  * If the dst_pmd is mapped as THP don't
 559                  * override it and just be strict.
 560                  */
 561                 if (unlikely(pmd_trans_huge(dst_pmdval))) {
 562                         err = -EEXIST;
 563                         break;
 564                 }
 565                 if (unlikely(pmd_none(dst_pmdval)) &&
 566                     unlikely(__pte_alloc(dst_mm, dst_pmd))) {
 567                         err = -ENOMEM;
 568                         break;
 569                 }
 570                 /* If an huge pmd materialized from under us fail */
 571                 if (unlikely(pmd_trans_huge(*dst_pmd))) {
 572                         err = -EFAULT;
 573                         break;
 574                 }
 575
 576                 BUG_ON(pmd_none(*dst_pmd));
 577                 BUG_ON(pmd_trans_huge(*dst_pmd));
 578
 579                 err = mfill_atomic_pte(dst_mm, dst_pmd, dst_vma, dst_addr,
 580                                        src_addr, &page, zeropage, wp_copy);
 581                 cond_resched();
 582
 583                 if (unlikely(err == -ENOENT)) {
 584                         void *page_kaddr;
 585
 586                         mmap_read_unlock(dst_mm);
 587                         BUG_ON(!page);
 588
 589                         page_kaddr = kmap(page);
 590                         err = copy_from_user(page_kaddr,
 591                                              (const void __user *) src_addr,
 592                                              PAGE_SIZE);
 593                         kunmap(page);
 594                         if (unlikely(err)) {
 595                                 err = -EFAULT;
 596                                 goto out;
 597                         }
 598                         goto retry;
 599                 } else
 600                         BUG_ON(page);
 601
 602                 if (!err) {
 603                         dst_addr += PAGE_SIZE;
 604                         src_addr += PAGE_SIZE;
 605                         copied += PAGE_SIZE;
 606
 607                         if (fatal_signal_pending(current))
 608                                 err = -EINTR;
 609                 }
 610                 if (err)
 611                         break;
 612         }
 613
 614 out_unlock:
 615         mmap_read_unlock(dst_mm);
 616 out:
 617         if (page)
 618                 put_page(page);
 619         BUG_ON(copied < 0);
 620         BUG_ON(err > 0);
 621         BUG_ON(!copied && !err);
 622         return copied ? copied : err;
 623 }
 624
 625 ssize_t mcopy_atomic(struct mm_struct *dst_mm, unsigned long dst_start,
 626                      unsigned long src_start, unsigned long len,
 627                      bool *mmap_changing, __u64 mode)
 628 {
 629         return __mcopy_atomic(dst_mm, dst_start, src_start, len, false,
 630                               mmap_changing, mode);
 631 }
 632
 633 ssize_t mfill_zeropage(struct mm_struct *dst_mm, unsigned long start,
 634                        unsigned long len, bool *mmap_changing)
 635 {
 636         return __mcopy_atomic(dst_mm, start, 0, len, true, mmap_changing, 0);
 637 }
 638
 639 int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start,
 640                         unsigned long len, bool enable_wp, bool *mmap_changing)
 641 {
 642         struct vm_area_struct *dst_vma;
 643         pgprot_t newprot;
 644         int err;
 645
 646         /*
 647          * Sanitize the command parameters:
 648          */
 649         BUG_ON(start & ~PAGE_MASK);
 650         BUG_ON(len & ~PAGE_MASK);
 651
 652         /* Does the address range wrap, or is the span zero-sized? */
 653         BUG_ON(start + len <= start);
 654
 655         mmap_read_lock(dst_mm);
 656
 657         /*
 658          * If memory mappings are changing because of non-cooperative
 659          * operation (e.g. mremap) running in parallel, bail out and
 660          * request the user to retry later
 661          */
 662         err = -EAGAIN;
 663         if (mmap_changing && READ_ONCE(*mmap_changing))
 664                 goto out_unlock;
 665
 666         err = -ENOENT;
 667         dst_vma = find_dst_vma(dst_mm, start, len);
 668         /*
 669          * Make sure the vma is not shared, that the dst range is
 670          * both valid and fully within a single existing vma.
 671          */
 672         if (!dst_vma || (dst_vma->vm_flags & VM_SHARED))
 673                 goto out_unlock;
 674         if (!userfaultfd_wp(dst_vma))
 675                 goto out_unlock;
 676         if (!vma_is_anonymous(dst_vma))
 677                 goto out_unlock;
 678
 679         if (enable_wp)
 680                 newprot = vm_get_page_prot(dst_vma->vm_flags & ~(VM_WRITE));
 681         else
 682                 newprot = vm_get_page_prot(dst_vma->vm_flags);
 683
 684         change_protection(dst_vma, start, start + len, newprot,
 685                           enable_wp ? MM_CP_UFFD_WP : MM_CP_UFFD_WP_RESOLVE);
 686
 687         err = 0;
 688 out_unlock:
 689         mmap_read_unlock(dst_mm);
 690         return err;
 691 }