[PATCH v4] userfaultfd/shmem: fix MCOPY_ATOMIC_CONTINUE behavior

1 Apr 2021

Previously, the continue implementation in shmem_mcopy_atomic_pte was
incorrect for two main reasons:
- It didn't correctly skip some sections of code which make sense for
  newly allocated pages, but absolutely don't make sense for
  pre-existing page cache pages.
- Because shmem_mcopy_continue_pte is called only if VM_SHARED is
  detected in mm/userfaultd.c, we were incorrectly not supporting the
  case where a tmpfs file had been mmap()-ed with MAP_PRIVATE.
So, this patch does the following:
In mm/userfaultfd.c, break the logic to install PTEs out of
mcopy_atomic_pte, into a separate helper function.
In mfill_atomic_pte, for the MCOPY_ATOMIC_CONTINUE case, simply look
up the existing page in the page cache, and then use the PTE
installation helper to setup the mapping. This addresses the two issues
noted above.
The previous code's bugs manifested clearly in the error handling path.
So, to detect this kind of issue in the future, modify the selftest to
exercise the error handling path as well.
Note that this patch is based on linux-next/akpm; the "fixes" line
refers to a SHA1 in that branch.
Changes since v3:
- Significantly refactored the patch. Continue handling now happens in
  mm/userfaultfd.c, via a PTE installation helper. Most of the
  mm/shmem.c changes from the patch being fixed [1] are reverted.
Changes since v2:
- Drop the ClearPageDirty() entirely, instead of trying to remember the
  old value.
- Modify both pgoff / max_off checks to use pgoff. It's equivalent to
  offset, but offset wasn't initialized until the first check (which
  we're skipping).
- Keep the second pgoff / max_off check in the continue case.
Changes since v1:
- Refactor to skip ahead with goto, instead of adding several more
  "if (!is_continue)".
- Fix unconditional ClearPageDirty().
- Don't pte_mkwrite() when is_continue && !VM_SHARED.
[1] https://lore.kernel.org/patchwork/patch/1392464/
Fixes: 00da60b9d0a0 ("userfaultfd: support minor fault handling for shmem")
Signed-off-by: Axel Rasmussen axelrasmussen@google.com
---
 mm/shmem.c       |  56 ++++++---------
 mm/userfaultfd.c | 183 ++++++++++++++++++++++++++++++++++-------------
 2 files changed, 156 insertions(+), 83 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 5cfd2fb6e52b..9d9a9f254f33 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2366,7 +2366,6 @@ int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
    		   unsigned long dst_addr, unsigned long src_addr,
    		   enum mcopy_atomic_mode mode, struct page **pagep)
 {
-	bool is_continue = (mode == MCOPY_ATOMIC_CONTINUE);
    struct inode *inode = file_inode(dst_vma->vm_file);
    struct shmem_inode_info *info = SHMEM_I(inode);
    struct address_space *mapping = inode->i_mapping;
@@ -2377,18 +2376,17 @@ int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
    struct page *page;
    pte_t _dst_pte, *dst_pte;
    int ret;
-	pgoff_t offset, max_off;
+	pgoff_t max_off;
+
+	/* Handled by mcontinue_atomic_pte instead. */
+	if (WARN_ON_ONCE(mode == MCOPY_ATOMIC_CONTINUE))
+		return -EINVAL;
ret = -ENOMEM;
    if (!shmem_inode_acct_block(inode, 1))
    	goto out;
-	if (is_continue) {
-		ret = -EFAULT;
-		page = find_lock_page(mapping, pgoff);
-		if (!page)
-			goto out_unacct_blocks;
-	} else if (!*pagep) {
+	if (!*pagep) {
    	page = shmem_alloc_page(gfp, info, pgoff);
    	if (!page)
    		goto out_unacct_blocks;
@@ -2415,27 +2413,21 @@ int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
    	*pagep = NULL;
    }
-	if (!is_continue) {
-		VM_BUG_ON(PageSwapBacked(page));
-		VM_BUG_ON(PageLocked(page));
-		__SetPageLocked(page);
-		__SetPageSwapBacked(page);
-		__SetPageUptodate(page);
-	}
+	VM_BUG_ON(PageSwapBacked(page));
+	VM_BUG_ON(PageLocked(page));
+	__SetPageLocked(page);
+	__SetPageSwapBacked(page);
+	__SetPageUptodate(page);
ret = -EFAULT;
-	offset = linear_page_index(dst_vma, dst_addr);
    max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
-	if (unlikely(offset >= max_off))
+	if (unlikely(pgoff >= max_off))
    	goto out_release;
-	/* If page wasn't already in the page cache, add it. */
-	if (!is_continue) {
-		ret = shmem_add_to_page_cache(page, mapping, pgoff, NULL,
-					      gfp & GFP_RECLAIM_MASK, dst_mm);
-		if (ret)
-			goto out_release;
-	}
+	ret = shmem_add_to_page_cache(page, mapping, pgoff, NULL,
+				      gfp & GFP_RECLAIM_MASK, dst_mm);
+	if (ret)
+		goto out_release;
_dst_pte = mk_pte(page, dst_vma->vm_page_prot);
    if (dst_vma->vm_flags & VM_WRITE)
@@ -2455,22 +2447,20 @@ int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
ret = -EFAULT;
    max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
-	if (unlikely(offset >= max_off))
+	if (unlikely(pgoff >= max_off))
    	goto out_release_unlock;
ret = -EEXIST;
    if (!pte_none(*dst_pte))
    	goto out_release_unlock;
-	if (!is_continue) {
-		lru_cache_add(page);
+	lru_cache_add(page);
-		spin_lock_irq(&info->lock);
-		info->alloced++;
-		inode->i_blocks += BLOCKS_PER_PAGE;
-		shmem_recalc_inode(inode);
-		spin_unlock_irq(&info->lock);
-	}
+	spin_lock_irq(&info->lock);
+	info->alloced++;
+	inode->i_blocks += BLOCKS_PER_PAGE;
+	shmem_recalc_inode(inode);
+	spin_unlock_irq(&info->lock);
inc_mm_counter(dst_mm, mm_counter_file(page));
    page_add_file_rmap(page, false);
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index cbb7c8d79a4d..286d0657fbe2 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -48,21 +48,103 @@ struct vm_area_struct *find_dst_vma(struct mm_struct *dst_mm,
    return dst_vma;
 }
+/*
+ * Install PTEs, to map dst_addr (within dst_vma) to page.
+ *
+ * This function handles MCOPY_ATOMIC_CONTINUE (which is always file-backed),
+ * whether or not dst_vma is VM_SHARED. It also handles the more general
+ * MCOPY_ATOMIC_NORMAL case, when dst_vma is *not* VM_SHARED (it may be file
+ * backed, or not).
+ *
+ * Note that MCOPY_ATOMIC_NORMAL for a VM_SHARED dst_vma is handled by
+ * shmem_mcopy_atomic_pte instead.
+ */
+static int mcopy_atomic_install_ptes(struct mm_struct *dst_mm, pmd_t *dst_pmd,
+				     struct vm_area_struct *dst_vma,
+				     unsigned long dst_addr, struct page *page,
+				     enum mcopy_atomic_mode mode, bool wp_copy)
+{
+	int ret;
+	pte_t _dst_pte, *dst_pte;
+	bool is_continue = mode == MCOPY_ATOMIC_CONTINUE;
+	int writable;
+	bool vm_shared = dst_vma->vm_flags & VM_SHARED;
+	bool is_file_backed = dst_vma->vm_file;
+	spinlock_t *ptl;
+	struct inode *inode;
+	pgoff_t offset, max_off;
+
+	_dst_pte = mk_pte(page, dst_vma->vm_page_prot);
+	writable = dst_vma->vm_flags & VM_WRITE;
+	/* For CONTINUE on a non-shared VMA, don't pte_mkwrite for CoW. */
+	if (is_continue && !vm_shared)
+		writable = 0;
+
+	if (writable) {
+		_dst_pte = pte_mkdirty(_dst_pte);
+		if (wp_copy)
+			_dst_pte = pte_mkuffd_wp(_dst_pte);
+		else
+			_dst_pte = pte_mkwrite(_dst_pte);
+	} else if (vm_shared) {
+		/*
+		 * Since we didn't pte_mkdirty(), mark the page dirty or it
+		 * could be freed from under us. We could do this
+		 * unconditionally, but doing it only if !writable is faster.
+		 */
+		set_page_dirty(page);
+	}
+
+	dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
+
+	if (is_file_backed) {
+		/* The shmem MAP_PRIVATE case requires checking the i_size */
+		inode = dst_vma->vm_file->f_inode;
+		offset = linear_page_index(dst_vma, dst_addr);
+		max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
+		ret = -EFAULT;
+		if (unlikely(offset >= max_off))
+			goto out_unlock;
+	}
+
+	ret = -EEXIST;
+	if (!pte_none(*dst_pte))
+		goto out_unlock;
+
+	inc_mm_counter(dst_mm, mm_counter(page));
+	if (is_file_backed)
+		page_add_file_rmap(page, false);
+	else
+		page_add_new_anon_rmap(page, dst_vma, dst_addr, false);
+
+	if (!is_continue)
+		lru_cache_add_inactive_or_unevictable(page, dst_vma);
+
+	set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);
+
+	/* No need to invalidate - it was non-present before */
+	update_mmu_cache(dst_vma, dst_addr, dst_pte);
+	pte_unmap_unlock(dst_pte, ptl);
+	ret = 0;
+out:
+	return ret;
+out_unlock:
+	pte_unmap_unlock(dst_pte, ptl);
+	goto out;
+}
+
 static int mcopy_atomic_pte(struct mm_struct *dst_mm,
    		    pmd_t *dst_pmd,
    		    struct vm_area_struct *dst_vma,
    		    unsigned long dst_addr,
    		    unsigned long src_addr,
    		    struct page **pagep,
+			    enum mcopy_atomic_mode mode,
    		    bool wp_copy)
 {
-	pte_t _dst_pte, *dst_pte;
-	spinlock_t *ptl;
    void *page_kaddr;
    int ret;
    struct page *page;
-	pgoff_t offset, max_off;
-	struct inode *inode;
if (!*pagep) {
    	ret = -ENOMEM;
@@ -99,43 +181,12 @@ static int mcopy_atomic_pte(struct mm_struct *dst_mm,
    if (mem_cgroup_charge(page, dst_mm, GFP_KERNEL))
    	goto out_release;
-	_dst_pte = pte_mkdirty(mk_pte(page, dst_vma->vm_page_prot));
-	if (dst_vma->vm_flags & VM_WRITE) {
-		if (wp_copy)
-			_dst_pte = pte_mkuffd_wp(_dst_pte);
-		else
-			_dst_pte = pte_mkwrite(_dst_pte);
-	}
-
-	dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
-	if (dst_vma->vm_file) {
-		/* the shmem MAP_PRIVATE case requires checking the i_size */
-		inode = dst_vma->vm_file->f_inode;
-		offset = linear_page_index(dst_vma, dst_addr);
-		max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
-		ret = -EFAULT;
-		if (unlikely(offset >= max_off))
-			goto out_release_uncharge_unlock;
-	}
-	ret = -EEXIST;
-	if (!pte_none(*dst_pte))
-		goto out_release_uncharge_unlock;
-
-	inc_mm_counter(dst_mm, MM_ANONPAGES);
-	page_add_new_anon_rmap(page, dst_vma, dst_addr, false);
-	lru_cache_add_inactive_or_unevictable(page, dst_vma);
-
-	set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);
-
-	/* No need to invalidate - it was non-present before */
-	update_mmu_cache(dst_vma, dst_addr, dst_pte);
-
-	pte_unmap_unlock(dst_pte, ptl);
-	ret = 0;
+	ret = mcopy_atomic_install_ptes(dst_mm, dst_pmd, dst_vma, dst_addr,
+					page, mode, wp_copy);
+	if (ret)
+		goto out_release;
 out:
    return ret;
-out_release_uncharge_unlock:
-	pte_unmap_unlock(dst_pte, ptl);
 out_release:
    put_page(page);
    goto out;
@@ -176,6 +227,38 @@ static int mfill_zeropage_pte(struct mm_struct *dst_mm,
    return ret;
 }
+static int mcontinue_atomic_pte(struct mm_struct *dst_mm,
+				pmd_t *dst_pmd,
+				struct vm_area_struct *dst_vma,
+				unsigned long dst_addr,
+				bool wp_copy)
+{
+	struct inode *inode = file_inode(dst_vma->vm_file);
+	struct address_space *mapping = inode->i_mapping;
+	pgoff_t pgoff = linear_page_index(dst_vma, dst_addr);
+	struct page *page;
+	int ret;
+
+	ret = -EFAULT;
+	page = find_lock_page(mapping, pgoff);
+	if (!page)
+		goto out;
+
+	ret = mcopy_atomic_install_ptes(dst_mm, dst_pmd, dst_vma, dst_addr,
+					page, MCOPY_ATOMIC_CONTINUE, wp_copy);
+	if (ret)
+		goto out_release;
+
+	unlock_page(page);
+	ret = 0;
+out:
+	return ret;
+out_release:
+	unlock_page(page);
+	put_page(page);
+	goto out;
+}
+
 static pmd_t *mm_alloc_pmd(struct mm_struct *mm, unsigned long address)
 {
    pgd_t *pgd;
@@ -418,7 +501,13 @@ static __always_inline ssize_t mfill_atomic_pte(struct mm_struct *dst_mm,
    					enum mcopy_atomic_mode mode,
    					bool wp_copy)
 {
-	ssize_t err;
+	ssize_t err = 0;
+
+	if (mode == MCOPY_ATOMIC_CONTINUE) {
+		err = mcontinue_atomic_pte(dst_mm, dst_pmd, dst_vma, dst_addr,
+					   wp_copy);
+		goto out;
+	}
/*
     * The normal page fault path for a shmem will invoke the
@@ -431,26 +520,20 @@ static __always_inline ssize_t mfill_atomic_pte(struct mm_struct *dst_mm,
     * and not in the radix tree.
     */
    if (!(dst_vma->vm_flags & VM_SHARED)) {
-		switch (mode) {
-		case MCOPY_ATOMIC_NORMAL:
+		if (mode == MCOPY_ATOMIC_NORMAL)
    		err = mcopy_atomic_pte(dst_mm, dst_pmd, dst_vma,
    				       dst_addr, src_addr, page,
-					       wp_copy);
-			break;
-		case MCOPY_ATOMIC_ZEROPAGE:
+					       mode, wp_copy);
+		else if (mode == MCOPY_ATOMIC_ZEROPAGE)
    		err = mfill_zeropage_pte(dst_mm, dst_pmd,
    					 dst_vma, dst_addr);
-			break;
-		case MCOPY_ATOMIC_CONTINUE:
-			err = -EINVAL;
-			break;
-		}
    } else {
    	VM_WARN_ON_ONCE(wp_copy);
    	err = shmem_mcopy_atomic_pte(dst_mm, dst_pmd, dst_vma, dst_addr,
    				     src_addr, mode, page);
    }
+out:
    return err;
 }
--
2.31.0.208.g409f899ff0-goog

    

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[PATCH v4] userfaultfd/shmem: fix MCOPY_ATOMIC_CONTINUE behavior