mm: softdirty: enable write notifications on VMAs after VM_SOFTDIRTY cleared

For VMAs that don't want write notifications, PTEs created for read faults
have their write bit set.  If the read fault happens after VM_SOFTDIRTY is
cleared, then the PTE's softdirty bit will remain clear after subsequent
writes.

Here's a simple code snippet to demonstrate the bug:

  char* m = mmap(NULL, getpagesize(), PROT_READ | PROT_WRITE,
                 MAP_ANONYMOUS | MAP_SHARED, -1, 0);
  system("echo 4 > /proc/$PPID/clear_refs"); /* clear VM_SOFTDIRTY */
  assert(*m == '\0');     /* new PTE allows write access */
  assert(!soft_dirty(x));
  *m = 'x';               /* should dirty the page */
  assert(soft_dirty(x));  /* fails */

With this patch, write notifications are enabled when VM_SOFTDIRTY is
cleared.  Furthermore, to avoid unnecessary faults, write notifications
are disabled when VM_SOFTDIRTY is set.

As a side effect of enabling and disabling write notifications with
care, this patch fixes a bug in mprotect where vm_page_prot bits set by
drivers were zapped on mprotect.  An analogous bug was fixed in mmap by
commit c9d0bf241451 ("mm: uncached vma support with writenotify").

Signed-off-by: Peter Feiner <pfeiner@google.com>
Reported-by: Peter Feiner <pfeiner@google.com>
Suggested-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Jamie Liu <jamieliu@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 14 insertions, 5 deletions

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index b7a7dc963a3..4e0388cffe3 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -827,8 +827,21 @@ static ssize_t clear_refs_write(struct file *file, const char __user *buf,
 			.private = &cp,
 		};
 		down_read(&mm->mmap_sem);
-		if (type == CLEAR_REFS_SOFT_DIRTY)
+		if (type == CLEAR_REFS_SOFT_DIRTY) {
+			for (vma = mm->mmap; vma; vma = vma->vm_next) {
+				if (!(vma->vm_flags & VM_SOFTDIRTY))
+					continue;
+				up_read(&mm->mmap_sem);
+				down_write(&mm->mmap_sem);
+				for (vma = mm->mmap; vma; vma = vma->vm_next) {
+					vma->vm_flags &= ~VM_SOFTDIRTY;
+					vma_set_page_prot(vma);
+				}
+				downgrade_write(&mm->mmap_sem);
+				break;
+			}
 			mmu_notifier_invalidate_range_start(mm, 0, -1);
+		}
 		for (vma = mm->mmap; vma; vma = vma->vm_next) {
 			cp.vma = vma;
 			if (is_vm_hugetlb_page(vma))
@@ -848,10 +861,6 @@ static ssize_t clear_refs_write(struct file *file, const char __user *buf,
 				continue;
 			if (type == CLEAR_REFS_MAPPED && !vma->vm_file)
 				continue;
-			if (type == CLEAR_REFS_SOFT_DIRTY) {
-				if (vma->vm_flags & VM_SOFTDIRTY)
-					vma->vm_flags &= ~VM_SOFTDIRTY;
-			}
 			walk_page_range(vma->vm_start, vma->vm_end,
 					&clear_refs_walk);
 		}