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>
diff --git a/mm/mmap.c b/mm/mmap.c
index 93d28c7..7f85520 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -89,6 +89,25 @@
}
EXPORT_SYMBOL(vm_get_page_prot);
+static pgprot_t vm_pgprot_modify(pgprot_t oldprot, unsigned long vm_flags)
+{
+ return pgprot_modify(oldprot, vm_get_page_prot(vm_flags));
+}
+
+/* Update vma->vm_page_prot to reflect vma->vm_flags. */
+void vma_set_page_prot(struct vm_area_struct *vma)
+{
+ unsigned long vm_flags = vma->vm_flags;
+
+ vma->vm_page_prot = vm_pgprot_modify(vma->vm_page_prot, vm_flags);
+ if (vma_wants_writenotify(vma)) {
+ vm_flags &= ~VM_SHARED;
+ vma->vm_page_prot = vm_pgprot_modify(vma->vm_page_prot,
+ vm_flags);
+ }
+}
+
+
int sysctl_overcommit_memory __read_mostly = OVERCOMMIT_GUESS; /* heuristic overcommit */
int sysctl_overcommit_ratio __read_mostly = 50; /* default is 50% */
unsigned long sysctl_overcommit_kbytes __read_mostly;
@@ -1475,11 +1494,16 @@
if (vma->vm_ops && vma->vm_ops->page_mkwrite)
return 1;
- /* The open routine did something to the protections already? */
+ /* The open routine did something to the protections that pgprot_modify
+ * won't preserve? */
if (pgprot_val(vma->vm_page_prot) !=
- pgprot_val(vm_get_page_prot(vm_flags)))
+ pgprot_val(vm_pgprot_modify(vma->vm_page_prot, vm_flags)))
return 0;
+ /* Do we need to track softdirty? */
+ if (IS_ENABLED(CONFIG_MEM_SOFT_DIRTY) && !(vm_flags & VM_SOFTDIRTY))
+ return 1;
+
/* Specialty mapping? */
if (vm_flags & VM_PFNMAP)
return 0;
@@ -1615,21 +1639,6 @@
goto free_vma;
}
- if (vma_wants_writenotify(vma)) {
- pgprot_t pprot = vma->vm_page_prot;
-
- /* Can vma->vm_page_prot have changed??
- *
- * Answer: Yes, drivers may have changed it in their
- * f_op->mmap method.
- *
- * Ensures that vmas marked as uncached stay that way.
- */
- vma->vm_page_prot = vm_get_page_prot(vm_flags & ~VM_SHARED);
- if (pgprot_val(pprot) == pgprot_val(pgprot_noncached(pprot)))
- vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
- }
-
vma_link(mm, vma, prev, rb_link, rb_parent);
/* Once vma denies write, undo our temporary denial count */
if (file) {
@@ -1663,6 +1672,8 @@
*/
vma->vm_flags |= VM_SOFTDIRTY;
+ vma_set_page_prot(vma);
+
return addr;
unmap_and_free_vma: