[PATCH] mm: softdirty: write protect PTEs created for read faults after VM_SOFTDIRTY cleared

[Peter Feiner <pfeiner@google.com>]

In readable+writable+shared VMAs, 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, new PTEs created for read faults are write protected
if the VMA has VM_SOFTDIRTY clear.

Signed-off-by: Peter Feiner <pfeiner@google.com>
---
 mm/memory.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/memory.c b/mm/memory.c
index ab3537b..282a959 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2755,6 +2755,8 @@ void do_set_pte(struct vm_area_struct *vma, unsigned long address,
 		entry = maybe_mkwrite(pte_mkdirty(entry), vma);
 	else if (pte_file(*pte) && pte_file_soft_dirty(*pte))
 		entry = pte_mksoft_dirty(entry);
+	else if (!(vma->vm_flags & VM_SOFTDIRTY))
+		entry = pte_wrprotect(entry);
 	if (anon) {
 		inc_mm_counter_fast(vma->vm_mm, MM_ANONPAGES);
 		page_add_new_anon_rmap(page, vma, address);
-- 
2.1.0.rc2.206.gedb03e5

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