Re: [patch] mm: fix race in COW logic

[Nick Piggin <npiggin@suse.de>]

On Mon, Jun 23, 2008 at 02:18:31PM +0200, Nick Piggin wrote:
> On Mon, Jun 23, 2008 at 11:04:31AM +0100, Hugh Dickins wrote:
> > moving the page_remove_rmap down was to be fully effective, it needed
> > to move through a suitable barrier; it hadn't occurred to me that it
> > was carrying the suitable barrier with it.  But if that is indeed
> > correct, I think it would be better to rely upon that, than resort
> > to more difficult arguments.
> 
> No I actually think you make a good point, and I'll resubmit the
> patch with a replacement comment to say we've got the ordering
> covered if nothing else then by the atomic op in rmap.

OK, this is a new comment. I don't actually know if it is any good.
It is hard to be coherent if you write these things in English.
Maybe it is best to illustrate with the interleaving diagram in the
changelog?

--
There is a race in the COW logic. It contains a shortcut to avoid the
COW and reuse the page if we have the sole reference on the page, however it
is possible to have two racing do_wp_page()ers with one causing the other to
mistakenly believe it is safe to take the shortcut when it is not. This could
lead to data corruption.

Process 1 and process2 each have a wp pte of the same anon page (ie. one
forked the other). The page's mapcount is 2. Then they both attempt to write
to it around the same time...

  proc1				proc2 thr1			proc2 thr2
  CPU0				CPU1				CPU3
  do_wp_page()			do_wp_page()
				 trylock_page()
				  can_share_swap_page()
				   load page mapcount (==2)
				  reuse = 0
				 pte unlock
				 copy page to new_page
				 pte lock
				 page_remove_rmap(page);
   trylock_page()	
    can_share_swap_page()
     load page mapcount (==1)
    reuse = 1
   ptep_set_access_flags (allow W)

  write private key into page
								read from page
				ptep_clear_flush()
				set_pte_at(pte of new_page)


Fix this by moving the page_remove_rmap of the old page after the pte clear
and flush. Potentially the entire branch could be moved down here, but in
order to stay consistent, I won't (should probably move all the *_mm_counter
stuff with one patch).

Signed-off-by: Nick Piggin <npiggin@suse.de>
---
Index: linux-2.6/mm/memory.c
===================================================================
--- linux-2.6.orig/mm/memory.c
+++ linux-2.6/mm/memory.c
@@ -1766,7 +1766,6 @@ gotten:
 	page_table = pte_offset_map_lock(mm, pmd, address, &ptl);
 	if (likely(pte_same(*page_table, orig_pte))) {
 		if (old_page) {
-			page_remove_rmap(old_page, vma);
 			if (!PageAnon(old_page)) {
 				dec_mm_counter(mm, file_rss);
 				inc_mm_counter(mm, anon_rss);
@@ -1788,6 +1787,32 @@ gotten:
 		lru_cache_add_active(new_page);
 		page_add_new_anon_rmap(new_page, vma, address);
 
+		if (old_page) {
+			/*
+			 * Only after switching the pte to the new page may
+			 * we remove the mapcount here. Otherwise another
+			 * process may come and find the rmap count decremented
+			 * before the pte is switched to the new page, and
+			 * "reuse" the old page writing into it while our pte
+			 * here still points into it and can be read by other
+			 * threads.
+			 *
+			 * The critical issue is to order this
+			 * page_remove_rmap with the ptp_clear_flush above.
+			 * Those stores are ordered by (if nothing else,)
+			 * the barrier present in the atomic_add_negative
+			 * in page_remove_rmap.
+			 *
+			 * Then the TLB flush in ptep_clear_flush ensures that
+			 * no process can access the old page before the
+			 * decremented mapcount is visible. And the old page
+			 * cannot be reused until after the decremented
+			 * mapcount is visible. So transitively, TLBs to
+			 * old page will be flushed before it can be reused.
+			 */
+			page_remove_rmap(old_page, vma);
+		}
+
 		/* Free the old page.. */
 		new_page = old_page;
 		ret |= VM_FAULT_WRITE;

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