Fix get_user_pages() race for write access

There's no real guarantee that handle_mm_fault() will always be able to
break a COW situation - if an update from another thread ends up
modifying the page table some way, handle_mm_fault() may end up
requiring us to re-try the operation.

That's normally fine, but get_user_pages() ended up re-trying it as a
read, and thus a write access could in theory end up losing the dirty
bit or be done on a page that had not been properly COW'ed.

This makes get_user_pages() always retry write accesses as write
accesses by making "follow_page()" require that a writable follow has
the dirty bit set.  That simplifies the code and solves the race: if the
COW break fails for some reason, we'll just loop around and try again.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 4 insertions, 17 deletions

diff --git a/mm/memory.c b/mm/memory.c
index 6fe77acbc1c..4e1c673784d 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -811,18 +811,15 @@ static struct page *__follow_page(struct mm_struct *mm, unsigned long address,
         pte = *ptep;
         pte_unmap(ptep);
         if (pte_present(pte)) {
-                if (write && !pte_write(pte))
+                if (write && !pte_dirty(pte))
                         goto out;
                 if (read && !pte_read(pte))
                         goto out;
                 pfn = pte_pfn(pte);
                 if (pfn_valid(pfn)) {
                         page = pfn_to_page(pfn);
-                        if (accessed) {
-                                if (write && !pte_dirty(pte) &&!PageDirty(page))
-                                        set_page_dirty(page);
+                        if (accessed)
                                 mark_page_accessed(page);
-                        }
                         return page;
                 }
         }
@@ -941,10 +938,9 @@ int get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
                 spin_lock(&mm->page_table_lock);
                 do {
                         struct page *page;
-                        int lookup_write = write;
 
                         cond_resched_lock(&mm->page_table_lock);
-                        while (!(page = follow_page(mm, start, lookup_write))) {
+                        while (!(page = follow_page(mm, start, write))) {
                                 /*
                                  * Shortcut for anonymous pages. We don't want
                                  * to force the creation of pages tables for
@@ -952,8 +948,7 @@ int get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
                                  * nobody touched so far. This is important
                                  * for doing a core dump for these mappings.
                                  */
-                                if (!lookup_write &&
-                                    untouched_anonymous_page(mm,vma,start)) {
+                                if (!write && untouched_anonymous_page(mm,vma,start)) {
                                         page = ZERO_PAGE(start);
                                         break;
                                 }
@@ -972,14 +967,6 @@ int get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
                                 default:
                                         BUG();
                                 }
-                                /*
-                                 * Now that we have performed a write fault
-                                 * and surely no longer have a shared page we
-                                 * shouldn't write, we shouldn't ignore an
-                                 * unwritable page in the page table if
-                                 * we are forcing write access.
-                                 */
-                                lookup_write = write && !force;
                                 spin_lock(&mm->page_table_lock);
                         }
                         if (pages) {