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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>
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Linus Torvalds
Linus Torvalds committed Aug 1, 2005
1 parent 8d894c4 commit 4ceb5db9757aaeadcf8fbbf97d76bd42aa4df0d6
Showing with 4 additions and 17 deletions.
  1. +4 −17 mm/memory.c
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@@ -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;
}
}
@@ -941,19 +938,17 @@ 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
* insanely big anonymously mapped areas that
* 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) {

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