diff options
authorLinus Torvalds <>2016-10-13 13:07:36 -0700
committerGreg Kroah-Hartman <>2016-10-20 10:03:30 +0200
commit89eeba1594ac641a30b91942961e80fae978f839 (patch)
parent0312017cbbb5f46dbf4932f65df0c2c66f9c8f27 (diff)
mm: remove gup_flags FOLL_WRITE games from __get_user_pages()
commit 19be0eaffa3ac7d8eb6784ad9bdbc7d67ed8e619 upstream. This is an ancient bug that was actually attempted to be fixed once (badly) by me eleven years ago in commit 4ceb5db9757a ("Fix get_user_pages() race for write access") but that was then undone due to problems on s390 by commit f33ea7f404e5 ("fix get_user_pages bug"). In the meantime, the s390 situation has long been fixed, and we can now fix it by checking the pte_dirty() bit properly (and do it better). The s390 dirty bit was implemented in abf09bed3cce ("s390/mm: implement software dirty bits") which made it into v3.9. Earlier kernels will have to look at the page state itself. Also, the VM has become more scalable, and what used a purely theoretical race back then has become easier to trigger. To fix it, we introduce a new internal FOLL_COW flag to mark the "yes, we already did a COW" rather than play racy games with FOLL_WRITE that is very fundamental, and then use the pte dirty flag to validate that the FOLL_COW flag is still valid. Reported-and-tested-by: Phil "not Paul" Oester <> Acked-by: Hugh Dickins <> Reviewed-by: Michal Hocko <> Cc: Andy Lutomirski <> Cc: Kees Cook <> Cc: Oleg Nesterov <> Cc: Willy Tarreau <> Cc: Nick Piggin <> Cc: Greg Thelen <> Signed-off-by: Linus Torvalds <> Signed-off-by: Greg Kroah-Hartman <>
2 files changed, 13 insertions, 2 deletions
diff --git a/include/linux/mm.h b/include/linux/mm.h
index ef815b9cd426..277cd39a6399 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2234,6 +2234,7 @@ static inline struct page *follow_page(struct vm_area_struct *vma,
#define FOLL_TRIED 0x800 /* a retry, previous pass started an IO */
#define FOLL_MLOCK 0x1000 /* lock present pages */
#define FOLL_REMOTE 0x2000 /* we are working on non-current tsk/mm */
+#define FOLL_COW 0x4000 /* internal GUP flag */
typedef int (*pte_fn_t)(pte_t *pte, pgtable_t token, unsigned long addr,
void *data);
diff --git a/mm/gup.c b/mm/gup.c
index 96b2b2fd0fbd..22cc22e7432f 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -60,6 +60,16 @@ static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address,
return -EEXIST;
+ * FOLL_FORCE can write to even unwritable pte's, but only
+ * after we've gone through a COW cycle and they are dirty.
+ */
+static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
+ return pte_write(pte) ||
+ ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
static struct page *follow_page_pte(struct vm_area_struct *vma,
unsigned long address, pmd_t *pmd, unsigned int flags)
@@ -95,7 +105,7 @@ retry:
if ((flags & FOLL_NUMA) && pte_protnone(pte))
goto no_page;
- if ((flags & FOLL_WRITE) && !pte_write(pte)) {
+ if ((flags & FOLL_WRITE) && !can_follow_write_pte(pte, flags)) {
pte_unmap_unlock(ptep, ptl);
return NULL;
@@ -412,7 +422,7 @@ static int faultin_page(struct task_struct *tsk, struct vm_area_struct *vma,
* reCOWed by userspace write).
if ((ret & VM_FAULT_WRITE) && !(vma->vm_flags & VM_WRITE))
- *flags &= ~FOLL_WRITE;
+ *flags |= FOLL_COW;
return 0;