[PATCH 4.4 2/2] mm: remove gup_flags FOLL_WRITE games from __get_user_pages()

From: Greg Kroah-Hartman
Date: Wed Oct 19 2016 - 14:33:57 EST

4.4-stable review patch. If anyone has any objections, please let me know.


From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>

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 <kernel@xxxxxxxxxxxx>
Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>
Reviewed-by: Michal Hocko <mhocko@xxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Willy Tarreau <w@xxxxxx>
Cc: Nick Piggin <npiggin@xxxxxxxxx>
Cc: Greg Thelen <gthelen@xxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

include/linux/mm.h | 1 +
mm/gup.c | 14 ++++++++++++--
2 files changed, 13 insertions(+), 2 deletions(-)

--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2112,6 +2112,7 @@ static inline struct page *follow_page(s
#define FOLL_MIGRATION 0x400 /* wait for page to replace migration entry */
#define FOLL_TRIED 0x800 /* a retry, previous pass started an IO */
#define FOLL_MLOCK 0x1000 /* lock present pages */
+#define FOLL_COW 0x4000 /* internal GUP flag */

typedef int (*pte_fn_t)(pte_t *pte, pgtable_t token, unsigned long addr,
void *data);
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -58,6 +58,16 @@ static int follow_pfn_pte(struct vm_area
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)
@@ -92,7 +102,7 @@ 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;
@@ -352,7 +362,7 @@ static int faultin_page(struct task_stru
* reCOWed by userspace write).
if ((ret & VM_FAULT_WRITE) && !(vma->vm_flags & VM_WRITE))
- *flags &= ~FOLL_WRITE;
+ *flags |= FOLL_COW;
return 0;