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mm: remove gup_flags FOLL_WRITE games from __get_user_pages()

This is an ancient bug that was actually attempted to be fixed once
(badly) by me eleven years ago in commit 4ceb5db ("Fix
get_user_pages() race for write access") but that was then undone due to
problems on s390 by commit f33ea7f ("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 abf09be ("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 <>
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torvalds committed Oct 13, 2016
1 parent 6b25e21 commit 19be0eaffa3ac7d8eb6784ad9bdbc7d67ed8e619
Showing with 13 additions and 2 deletions.
  1. +1 −0 include/linux/mm.h
  2. +12 −2 mm/gup.c
@@ -2232,6 +2232,7 @@ 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);
@@ -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 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
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;

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