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mm: remove gup_flags FOLL_WRITE games from __get_user_pages()

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This is an ancient bug that was actually attempted to be fixed once
(badly) by me eleven years ago in commit 4ceb5db975 ("Fix
get_user_pages() race for write access") but that was then undone due to
problems on s390 by commit f33ea7f404 ("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 abf09bed3c ("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 <[email protected]>
Acked-by: Hugh Dickins <[email protected]>
Reviewed-by: Michal Hocko <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Willy Tarreau <[email protected]>
Cc: Nick Piggin <[email protected]>
Cc: Greg Thelen <[email protected]>
Cc: [email protected]
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Linus Torvalds 2016-10-13 13:07:36 -07:00
commit 19be0eaffa
2 changed files with 13 additions and 2 deletions
Download patch file Download diff file Expand all files Collapse all files

14
mm/gup.c
View file

@ -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;
}