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- From 1294d355881cc5c3421d24fee512f16974addb6c Mon Sep 17 00:00:00 2001
- From: Linus Torvalds <torvalds@linux-foundation.org>
- Date: Thu, 13 Oct 2016 13:07:36 -0700
- Subject: 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 <kernel@linuxace.com>
- Acked-by: Hugh Dickins <hughd@google.com>
- Reviewed-by: Michal Hocko <mhocko@suse.com>
- Cc: Andy Lutomirski <luto@kernel.org>
- Cc: Kees Cook <keescook@chromium.org>
- Cc: Oleg Nesterov <oleg@redhat.com>
- Cc: Willy Tarreau <w@1wt.eu>
- Cc: Nick Piggin <npiggin@gmail.com>
- Cc: Greg Thelen <gthelen@google.com>
- Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
- Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
- ---
- include/linux/mm.h | 1 +
- mm/gup.c | 14 ++++++++++++--
- 2 files changed, 13 insertions(+), 2 deletions(-)
- diff --git a/include/linux/mm.h b/include/linux/mm.h
- index cfebb74..f0ffa01 100644
- --- a/include/linux/mm.h
- +++ b/include/linux/mm.h
- @@ -2112,6 +2112,7 @@ static inline struct page *follow_page(struct vm_area_struct *vma,
- #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);
- diff --git a/mm/gup.c b/mm/gup.c
- index deafa2c..4b0b7e7 100644
- --- a/mm/gup.c
- +++ b/mm/gup.c
- @@ -58,6 +58,16 @@ 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)
- {
- @@ -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_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;
- }
-
- --
- cgit v0.12
|
