-From e45a502bdeae5a075257c4f061d1ff4ff0821354 Mon Sep 17 00:00:00 2001
-From: Linus Torvalds <torvalds@linux-foundation.org>
-Date: Thu, 13 Oct 2016 13:07:36 -0700
-Subject: [PATCH] mm: remove gup_flags FOLL_WRITE games from __get_user_pages()
-
-[ Upstream commit 19be0eaffa3ac7d8eb6784ad9bdbc7d67ed8e619 ]
-
-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>
-Cc: stable@vger.kernel.org
-Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
----
- 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
-@@ -2029,6 +2029,7 @@ static inline struct page *follow_page(s
- #define FOLL_NUMA 0x200 /* force NUMA hinting page fault */
- #define FOLL_MIGRATION 0x400 /* wait for page to replace migration entry */
- #define FOLL_TRIED 0x800 /* a retry, previous pass started an IO */
-+#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
-@@ -32,6 +32,16 @@ static struct page *no_page_table(struct
- return NULL;
- }
-
-+/*
-+ * 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)
- {
-@@ -66,7 +76,7 @@ retry:
- }
- if ((flags & FOLL_NUMA) && pte_numa(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;
- }
-@@ -315,7 +325,7 @@ 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;
- }
-
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