95 lines
3.4 KiB
Diff
95 lines
3.4 KiB
Diff
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From 19be0eaffa3ac7d8eb6784ad9bdbc7d67ed8e619 Mon Sep 17 00:00:00 2001
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From: Linus Torvalds <torvalds@linux-foundation.org>
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Date: Thu, 13 Oct 2016 13:07:36 -0700
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Subject: 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
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(badly) by me eleven years ago in commit 4ceb5db9757a ("Fix
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get_user_pages() race for write access") but that was then undone due to
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problems on s390 by commit f33ea7f404e5 ("fix get_user_pages bug").
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In the meantime, the s390 situation has long been fixed, and we can now
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fix it by checking the pte_dirty() bit properly (and do it better). The
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s390 dirty bit was implemented in abf09bed3cce ("s390/mm: implement
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software dirty bits") which made it into v3.9. Earlier kernels will
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have to look at the page state itself.
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Also, the VM has become more scalable, and what used a purely
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theoretical race back then has become easier to trigger.
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To fix it, we introduce a new internal FOLL_COW flag to mark the "yes,
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we already did a COW" rather than play racy games with FOLL_WRITE that
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is very fundamental, and then use the pte dirty flag to validate that
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the FOLL_COW flag is still valid.
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Reported-and-tested-by: Phil "not Paul" Oester <kernel@linuxace.com>
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Acked-by: Hugh Dickins <hughd@google.com>
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Reviewed-by: Michal Hocko <mhocko@suse.com>
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Cc: Andy Lutomirski <luto@kernel.org>
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Cc: Kees Cook <keescook@chromium.org>
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Cc: Oleg Nesterov <oleg@redhat.com>
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Cc: Willy Tarreau <w@1wt.eu>
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Cc: Nick Piggin <npiggin@gmail.com>
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Cc: Greg Thelen <gthelen@google.com>
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Cc: stable@vger.kernel.org
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Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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---
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include/linux/mm.h | 1 +
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mm/gup.c | 14 ++++++++++++--
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2 files changed, 13 insertions(+), 2 deletions(-)
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diff --git a/include/linux/mm.h b/include/linux/mm.h
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index e9caec6..ed85879 100644
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--- a/include/linux/mm.h
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+++ b/include/linux/mm.h
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@@ -2232,6 +2232,7 @@ static inline struct page *follow_page(struct vm_area_struct *vma,
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#define FOLL_TRIED 0x800 /* a retry, previous pass started an IO */
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#define FOLL_MLOCK 0x1000 /* lock present pages */
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#define FOLL_REMOTE 0x2000 /* we are working on non-current tsk/mm */
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+#define FOLL_COW 0x4000 /* internal GUP flag */
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typedef int (*pte_fn_t)(pte_t *pte, pgtable_t token, unsigned long addr,
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void *data);
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diff --git a/mm/gup.c b/mm/gup.c
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index 96b2b2f..22cc22e 100644
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--- a/mm/gup.c
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+++ b/mm/gup.c
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@@ -60,6 +60,16 @@ static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address,
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return -EEXIST;
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}
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+/*
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+ * FOLL_FORCE can write to even unwritable pte's, but only
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+ * after we've gone through a COW cycle and they are dirty.
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+ */
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+static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
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+{
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+ return pte_write(pte) ||
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+ ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
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+}
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+
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static struct page *follow_page_pte(struct vm_area_struct *vma,
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unsigned long address, pmd_t *pmd, unsigned int flags)
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{
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@@ -95,7 +105,7 @@ retry:
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}
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if ((flags & FOLL_NUMA) && pte_protnone(pte))
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goto no_page;
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- if ((flags & FOLL_WRITE) && !pte_write(pte)) {
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+ if ((flags & FOLL_WRITE) && !can_follow_write_pte(pte, flags)) {
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pte_unmap_unlock(ptep, ptl);
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return NULL;
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}
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@@ -412,7 +422,7 @@ static int faultin_page(struct task_struct *tsk, struct vm_area_struct *vma,
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* reCOWed by userspace write).
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*/
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if ((ret & VM_FAULT_WRITE) && !(vma->vm_flags & VM_WRITE))
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- *flags &= ~FOLL_WRITE;
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+ *flags |= FOLL_COW;
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return 0;
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}
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--
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cgit v0.12
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