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Subject[PATCH 3.10 16/16] mm: remove gup_flags FOLL_WRITE games from __get_user_pages()
Date
From: Linus Torvalds <torvalds@linux-foundation.org>

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>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[wt: s/gup.c/memory.c; s/follow_page_pte/follow_page_mask;
s/faultin_page/__get_user_page]
Signed-off-by: Willy Tarreau <w@1wt.eu>
---
include/linux/mm.h | 1 +
mm/memory.c | 14 ++++++++++++--
2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 53b0d70..55590f4 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1715,6 +1715,7 @@ static inline struct page *follow_page(struct vm_area_struct *vma,
#define FOLL_HWPOISON 0x100 /* check page is hwpoisoned */
#define FOLL_NUMA 0x200 /* force NUMA hinting page fault */
#define FOLL_MIGRATION 0x400 /* wait for page to replace migration entry */
+#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/memory.c b/mm/memory.c
index 10cdade..2ca2ee1 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1462,6 +1462,16 @@ int zap_vma_ptes(struct vm_area_struct *vma, unsigned long address,
}
EXPORT_SYMBOL_GPL(zap_vma_ptes);

+/*
+ * 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));
+}
+
/**
* follow_page_mask - look up a page descriptor from a user-virtual address
* @vma: vm_area_struct mapping @address
@@ -1569,7 +1579,7 @@ split_fallthrough:
}
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))
goto unlock;

page = vm_normal_page(vma, address, pte);
@@ -1877,7 +1887,7 @@ long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
*/
if ((ret & VM_FAULT_WRITE) &&
!(vma->vm_flags & VM_WRITE))
- foll_flags &= ~FOLL_WRITE;
+ foll_flags |= FOLL_COW;

cond_resched();
}
--
2.8.0.rc2.1.gbe9624a
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