linux-kernel - [PATCH] ksm: Fix unlocked iteration over vmas in cmp_and_merge

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Date:   Mon, 11 Sep 2017 14:05:05 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     akpm@...ux-foundation.org, aarcange@...hat.com, minchan@...nel.org,
        mhocko@...e.com, zhongjiang@...wei.com, mingo@...nel.org,
        imbrenda@...ux.vnet.ibm.com, kirill.shutemov@...ux.intel.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        ktkhai@...tuozzo.com
Subject: [PATCH] ksm: Fix unlocked iteration over vmas in
 cmp_and_merge_page()

In this place mm is unlocked, so vmas or list may change.
Down read mmap_sem to protect them from modifications.

Signed-off-by: Kirill Tkhai <ktkhai@...tuozzo.com>
(and compile-tested-by)
---
 mm/ksm.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index db20f8436bc3..86f0db3d6cdb 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1990,6 +1990,7 @@ static void stable_tree_append(struct rmap_item *rmap_item,
  */
 static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item)
 {
+	struct mm_struct *mm = rmap_item->mm;
 	struct rmap_item *tree_rmap_item;
 	struct page *tree_page = NULL;
 	struct stable_node *stable_node;
@@ -2062,9 +2063,11 @@ static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item)
 	if (ksm_use_zero_pages && (checksum == zero_checksum)) {
 		struct vm_area_struct *vma;
 
-		vma = find_mergeable_vma(rmap_item->mm, rmap_item->address);
+		down_read(&mm->mmap_sem);
+		vma = find_mergeable_vma(mm, rmap_item->address);
 		err = try_to_merge_one_page(vma, page,
 					    ZERO_PAGE(rmap_item->address));
+		up_read(&mm->mmap_sem);
 		/*
 		 * In case of failure, the page was not really empty, so we
 		 * need to continue. Otherwise we're done.