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SubjectRe: [PATCH] ksm: Fix unlocked iteration over vmas in cmp_and_merge_page()
[CC Claudio and Hugh]

On Mon 11-09-17 14:05:05, Kirill Tkhai wrote:
> 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@virtuozzo.com>
> (and compile-tested-by)

Fixes: e86c59b1b12d ("mm/ksm: improve deduplication of zero pages with colouring")
AFAICS. Maybe even CC: stable as unstable vma can cause large variety of
issues including memory corruption.

The fix lookds good to me
Acked-by: Michal Hocko <mhocko@suse.com>

> ---
> 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.
>

--
Michal Hocko
SUSE Labs

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