On Thu, 16 Apr 2020 10:50:34 +0800 Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
> The find_mergeable_vma can return NULL. In this case, it leads
> to a crash when we access vm_mm(its offset is 0x40) later in
> write_protect_page. And this case did happen on our server. The
> following call trace is captured in kernel 4.19 with the following
> patch applied and KSM zero page enabled on our server.
>
> commit e86c59b1b12d ("mm/ksm: improve deduplication of zero pages with colouring")
>
> So add a vma check to fix it.
>
> ...
>
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -2112,8 +2112,15 @@ static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item)
>
> 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));
> + if (vma)
> + err = try_to_merge_one_page(vma, page,
> + ZERO_PAGE(rmap_item->address));
> + else
> + /**
> + * If the vma is out of date, we do not need to
> + * continue.
> + */
> + err = 0;
> up_read(&mm->mmap_sem);
> /*
> * In case of failure, the page was not really empty, so we
Thanks.
It's conventional to put braces around multi-line blocks such as this.
Also, /** is specifically used to introduce kerneldoc comments. This
comment is not a kerneldoc one so use /*.
--- a/mm/ksm.c~mm-ksm-fix-null-pointer-dereference-when-ksm-zero-page-is-enabled-v4-fix
+++ a/mm/ksm.c
@@ -2112,15 +2112,16 @@ static void cmp_and_merge_page(struct pa
down_read(&mm->mmap_sem);
vma = find_mergeable_vma(mm, rmap_item->address);
- if (vma)
+ if (vma) {
err = try_to_merge_one_page(vma, page,
ZERO_PAGE(rmap_item->address));
- else
- /**
+ } else {
+ /*
* If the vma is out of date, we do not need to
* continue.
*/
err = 0;
+ }
up_read(&mm->mmap_sem);
/*
* In case of failure, the page was not really empty, so we
_
