Re: [PATCH] hugetlb: fix race condition in hugetlb_fault()

From: Hillf Danton
Date: Sat Mar 31 2012 - 08:27:47 EST

On Sat, Mar 31, 2012 at 4:07 AM, Chris Metcalf <cmetcalf@xxxxxxxxxx> wrote:
> The race is as follows. ÂSuppose a multi-threaded task forks a new
> process, thus bumping up the ref count on all the pages. ÂWhile the fork
> is occurring (and thus we have marked all the PTEs as read-only), another
> thread in the original process tries to write to a huge page, taking an
> access violation from the write-protect and calling hugetlb_cow(). ÂNow,
> suppose the fork() fails. ÂIt will undo the COW and decrement the ref
> count on the pages, so the ref count on the huge page drops back to 1.
> Meanwhile hugetlb_cow() also decrements the ref count by one on the
> original page, since the original address space doesn't need it any more,
> having copied a new page to replace the original page. ÂThis leaves the
> ref count at zero, and when we call unlock_page(), we panic.
> The solution is to take an extra reference to the page while we are
> holding the lock on it.
> Cc: stable@xxxxxxxxxx
> Signed-off-by: Chris Metcalf <cmetcalf@xxxxxxxxxx>
> ---
> Âmm/hugetlb.c | Â Â8 ++++++--
> Â1 files changed, 6 insertions(+), 2 deletions(-)
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 4531be2..ab674fc 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -2703,8 +2703,10 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> Â Â Â Â * so no worry about deadlock.
> Â Â Â Â */
> Â Â Â Âpage = pte_page(entry);
> - Â Â Â if (page != pagecache_page)
> + Â Â Â if (page != pagecache_page) {
> + Â Â Â Â Â Â Â get_page(page);
> Â Â Â Â Â Â Â Âlock_page(page);
> + Â Â Â }
Perhaps, directly get page?

page = pte_page(entry);
+ get_page(page);
if (page != pagecache_page)
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