[PATCH] mm/khugepaged: Check again on anon uffd-wp during isolation

Peter Xu
2023-04-05T15:51:29Z

Khugepaged collapse an anonymous thp in two rounds of scans. The 2nd round
done in __collapse_huge_page_isolate() after hpage_collapse_scan_pmd(),
during which all the locks will be released temporarily. It means the
pgtable can change during this phase before 2nd round starts.

It's logically possible some ptes got wr-protected during this phase, and
we can errornously collapse a thp without noticing some ptes are
wr-protected by userfault. e1e267c7928f wanted to avoid it but it only did
that for the 1st phase, not the 2nd phase.

Since __collapse_huge_page_isolate() happens after a round of small page
swapins, we don't need to worry on any !present ptes - if it existed
khugepaged will already bail out. So we only need to check present ptes
with uffd-wp bit set there.

This is something I found only but never had a reproducer, I thought it was
one caused a bug in Muhammad's recent pagemap new ioctl work, but it turns
out it's not the cause of that but an userspace bug. However this seems to
still be a real bug even with a very small race window, still worth to have
it fixed and copy stable.

Cc: linux-stable <stable@vger.kernel.org>
Fixes: e1e267c7928f ("khugepaged: skip collapse if uffd-wp detected")
Signed-off-by: Peter Xu <peterx@redhat.com>
---
mm/khugepaged.c | 4 ++++
1 file changed, 4 insertions(+)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index a19aa140fd52..42ac93b4bd87 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -575,6 +575,10 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
result = SCAN_PTE_NON_PRESENT;
goto out;
}
+ if (pte_uffd_wp(pteval)) {
+ result = SCAN_PTE_UFFD_WP;
+ goto out;
+ }
page = vm_normal_page(vma, address, pteval);
if (unlikely(!page) || unlikely(is_zone_device_page(page))) {
result = SCAN_PAGE_NULL;
--
2.39.1