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authorPeter Xu <peterx@redhat.com>2022-10-04 15:33:58 -0400
committerAndrew Morton <akpm@linux-foundation.org>2022-10-12 18:51:50 -0700
commit2ea7ff1e39cbe3753d3c649beb70f2cf861dca75 (patch)
treecf2556c3661e0cfefabab687e5d45a5cd89af941 /mm
parent94541bc3fbde45bbd40e7989995246b22732679a (diff)
downloadlwn-2ea7ff1e39cbe3753d3c649beb70f2cf861dca75.tar.gz
lwn-2ea7ff1e39cbe3753d3c649beb70f2cf861dca75.zip
mm/hugetlb: fix race condition of uffd missing/minor handling
Patch series "mm/hugetlb: Fix selftest failures with write check", v3. Currently akpm mm-unstable fails with uffd hugetlb private mapping test randomly on a write check. The initial bisection of that points to the recent pmd unshare series, but it turns out there's no direction relationship with the series but only some timing change caused the race to start trigger. The race should be fixed in patch 1. Patch 2 is a trivial cleanup on the similar race with hugetlb migrations, patch 3 comment on the write check so when anyone read it again it'll be clear why it's there. This patch (of 3): After the recent rework patchset of hugetlb locking on pmd sharing, kselftest for userfaultfd sometimes fails on hugetlb private tests with unexpected write fault checks. It turns out there's nothing wrong within the locking series regarding this matter, but it could have changed the timing of threads so it can trigger an old bug. The real bug is when we call hugetlb_no_page() we're not with the pgtable lock. It means we're reading the pte values lockless. It's perfectly fine in most cases because before we do normal page allocations we'll take the lock and check pte_same() again. However before that, there are actually two paths on userfaultfd missing/minor handling that may directly move on with the fault process without checking the pte values. It means for these two paths we may be generating an uffd message based on an unstable pte, while an unstable pte can legally be anything as long as the modifier holds the pgtable lock. One example, which is also what happened in the failing kselftest and caused the test failure, is that for private mappings wr-protection changes can happen on one page. While hugetlb_change_protection() generally requires pte being cleared before being changed, then there can be a race condition like: thread 1 thread 2 -------- -------- UFFDIO_WRITEPROTECT hugetlb_fault hugetlb_change_protection pgtable_lock() huge_ptep_modify_prot_start pte==NULL hugetlb_no_page generate uffd missing event even if page existed!! huge_ptep_modify_prot_commit pgtable_unlock() Fix this by rechecking the pte after pgtable lock for both userfaultfd missing & minor fault paths. This bug should have been around starting from uffd hugetlb introduced, so attaching a Fixes to the commit. Also attach another Fixes to the minor support commit for easier tracking. Note that userfaultfd is actually fine with false positives (e.g. caused by pte changed), but not wrong logical events (e.g. caused by reading a pte during changing). The latter can confuse the userspace, so the strictness is very much preferred. E.g., MISSING event should never happen on the page after UFFDIO_COPY has correctly installed the page and returned. Link: https://lkml.kernel.org/r/20221004193400.110155-1-peterx@redhat.com Link: https://lkml.kernel.org/r/20221004193400.110155-2-peterx@redhat.com Fixes: 1a1aad8a9b7b ("userfaultfd: hugetlbfs: add userfaultfd hugetlb hook") Fixes: 7677f7fd8be7 ("userfaultfd: add minor fault registration mode") Signed-off-by: Peter Xu <peterx@redhat.com> Co-developed-by: Mike Kravetz <mike.kravetz@oracle.com> Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Axel Rasmussen <axelrasmussen@google.com> Cc: Nadav Amit <nadav.amit@gmail.com> Cc: David Hildenbrand <david@redhat.com> Cc: Mike Rapoport <rppt@linux.vnet.ibm.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Diffstat (limited to 'mm')
-rw-r--r--mm/hugetlb.c59
1 files changed, 52 insertions, 7 deletions
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 9a910612336d..bf9d8d04bf4f 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5535,6 +5535,23 @@ static inline vm_fault_t hugetlb_handle_userfault(struct vm_area_struct *vma,
return handle_userfault(&vmf, reason);
}
+/*
+ * Recheck pte with pgtable lock. Returns true if pte didn't change, or
+ * false if pte changed or is changing.
+ */
+static bool hugetlb_pte_stable(struct hstate *h, struct mm_struct *mm,
+ pte_t *ptep, pte_t old_pte)
+{
+ spinlock_t *ptl;
+ bool same;
+
+ ptl = huge_pte_lock(h, mm, ptep);
+ same = pte_same(huge_ptep_get(ptep), old_pte);
+ spin_unlock(ptl);
+
+ return same;
+}
+
static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
struct vm_area_struct *vma,
struct address_space *mapping, pgoff_t idx,
@@ -5575,10 +5592,33 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
if (idx >= size)
goto out;
/* Check for page in userfault range */
- if (userfaultfd_missing(vma))
- return hugetlb_handle_userfault(vma, mapping, idx,
- flags, haddr, address,
- VM_UFFD_MISSING);
+ if (userfaultfd_missing(vma)) {
+ /*
+ * Since hugetlb_no_page() was examining pte
+ * without pgtable lock, we need to re-test under
+ * lock because the pte may not be stable and could
+ * have changed from under us. Try to detect
+ * either changed or during-changing ptes and retry
+ * properly when needed.
+ *
+ * Note that userfaultfd is actually fine with
+ * false positives (e.g. caused by pte changed),
+ * but not wrong logical events (e.g. caused by
+ * reading a pte during changing). The latter can
+ * confuse the userspace, so the strictness is very
+ * much preferred. E.g., MISSING event should
+ * never happen on the page after UFFDIO_COPY has
+ * correctly installed the page and returned.
+ */
+ if (!hugetlb_pte_stable(h, mm, ptep, old_pte)) {
+ ret = 0;
+ goto out;
+ }
+
+ return hugetlb_handle_userfault(vma, mapping, idx, flags,
+ haddr, address,
+ VM_UFFD_MISSING);
+ }
page = alloc_huge_page(vma, haddr, 0);
if (IS_ERR(page)) {
@@ -5644,9 +5684,14 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
if (userfaultfd_minor(vma)) {
unlock_page(page);
put_page(page);
- return hugetlb_handle_userfault(vma, mapping, idx,
- flags, haddr, address,
- VM_UFFD_MINOR);
+ /* See comment in userfaultfd_missing() block above */
+ if (!hugetlb_pte_stable(h, mm, ptep, old_pte)) {
+ ret = 0;
+ goto out;
+ }
+ return hugetlb_handle_userfault(vma, mapping, idx, flags,
+ haddr, address,
+ VM_UFFD_MINOR);
}
}