mm/hugetlb: move swap entry handling into vma lock when faulted

In hugetlb_fault(), there used to have a special path to handle swap entry
at the entrance using huge_pte_offset().  That's unsafe because
huge_pte_offset() for a pmd sharable range can access freed pgtables if
without any lock to protect the pgtable from being freed after pmd
unshare.

Here the simplest solution to make it safe is to move the swap handling to
be after the vma lock being held.  We may need to take the fault mutex on
either migration or hwpoison entries now (also the vma lock, but that's
really needed), however neither of them is hot path.

Note that the vma lock cannot be released in hugetlb_fault() when the
migration entry is detected, because in migration_entry_wait_huge() the
pgtable page will be used again (by taking the pgtable lock), so that also
need to be protected by the vma lock.  Modify migration_entry_wait_huge()
so that it must be called with vma read lock held, and properly release
the lock in __migration_entry_wait_huge().

Link: https://lkml.kernel.org/r/20221216155100.2043537-5-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: James Houghton <jthoughton@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Cc: Rik van Riel <riel@surriel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 21 insertions, 4 deletions

diff --git a/mm/migrate.c b/mm/migrate.c
index a4d3fc65085f..98de7ce2b576 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -329,24 +329,41 @@ void migration_entry_wait(struct mm_struct *mm, pmd_t *pmd,
 }
 
 #ifdef CONFIG_HUGETLB_PAGE
-void __migration_entry_wait_huge(pte_t *ptep, spinlock_t *ptl)
+/*
+ * The vma read lock must be held upon entry. Holding that lock prevents either
+ * the pte or the ptl from being freed.
+ *
+ * This function will release the vma lock before returning.
+ */
+void __migration_entry_wait_huge(struct vm_area_struct *vma,
+				 pte_t *ptep, spinlock_t *ptl)
 {
 	pte_t pte;
 
+	hugetlb_vma_assert_locked(vma);
 	spin_lock(ptl);
 	pte = huge_ptep_get(ptep);
 
-	if (unlikely(!is_hugetlb_entry_migration(pte)))
+	if (unlikely(!is_hugetlb_entry_migration(pte))) {
 		spin_unlock(ptl);
-	else
+		hugetlb_vma_unlock_read(vma);
+	} else {
+		/*
+		 * If migration entry existed, safe to release vma lock
+		 * here because the pgtable page won't be freed without the
+		 * pgtable lock released.  See comment right above pgtable
+		 * lock release in migration_entry_wait_on_locked().
+		 */
+		hugetlb_vma_unlock_read(vma);
 		migration_entry_wait_on_locked(pte_to_swp_entry(pte), NULL, ptl);
+	}
 }
 
 void migration_entry_wait_huge(struct vm_area_struct *vma, pte_t *pte)
 {
 	spinlock_t *ptl = huge_pte_lockptr(hstate_vma(vma), vma->vm_mm, pte);
 
-	__migration_entry_wait_huge(pte, ptl);
+	__migration_entry_wait_huge(vma, pte, ptl);
 }
 #endif