Re: [PATCH 10/12] mm/khugepaged: collapse_pte_mapped_thp() with mmap_read_lock()

[Jann Horn]

On Mon, May 29, 2023 at 8:26 AM Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> Bring collapse_and_free_pmd() back into collapse_pte_mapped_thp().
> It does need mmap_read_lock(), but it does not need mmap_write_lock(),
> nor vma_start_write() nor i_mmap lock nor anon_vma lock.  All racing
> paths are relying on pte_offset_map_lock() and pmd_lock(), so use those.

I think there's a weirdness in the existing code, and this change
probably turns that into a UAF bug.

collapse_pte_mapped_thp() can be called on an address that might not
be associated with a VMA anymore, and after this change, the page
tables for that address might be in the middle of page table teardown
in munmap(), right? The existing mmap_write_lock() guards against
concurrent munmap() (so in the old code we are guaranteed to either
see a normal VMA or not see the page tables anymore), but
mmap_read_lock() only guards against the part of munmap() up to the
mmap_write_downgrade() in do_vmi_align_munmap(), and unmap_region()
(including free_pgtables()) happens after that.

So we can now enter collapse_pte_mapped_thp() and race with concurrent
free_pgtables() such that a PUD disappears under us while we're
walking it or something like that:


int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
          bool install_pmd)
{
  struct mmu_notifier_range range;
  unsigned long haddr = addr & HPAGE_PMD_MASK;
  struct vm_area_struct *vma = vma_lookup(mm, haddr); // <<< returns NULL
  struct page *hpage;
  pte_t *start_pte, *pte;
  pmd_t *pmd, pgt_pmd;
  spinlock_t *pml, *ptl;
  int nr_ptes = 0, result = SCAN_FAIL;
  int i;

  mmap_assert_locked(mm);

  /* Fast check before locking page if already PMD-mapped */
  result = find_pmd_or_thp_or_none(mm, haddr, &pmd); // <<< PUD UAF in here
  if (result == SCAN_PMD_MAPPED)
    return result;

  if (!vma || !vma->vm_file || // <<< bailout happens too late
      !range_in_vma(vma, haddr, haddr + HPAGE_PMD_SIZE))
    return SCAN_VMA_CHECK;


I guess the right fix here is to make sure that at least the basic VMA
revalidation stuff (making sure there still is a VMA covering this
range) happens before find_pmd_or_thp_or_none()? Like:


diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 301c0e54a2ef..5db365587556 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1481,15 +1481,15 @@ int collapse_pte_mapped_thp(struct mm_struct
*mm, unsigned long addr,

         mmap_assert_locked(mm);

+        if (!vma || !vma->vm_file ||
+            !range_in_vma(vma, haddr, haddr + HPAGE_PMD_SIZE))
+                return SCAN_VMA_CHECK;
+
         /* Fast check before locking page if already PMD-mapped */
         result = find_pmd_or_thp_or_none(mm, haddr, &pmd);
         if (result == SCAN_PMD_MAPPED)
                 return result;

-        if (!vma || !vma->vm_file ||
-            !range_in_vma(vma, haddr, haddr + HPAGE_PMD_SIZE))
-                return SCAN_VMA_CHECK;
-
         /*
          * If we are here, we've succeeded in replacing all the native pages
          * in the page cache with a single hugepage. If a mm were to fault-in