Re: [PATCH 1/1] userfaultfd: fix a crash when UFFDIO_MOVE handles a THP hole

On Wed, Jul 30, 2025 at 6:58 PM Hillf Danton hdanton@sina.com wrote:

#syz test

When UFFDIO_MOVE is used with UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES and it encounters a non-present THP, it fails to properly recognize an unmapped hole and tries to access a non-existent folio, resulting in a crash. Add a check to skip non-present THPs.

Thanks Suren for promptly addressing this issue.

Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI") Reported-by: syzbot+b446dbe27035ef6bd6c2@syzkaller.appspotmail.com Closes: https://lore.kernel.org/all/68794b5c.a70a0220.693ce.0050.GAE@google.com/ Signed-off-by: Suren Baghdasaryan surenb@google.com Cc: stable@vger.kernel.org

---

mm/userfaultfd.c | 38 +++++++++++++++++++++++--------------- 1 file changed, 23 insertions(+), 15 deletions(-)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c index cbed91b09640..60be8080ddd0 100644 --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -1818,27 +1818,35 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start,

            ptl = pmd_trans_huge_lock(src_pmd, src_vma);
            if (ptl) {

•                   /* Check if we can move the pmd without splitting it. */
  

•                   if (move_splits_huge_pmd(dst_addr, src_addr, src_start + len) ||
  

•                       !pmd_none(dst_pmdval)) {
  

•                           struct folio *folio = pmd_folio(*src_pmd);
  

•                   if (pmd_present(*src_pmd) || is_pmd_migration_entry(*src_pmd)) {
  

•                           /* Check if we can move the pmd without splitting it. */
  

•                           if (move_splits_huge_pmd(dst_addr, src_addr, src_start + len) ||
  

•                               !pmd_none(dst_pmdval)) {
  

•                                   if (pmd_present(*src_pmd)) {
  

•                                           struct folio *folio = pmd_folio(*src_pmd);
  

•                                           if (!folio || (!is_huge_zero_folio(folio) &&
  

•                                                          !PageAnonExclusive(&folio->page))) {
  

•                                                   spin_unlock(ptl);
  

•                                                   err = -EBUSY;
  

•                                                   break;
  

•                                           }
  

•                                   }
  
  

•                           if (!folio || (!is_huge_zero_folio(folio) &&
  

•                                          !PageAnonExclusive(&folio->page))) {
                                    spin_unlock(ptl);
  

•                                   err = -EBUSY;
  

•                                   break;
  

•                                   split_huge_pmd(src_vma, src_pmd, src_addr);
  

•                                   /* The folio will be split by move_pages_pte() */
  

•                                   continue;
                            }
  
  

•                           err = move_pages_huge_pmd(mm, dst_pmd, src_pmd,
  

•                                                     dst_pmdval, dst_vma, src_vma,
  

•                                                     dst_addr, src_addr);
  

•                   } else {
  

•                           /* nothing to do to move a hole */
                            spin_unlock(ptl);
  

•                           split_huge_pmd(src_vma, src_pmd, src_addr);
  

•                           /* The folio will be split by move_pages_pte() */
  

•                           continue;
  

•                           err = 0;
  

I think we need to act here depending on whether UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES is set or not.

err = (mode & UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES) ? 0 : -ENOENT;

Also, IMO, the step_size in this case should be the minimum of remaining length and HPAGE_PMD_SIZE.

                    }

•                   err = move_pages_huge_pmd(mm, dst_pmd, src_pmd,
  

•                                             dst_pmdval, dst_vma, src_vma,
  

•                                             dst_addr, src_addr);
                    step_size = HPAGE_PMD_SIZE;
            } else {
                    if (pmd_none(*src_pmd)) {
  

I have a related question/doubt: why do we populate the page-table hierarchy on the src side [1] (and then also at line 1857) when a hole is found? IMHO, it shouldn't be needed. Depending on whether UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES is set or not, it should either return -ENOENT, or continue past the hole. Please correct me if I'm wrong.

[1] https://elixir.bootlin.com/linux/v6.16/source/mm/userfaultfd.c#L1797

base-commit: 01da54f10fddf3b01c5a3b80f6b16bbad390c302

2.50.1.552.g942d659e1b-goog