On Thu, Jul 31, 2025 at 8:44 AM Suren Baghdasaryan surenb@google.com wrote:
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.
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
Reviewed-by: Lokesh Gidra lokeshgidra@google.com
Cc: stable@vger.kernel.org
Changes since v1 [1]
- Fixed step size calculation, per Lokesh Gidra
- Added missing check for UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES, per Lokesh Gidra
[1] https://lore.kernel.org/all/20250730170733.3829267-1-surenb@google.com/
mm/userfaultfd.c | 45 +++++++++++++++++++++++++++++---------------- 1 file changed, 29 insertions(+), 16 deletions(-)
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c index cbed91b09640..b5af31c22731 100644 --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -1818,28 +1818,41 @@ 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);
step_size = HPAGE_PMD_SIZE;
} else { spin_unlock(ptl);
split_huge_pmd(src_vma, src_pmd, src_addr);
/* The folio will be split by move_pages_pte() */
continue;
if (!(mode & UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES)) {
err = -ENOENT;
break;
}
/* nothing to do to move a hole */
err = 0;
step_size = min(HPAGE_PMD_SIZE, src_start + len - src_addr); }
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)) { if (!(mode & UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES)) {
base-commit: 01da54f10fddf3b01c5a3b80f6b16bbad390c302
2.50.1.552.g942d659e1b-goog