Commit aba6faec authored Aug 06, 2025 by Suren Baghdasaryan Committed by Andrew Morton Aug 11, 2025

userfaultfd: fix a crash in UFFDIO_MOVE when PMD is a migration entry

When UFFDIO_MOVE encounters a migration PMD entry, it proceeds with
obtaining a folio and accessing it even though the entry is swp_entry_t. 
Add the missing check and let split_huge_pmd() handle migration entries. 
While at it also remove unnecessary folio check.

[surenb@google.com: remove extra folio check, per David]
  Link: https://lkml.kernel.org/r/20250807200418.1963585-1-surenb@google.com
Link: https://lkml.kernel.org/r/20250806220022.926763-1-surenb@google.com


Fixes: adef4406 ("userfaultfd: UFFDIO_MOVE uABI")
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Reported-by:  <syzbot+b446dbe27035ef6bd6c2@syzkaller.appspotmail.com>
Closes: https://lore.kernel.org/all/68794b5c.a70a0220.693ce.0050.GAE@google.com/


Reviewed-by: Peter Xu <peterx@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Lokesh Gidra <lokeshgidra@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

parent cf1b80dc

mm/userfaultfd.c

+10 −7

Original line number	Diff line number	Diff line
		@@ -1821,14 +1821,17 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start,
		/* 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)) {
		/* Can be a migration entry */
		if (pmd_present(*src_pmd)) {
		struct folio folio = pmd_folio(src_pmd);

		if (!folio \|\| (!is_huge_zero_folio(folio) &&
		!PageAnonExclusive(&folio->page))) {
		if (!is_huge_zero_folio(folio) &&
		!PageAnonExclusive(&folio->page)) {
		spin_unlock(ptl);
		err = -EBUSY;
		break;
		}
		}

		spin_unlock(ptl);
		split_huge_pmd(src_vma, src_pmd, src_addr);