linux-kernel - [PATCH v5 1/1] userfaultfd: fix a crash in UFFDIO

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Message-ID: <20250807200418.1963585-1-surenb@google.com>
Date: Thu,  7 Aug 2025 13:04:18 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: akpm@...ux-foundation.org
Cc: peterx@...hat.com, david@...hat.com, aarcange@...hat.com, 
	lokeshgidra@...gle.com, surenb@...gle.com, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, 
	syzbot+b446dbe27035ef6bd6c2@...kaller.appspotmail.com, stable@...r.kernel.org
Subject: [PATCH v5 1/1] 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.

Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")
Reported-by: syzbot+b446dbe27035ef6bd6c2@...kaller.appspotmail.com
Closes: https://lore.kernel.org/all/68794b5c.a70a0220.693ce.0050.GAE@google.com/
Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
Reviewed-by: Peter Xu <peterx@...hat.com>
Cc: stable@...r.kernel.org
---
Applies to mm-unstable after reverting older v4 [1] version.

Changes since v4 [1]
- Removed extra folio check, per David Hildenbrand

[1] https://lore.kernel.org/all/20250806220022.926763-1-surenb@google.com/

 mm/userfaultfd.c | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 5431c9dd7fd7..aefdf3a812a1 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -1826,13 +1826,16 @@ 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)) {
-				struct folio *folio = pmd_folio(*src_pmd);
-
-				if (!folio || (!is_huge_zero_folio(folio) &&
-					       !PageAnonExclusive(&folio->page))) {
-					spin_unlock(ptl);
-					err = -EBUSY;
-					break;
+				/* Can be a migration entry */
+				if (pmd_present(*src_pmd)) {
+					struct folio *folio = pmd_folio(*src_pmd);
+
+					if (!is_huge_zero_folio(folio) &&
+					    !PageAnonExclusive(&folio->page)) {
+						spin_unlock(ptl);
+						err = -EBUSY;
+						break;
+					}
 				}
 
 				spin_unlock(ptl);
-- 
2.50.1.703.g449372360f-goog