[PATCH 1/2] mm/memfd: handle GUP pins more reliably when write-sealing

["David Hildenbrand (Arm)" <david@kernel.org>]

memfd_wait_for_pins() detects GUP references by comparing a folio's
actual refcount with its expected refcount. As spelled out by
folio_expected_ref_count(), obtaining the expected refcount when there
can be concurrent folio (un)mapping is racy.

So it is possible that a changing folio mapcount can race with our
check for GUP references, and F_SEAL_WRITE might succeed even if there
are some, allowing file contents to be modified after sealing.

For folios with a reliable pincount, detect FOLL_PIN references using
folio_maybe_dma_pinned() before falling back to the refcount comparison.
For small folios without that, the 1024 pinning BIAS makes it very hard
(although not impossible) to misguide the refcount comparison.

This improves FOLL_PIN detection for folios with a pincount. Handling
FOLL_GET and small folios requires a more fundamental change.

Fortunately, most interfaces that allow for GUP'ing such pages use
FOLL_PIN, in particular interfaces using FOLL_LONGTERM. However, we
still have some FOLL_GET leftovers ...

Fixes: 05f65b5c7090 ("shm: wait for pins to be released when sealing")
Cc: stable@vger.kernel.org
Cc: Chris Mason <mason@kernel.org>
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 mm/memfd.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/mm/memfd.c b/mm/memfd.c
index 0f6fff004f5e4..9eda4c3eeaca5 100644
--- a/mm/memfd.c
+++ b/mm/memfd.c
@@ -32,6 +32,13 @@
 
 static bool memfd_folio_has_extra_refs(struct folio *folio)
 {
+	/*
+	 * Use the precise pincount where available to avoid racy refcount
+	 * comparisons.
+	 */
+	if (folio_has_pincount(folio) && folio_maybe_dma_pinned(folio))
+		return true;
+
 	return folio_ref_count(folio) != folio_expected_ref_count(folio);
 }
 

-- 
2.43.0