From: Kiryl Shutsemau <kas@kernel.org>

Accesses within VMA, but beyond i_size rounded up to PAGE_SIZE are
supposed to generate SIGBUS.

This behavior might not be respected on truncation.

During truncation, the kernel splits a large folio in order to reclaim
memory. As a side effect, it unmaps the folio and destroys PMD mappings
of the folio. The folio will be refaulted as PTEs and SIGBUS semantics
are preserved.

However, if the split fails, PMD mappings are preserved and the user
will not receive SIGBUS on any accesses within the PMD.

Unmap the folio on split failure. It will lead to refault as PTEs and
preserve SIGBUS semantics.

Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
---
 mm/truncate.c | 29 ++++++++++++++++++++++++++---
 1 file changed, 26 insertions(+), 3 deletions(-)

diff --git a/mm/truncate.c b/mm/truncate.c
index 91eb92a5ce4f..cdb698b5f7fa 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -177,6 +177,28 @@ int truncate_inode_folio(struct address_space *mapping, struct folio *folio)
 	return 0;
 }
 
+static int try_folio_split_or_unmap(struct folio *folio, struct page *split_at)
+{
+	enum ttu_flags ttu_flags =
+		TTU_RMAP_LOCKED |
+		TTU_SYNC |
+		TTU_BATCH_FLUSH |
+		TTU_SPLIT_HUGE_PMD |
+		TTU_IGNORE_MLOCK;
+	int ret;
+
+	ret = try_folio_split(folio, split_at, NULL);
+
+	/*
+	 * If the split fails, unmap the folio, so it will be refaulted
+	 * with PTEs to respect SIGBUS semantics.
+	 */
+	if (ret)
+		try_to_unmap(folio, ttu_flags);
+
+	return ret;
+}
+
 /*
  * Handle partial folios.  The folio may be entirely within the
  * range if a split has raced with us.  If not, we zero the part of the
@@ -224,7 +246,7 @@ bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end)
 		return true;
 
 	split_at = folio_page(folio, PAGE_ALIGN_DOWN(offset) / PAGE_SIZE);
-	if (!try_folio_split(folio, split_at, NULL)) {
+	if (!try_folio_split_or_unmap(folio, split_at)) {
 		/*
 		 * try to split at offset + length to make sure folios within
 		 * the range can be dropped, especially to avoid memory waste
@@ -249,12 +271,13 @@ bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end)
 			goto out;
 
 		/*
+		 * Split the folio.
+		 *
 		 * make sure folio2 is large and does not change its mapping.
-		 * Its split result does not matter here.
 		 */
 		if (folio_test_large(folio2) &&
 		    folio2->mapping == folio->mapping)
-			try_folio_split(folio2, split_at2, NULL);
+			try_folio_split_or_unmap(folio2, split_at2);
 
 		folio_unlock(folio2);
 out:
-- 
2.50.1