mm/damon/core-kunit: skip wrong region walk in commit_target_regions()

damon_test_commit_target_regions_for() traverses expected values array
after damon_commit_target_regions() call.  It assumes
damon_commit_target_regions() made expected number of regions.  It might
not.  Because the traversal is made based on the region count, it could do
out of bounds access to the expectation value array.

The consequent user impact (out-of-bound access) is quite bad.  The
realistic user impact would be limited, though.  It would affect only test
run setups.

Fix it by testing if the number of regions was also changed as expected
and exit early for the failure.

The issue was discovered [1] by Sashiko.

Link: https://lore.kernel.org/20260718001442.87129-8-sj@kernel.org
Link: https://lore.kernel.org/20260713144757.39740-1-sj@kernel.org [1]
Fixes: 603f67eb91e0 ("mm/damon/tests/core-kunit: add damon_commit_target_regions() test")
Signed-off-by: SJ Park <sj@kernel.org>
Cc: Brendan Higgins <brendan.higgins@linux.dev>
Cc: <stable@vger.kernel.org> # 6.19.x
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 7 insertions, 2 deletions

diff --git a/mm/damon/tests/core-kunit.h b/mm/damon/tests/core-kunit.h
index fbf986aee2de..4a536d41cdb2 100644
--- a/mm/damon/tests/core-kunit.h
+++ b/mm/damon/tests/core-kunit.h
@@ -1277,14 +1277,19 @@ static void damon_test_commit_target_regions_for(struct kunit *test,
 		kunit_skip(test, "src target setup fail");
 	}
 	damon_commit_target_regions(dst_target, src_target, 1);
+
+	KUNIT_EXPECT_EQ(test, damon_nr_regions(dst_target), nr_expect_regions);
+	if (damon_nr_regions(dst_target) != nr_expect_regions)
+		goto out;
+
 	i = 0;
 	damon_for_each_region(r, dst_target) {
 		KUNIT_EXPECT_EQ(test, r->ar.start, expect_start_end[i][0]);
 		KUNIT_EXPECT_EQ(test, r->ar.end, expect_start_end[i][1]);
 		i++;
 	}
-	KUNIT_EXPECT_EQ(test, damon_nr_regions(dst_target), nr_expect_regions);
-	KUNIT_EXPECT_EQ(test, i, nr_expect_regions);
+
+out:
 	damon_free_target(dst_target);
 	damon_free_target(src_target);
 }