[PATCH] selftests/mm: fix uffd move-pmd test crash on 64K base page systems

[Zenghui Yu]

From: "Zenghui Yu (Huawei)" <zenghui.yu@xxxxxxxxx>

uffd_move_pmd_test()/uffd_move_test_common() splits the test area into
chunks of chunk_size and moves them one by one.  On arm64 with 64K base
pages the chunk_size (read_pmd_pagesize()) is 512MB, which is larger than
the whole 32MB (UFFD_TEST_MEM_SIZE) test area:

  step_count = nr_pages / (chunk_size / page_size) == 0

Because chunk_size > page_size, the test then aligns both areas up to
512MB, which points far outside of the 32MB mappings, and decrements
step_count, which underflows to ULONG_MAX.  The verification loop then
runs away and the first dereference of area_src faults with a SIGSEGV.

Skip the test when a single chunk cannot fit into the test area.

Fixes: a2bf6a9ca805 ("selftests/mm: add UFFDIO_MOVE ioctl test")
Signed-off-by: Zenghui Yu (Huawei) <zenghui.yu@xxxxxxxxx>
---
 tools/testing/selftests/mm/uffd-unit-tests.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/tools/testing/selftests/mm/uffd-unit-tests.c b/tools/testing/selftests/mm/uffd-unit-tests.c
index ef9b3956bdcf..2a97ba1d6feb 100644
--- a/tools/testing/selftests/mm/uffd-unit-tests.c
+++ b/tools/testing/selftests/mm/uffd-unit-tests.c
@@ -1937,6 +1937,11 @@ uffd_move_test_common(uffd_global_test_opts_t *gopts,
 
 	args.gopts = gopts;
 
+	if (chunk_size > gopts->nr_pages * gopts->page_size) {
+		uffd_test_skip("chunk size does not fit in the test area");
+		return;
+	}
+
 	/* Prevent source pages from being mapped more than once */
 	if (madvise(gopts->area_src, gopts->nr_pages * gopts->page_size, MADV_DONTFORK))
 		err("madvise(MADV_DONTFORK) failure");
-- 
2.53.0