[PATCH] kasan: test: don't copy more than size bytes in memcpy test

[Peter Collingbourne]

With HW tag-based KASAN, error checks are performed implicitly by the load
and store instructions in the memcpy implementation.  A failed check results
in tag checks being disabled and execution will keep going. As a result,
under HW tag-based KASAN, this memcpy would end up corrupting memory until
it hits an inaccessible page and causes a kernel panic.

This is a pre-existing issue that was revealed by commit 285133040e6c ("arm64:
Import latest memcpy()/memmove() implementation") which changed the memcpy
implementation from using signed comparisons (incorrectly, resulting in
the memcpy being terminated early for negative sizes) to using unsigned
comparisons.

It is unclear how this could be handled by memcpy itself in a reasonable
way. One possibility would be to add an exception handler that would force
memcpy to return if a tag check fault is detected -- this would make the
behavior roughly similar to generic and SW tag-based KASAN. However, this
wouldn't solve the problem for asynchronous mode and also makes memcpy
behavior inconsistent with manually copying data.

It may be more accurate to consider this a bug in the test: what we really
want to test here is that a memcpy overflow, however small, is caught, and any
further copying after the initial overflow is unnecessary and may affect system
stability. Therefore, adjust the test to pass the allocation size as the memcpy
size, ensuring that the memcpy will not result in an out-of-bounds write.

Commit 1b0668be62cf ("kasan: test: disable kmalloc_memmove_invalid_size for
HW_TAGS") disabled this test in HW tags mode, but there is some value in
testing small memcpy overflows, so let's re-enable it with this fix.

Link: https://linux-review.googlesource.com/id/I048d1e6a9aff766c4a53f989fb0c83de68923882
Signed-off-by: Peter Collingbourne <pcc at google.com>
---
 lib/test_kasan.c | 9 +--------
 1 file changed, 1 insertion(+), 8 deletions(-)

diff --git a/lib/test_kasan.c b/lib/test_kasan.c
index 8835e0784578..9af51e1f692d 100644
--- a/lib/test_kasan.c
+++ b/lib/test_kasan.c
@@ -497,14 +497,7 @@ static void kmalloc_memmove_invalid_size(struct kunit *test)
 {
 	char *ptr;
 	size_t size = 64;
-	volatile size_t invalid_size = -2;
-
-	/*
-	 * Hardware tag-based mode doesn't check memmove for negative size.
-	 * As a result, this test introduces a side-effect memory corruption,
-	 * which can result in a crash.
-	 */
-	KASAN_TEST_NEEDS_CONFIG_OFF(test, CONFIG_KASAN_HW_TAGS);
+	volatile size_t invalid_size = size;
 
 	ptr = kmalloc(size, GFP_KERNEL);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
-- 
2.33.0.309.g3052b89438-goog