[PATCH v2] kasan: test: add memcpy test that avoids out-of-bounds write

[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, prior to commit 1b0668be62cf ("kasan:
test: disable kmalloc_memmove_invalid_size for HW_TAGS"), 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.

This test was added as a part of a series that taught KASAN to detect
negative sizes in memory operations, see commit 8cceeff48f23 ("kasan:
detect negative size in memory operation function"). Therefore we
should keep testing for negative sizes with generic and SW tag-based
KASAN. But there is some value in testing small memcpy overflows, so
let's add another test with memcpy that does not destabilize the kernel
by performing out-of-bounds writes, and run it in all modes.

Link: https://linux-review.googlesource.com/id/I048d1e6a9aff766c4a53f989fb0c83de68923882
Signed-off-by: Peter Collingbourne <pcc at google.com>
---
 lib/test_kasan.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/lib/test_kasan.c b/lib/test_kasan.c
index 8835e0784578..aa8e42250219 100644
--- a/lib/test_kasan.c
+++ b/lib/test_kasan.c
@@ -493,7 +493,7 @@ static void kmalloc_oob_in_memset(struct kunit *test)
 	kfree(ptr);
 }
 
-static void kmalloc_memmove_invalid_size(struct kunit *test)
+static void kmalloc_memmove_negative_size(struct kunit *test)
 {
 	char *ptr;
 	size_t size = 64;
@@ -515,6 +515,21 @@ static void kmalloc_memmove_invalid_size(struct kunit *test)
 	kfree(ptr);
 }
 
+static void kmalloc_memmove_invalid_size(struct kunit *test)
+{
+	char *ptr;
+	size_t size = 64;
+	volatile size_t invalid_size = size;
+
+	ptr = kmalloc(size, GFP_KERNEL);
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
+
+	memset((char *)ptr, 0, 64);
+	KUNIT_EXPECT_KASAN_FAIL(test,
+		memmove((char *)ptr, (char *)ptr + 4, invalid_size));
+	kfree(ptr);
+}
+
 static void kmalloc_uaf(struct kunit *test)
 {
 	char *ptr;
@@ -1129,6 +1144,7 @@ static struct kunit_case kasan_kunit_test_cases[] = {
 	KUNIT_CASE(kmalloc_oob_memset_4),
 	KUNIT_CASE(kmalloc_oob_memset_8),
 	KUNIT_CASE(kmalloc_oob_memset_16),
+	KUNIT_CASE(kmalloc_memmove_negative_size),
 	KUNIT_CASE(kmalloc_memmove_invalid_size),
 	KUNIT_CASE(kmalloc_uaf),
 	KUNIT_CASE(kmalloc_uaf_memset),
-- 
2.33.0.309.g3052b89438-goog