mm/damon/core: fix unitialized memory error from

damos_new_filter() is returning a damos_filter struct without initializing its ->list field. And the users of the function uses the struct without initializing the field. As a result, uninitialized memory access error is possible. Actually, a kernel NULL pointer dereference BUG can be triggered using DAMON user-space tool, like below.

    # damo start --damos_action stat --damos_filter anon matching
    # damo tune --damos_action stat --damos_filter anon matching --damos_filter anon nomatching
    # dmesg
    [...]
    [   36.908136] BUG: kernel NULL pointer dereference, address: 0000000000000008
    [   36.910483] #PF: supervisor write access in kernel mode
    [   36.912238] #PF: error_code(0x0002) - not-present page
    [   36.913415] PGD 0 P4D 0
    [   36.913978] Oops: 0002 [#1] PREEMPT SMP PTI
    [   36.914878] CPU: 32 PID: 1335 Comm: kdamond.0 Not tainted 6.5.0-rc3-mm-unstable-damon+ #1
    [   36.916621] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014
    [   36.919051] RIP: 0010:damos_destroy_filter (include/linux/list.h:114 include/linux/list.h:137 include/linux/list.h:148 mm/damon/core.c:345 mm/damon/core.c:355)
    [...]
    [   36.938247] Call Trace:
    [   36.938721]  <TASK>
    [...]
    [   36.950064] ? damos_destroy_filter (include/linux/list.h:114 include/linux/list.h:137 include/linux/list.h:148 mm/damon/core.c:345 mm/damon/core.c:355)
    [   36.950883] ? damon_sysfs_set_scheme_filters.isra.0 (mm/damon/sysfs-schemes.c:1573)
    [   36.952019] damon_sysfs_set_schemes (mm/damon/sysfs-schemes.c:1674 mm/damon/sysfs-schemes.c:1686)
    [   36.952875] damon_sysfs_apply_inputs (mm/damon/sysfs.c:1312 mm/damon/sysfs.c:1298)
    [   36.953757] ? damon_pa_check_accesses (mm/damon/paddr.c:168 mm/damon/paddr.c:179)
    [   36.954648] damon_sysfs_cmd_request_callback (mm/damon/sysfs.c:1329 mm/damon/sysfs.c:1359)
    [...]

The first patch of this patchset fixes the bug by initializing the field in damos_new_filter(). The second patch adds a unit test for the problem. Note that the second patch Cc stable@ without Fixes: tag, since it would be better to be ingested together for avoiding any future regression.

SeongJae Park (2):
mm/damon/core: initialize damo_filter->list from damos_new_filter() mm/damon/core-test: add a test for damos_new_filter()

 mm/damon/core-test.h | 13 +++++++++++++
 mm/damon/core.c      |  1 +
 2 files changed, 14 insertions(+)

2.25.1

damos_new_filter() is not initializing the list field of newly allocated filter object. However, DAMON sysfs interface and DAMON_RECLAIM are not initializing it after calling damos_new_filter(). As a result, accessing uninitialized memory is possible. Actually, adding multiple DAMOS filters via DAMON sysfs interface caused NULL pointer dereferencing. Initialize the field just after the allocation from damos_new_filter().

---
 mm/damon/core.c | 1 +
 1 file changed, 1 insertion(+)

index 91cff7f2997e..eb9580942a5c 100644
@@ -273,6 +273,7 @@ struct damos_filter *damos_new_filter(enum damos_filter_type type,
 		return NULL;
 	filter->type = type;
 	filter->matching = matching;
+	INIT_LIST_HEAD(&filter->list);
 	return filter;
 }
 

2.25.1

damos_new_filter() was having a bug that not initializing ->list field of the returning damos_filter struct, which results in access to uninitialized memory. Add a unit test for the function.

---
 mm/damon/core-test.h | 13 +++++++++++++
 1 file changed, 13 insertions(+)

index bb07721909e1..4bddbfe243c3 100644
@@ -341,6 +341,18 @@ static void damon_test_set_attrs(struct kunit *test)
 	KUNIT_EXPECT_EQ(test, damon_set_attrs(c, &invalid_attrs), -EINVAL);
 }
 
+static void damos_test_new_filter(struct kunit *test)
+{
+	struct damos_filter *filter;
+
+	filter = damos_new_filter(DAMOS_FILTER_TYPE_ANON, true);
+	KUNIT_EXPECT_EQ(test, filter->type, DAMOS_FILTER_TYPE_ANON);
+	KUNIT_EXPECT_EQ(test, filter->matching, true);
+	KUNIT_EXPECT_PTR_EQ(test, filter->list.prev, &filter->list);
+	KUNIT_EXPECT_PTR_EQ(test, filter->list.next, &filter->list);
+	damos_destroy_filter(filter);
+}
+
 static struct kunit_case damon_test_cases[] = {
 	KUNIT_CASE(damon_test_target),
 	KUNIT_CASE(damon_test_regions),

@@ -353,6 +365,7 @@ static struct kunit_case damon_test_cases[] = {
 	KUNIT_CASE(damon_test_set_regions),
 	KUNIT_CASE(damon_test_update_monitoring_result),
 	KUNIT_CASE(damon_test_set_attrs),
+	KUNIT_CASE(damos_test_new_filter),
 	{},
 };
 

2.25.1