[PATCH V3 3/3] livepatch: free klp_patch object synchronously

From: Ming Lei
Date: Mon Nov 01 2021 - 07:26:50 EST


klp_mutex isn't acquired before calling kobject_put(klp_patch), so it is
fine to free klp_patch object synchronously.

One issue is that enabled store() method, in which the klp_patch kobject
itself is deleted & released. However, sysfs has provided APIs for dealing
with this corner case, so use sysfs_break_active_protection() and
sysfs_unbreak_active_protection() for releasing klp_patch kobject from
enabled_store(), meantime the enabled attribute has to be removed
before deleting the klp_patch kobject.

Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
---
include/linux/livepatch.h | 1 -
kernel/livepatch/core.c | 35 +++++++++++++++--------------------
kernel/livepatch/core.h | 2 +-
kernel/livepatch/transition.c | 2 +-
4 files changed, 17 insertions(+), 23 deletions(-)

diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
index 9712818997c5..4dcebf52fac5 100644
--- a/include/linux/livepatch.h
+++ b/include/linux/livepatch.h
@@ -169,7 +169,6 @@ struct klp_patch {
struct list_head obj_list;
bool enabled;
bool forced;
- struct work_struct free_work;
};

#define klp_for_each_object_static(patch, obj) \
diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
index 9ede093d699a..bb573abecb90 100644
--- a/kernel/livepatch/core.c
+++ b/kernel/livepatch/core.c
@@ -337,6 +337,7 @@ static ssize_t enabled_store(struct kobject *kobj, struct kobj_attribute *attr,
int ret;
bool enabled;
LIST_HEAD(to_free);
+ struct kernfs_node *kn = NULL;

ret = kstrtobool(buf, &enabled);
if (ret)
@@ -369,10 +370,18 @@ static ssize_t enabled_store(struct kobject *kobj, struct kobj_attribute *attr,
out:
mutex_unlock(&klp_mutex);

- klp_free_patches_async(&to_free);
-
if (ret)
return ret;
+
+ if (!list_empty(&to_free)) {
+ kn = sysfs_break_active_protection(kobj, &attr->attr);
+ WARN_ON_ONCE(!kn);
+ sysfs_remove_file(kobj, &attr->attr);
+ klp_free_patches(&to_free);
+ if (kn)
+ sysfs_unbreak_active_protection(kn);
+ }
+
return count;
}

@@ -684,32 +693,19 @@ static void klp_free_patch_finish(struct klp_patch *patch)
kobject_put(&patch->kobj);
}

-/*
- * The livepatch might be freed from sysfs interface created by the patch.
- * This work allows to wait until the interface is destroyed in a separate
- * context.
- */
-static void klp_free_patch_work_fn(struct work_struct *work)
-{
- struct klp_patch *patch =
- container_of(work, struct klp_patch, free_work);
-
- klp_free_patch_finish(patch);
-}
-
-static void klp_free_patch_async(struct klp_patch *patch)
+static void klp_free_patch(struct klp_patch *patch)
{
klp_free_patch_start(patch);
- schedule_work(&patch->free_work);
+ klp_free_patch_finish(patch);
}

-void klp_free_patches_async(struct list_head *to_free)
+void klp_free_patches(struct list_head *to_free)
{
struct klp_patch *patch, *tmp_patch;

list_for_each_entry_safe(patch, tmp_patch, to_free, list) {
list_del_init(&patch->list);
- klp_free_patch_async(patch);
+ klp_free_patch(patch);
}
}

@@ -873,7 +869,6 @@ static int klp_init_patch_early(struct klp_patch *patch)
kobject_init(&patch->kobj, &klp_ktype_patch);
patch->enabled = false;
patch->forced = false;
- INIT_WORK(&patch->free_work, klp_free_patch_work_fn);

klp_for_each_object_static(patch, obj) {
if (!obj->funcs)
diff --git a/kernel/livepatch/core.h b/kernel/livepatch/core.h
index 8ff97745ba40..ea593f370049 100644
--- a/kernel/livepatch/core.h
+++ b/kernel/livepatch/core.h
@@ -13,7 +13,7 @@ extern struct list_head klp_patches;
#define klp_for_each_patch(patch) \
list_for_each_entry(patch, &klp_patches, list)

-void klp_free_patches_async(struct list_head *to_free);
+void klp_free_patches(struct list_head *to_free);
void klp_unpatch_replaced_patches(struct klp_patch *new_patch);
void klp_discard_nops(struct klp_patch *new_patch);

diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
index a9ebc9c5db02..3eff5fc0deee 100644
--- a/kernel/livepatch/transition.c
+++ b/kernel/livepatch/transition.c
@@ -41,7 +41,7 @@ static void klp_transition_work_fn(struct work_struct *work)

mutex_unlock(&klp_mutex);

- klp_free_patches_async(&to_free);
+ klp_free_patches(&to_free);
}
static DECLARE_DELAYED_WORK(klp_transition_work, klp_transition_work_fn);

--
2.31.1