linux-kernel - [PATCH v2] livepatch: Fix leak on klp_init_patch

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Date:   Tue, 14 Dec 2021 14:01:26 -0800
From:   David Vernet <void@...ifault.com>
To:     pmladek@...e.com, live-patching@...r.kernel.org,
        linux-kernel@...r.kernel.org, jpoimboe@...hat.com,
        jikos@...nel.org, mbenes@...e.cz, joe.lawrence@...hat.com,
        corbet@....net
Cc:     void@...ifault.com, songliubraving@...com,
        gregkh@...uxfoundation.org
Subject: [PATCH v2] livepatch: Fix leak on klp_init_patch_early failure path

When enabling a klp patch with klp_enable_patch(), klp_init_patch_early()
is invoked to initialize the kobjects for the patch itself, as well as the
'struct klp_object' and 'struct klp_func' objects that comprise it.
However, there are some error paths in klp_enable_patch() where some
kobjects may have been initialized with kobject_init(), but an error code
is still returned due to e.g. a 'struct klp_object' having a NULL funcs
pointer.

In these paths, the kobject of the 'struct klp_patch' may be leaked, along
with one or more of its objects and their functions, as kobject_put() is
not invoked on the cleanup path if klp_init_patch_early() returns an error
code.

For example, if an object entry such as the following were added to the
sample livepatch module's klp patch, it would cause the vmlinux klp_object,
and its klp_func which updates 'cmdline_proc_show', to be leaked:

static struct klp_object objs[] = {
	{
		/* name being NULL means vmlinux */
		.funcs = funcs,
	},
	{
		.name = "kvm",
		/* NULL funcs -- would cause leak */
	}, { }
};

Without this change, if CONFIG_DEBUG_KOBJECT is enabled, and the sample klp
patch is loaded, the kobjects (the patch, the vmlinux 'struct klp_obj', and
its func) are not observed to be released in the dmesg log output.  With
the change, the kobjects are observed to be released.

Signed-off-by: David Vernet <void@...ifault.com>
---
v2:
  - Move try_module_get() and the patch->objs NULL check out of
    klp_init_patch_early() to ensure that it's safe to jump to the 'err' label
    on the error path in klp_enable_patch().
  - Fix the patch description to not use markdown, and to use imperative
    language.

 kernel/livepatch/core.c | 17 ++++++-----------
 1 file changed, 6 insertions(+), 11 deletions(-)

diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
index 335d988bd811..98c2b0d02770 100644
--- a/kernel/livepatch/core.c
+++ b/kernel/livepatch/core.c
@@ -867,9 +867,6 @@ static int klp_init_patch_early(struct klp_patch *patch)
 	struct klp_object *obj;
 	struct klp_func *func;
 
-	if (!patch->objs)
-		return -EINVAL;
-
 	INIT_LIST_HEAD(&patch->list);
 	INIT_LIST_HEAD(&patch->obj_list);
 	kobject_init(&patch->kobj, &klp_ktype_patch);
@@ -889,9 +886,6 @@ static int klp_init_patch_early(struct klp_patch *patch)
 		}
 	}
 
-	if (!try_module_get(patch->mod))
-		return -ENODEV;
-
 	return 0;
 }
 
@@ -1025,7 +1019,7 @@ int klp_enable_patch(struct klp_patch *patch)
 {
 	int ret;
 
-	if (!patch || !patch->mod)
+	if (!patch || !patch->mod || !patch->objs)
 		return -EINVAL;
 
 	if (!is_livepatch_module(patch->mod)) {
@@ -1051,11 +1045,12 @@ int klp_enable_patch(struct klp_patch *patch)
 		return -EINVAL;
 	}
 
+	if (!try_module_get(patch->mod))
+		return -ENODEV;
+
 	ret = klp_init_patch_early(patch);
-	if (ret) {
-		mutex_unlock(&klp_mutex);
-		return ret;
-	}
+	if (ret)
+		goto err;
 
 	ret = klp_init_patch(patch);
 	if (ret)
-- 
2.30.2