* [PATCH v4 1/2] livepatch: Fix UAF of unregistered patch kobjects
2026-08-30 5:48 [PATCH v4 0/2] livepatch: Fix issues around CONFIG_DEBUG_KOBJECT_RELEASE Yafang Shao
@ 2026-08-30 5:48 ` Yafang Shao
2026-08-30 5:48 ` [PATCH v4 2/2] selftests/livepatch: filter debug messages in check_result() Yafang Shao
2026-09-04 10:26 ` [PATCH v4 0/2] livepatch: Fix issues around CONFIG_DEBUG_KOBJECT_RELEASE Petr Mladek
2 siblings, 0 replies; 4+ messages in thread
From: Yafang Shao @ 2026-08-30 5:48 UTC (permalink / raw)
To: jpoimboe, jikos, mbenes, pmladek, joe.lawrence, song
Cc: live-patching, Yafang Shao, sashiko-bot
The kobjects of a livepatch are released via kobject_put(). When
CONFIG_DEBUG_KOBJECT_RELEASE is enabled, kobject_put() does not release
the kobject synchronously but schedules a delayed release with a random
delay of up to 4 seconds (see kobject_release() in lib/kobject.c).
klp_free_patch_finish() only waits for the release of the patch kobject:
klp_free_patch_finish():
kobject_put(&patch->kobj);
wait_for_completion(&patch->finish);
patch->finish is completed by the patch kobject's release callback. If
the patch kobject was never added to sysfs, or if some child kobjects
were initialized but never added to sysfs (e.g. when klp_enable_patch()
fails after klp_init_patch_early()), those un-added children do not hold
a reference on the patch kobject. kobject_add() is what takes the parent
reference, so the patch kobject can be released first, completing
patch->finish while the child releases are still pending.
The caller then unloads the livepatch module, which destroys the static
klp_object and klp_func structures. The delayed child release callbacks
later access this freed memory, causing a use-after-free.
Fix it by making every child kobject hold an explicit reference on its
parent from the moment the object is initialized: klp_init_object_early()
takes a reference on the patch kobject and klp_init_func_early() takes a
reference on the object kobject. Unlike the reference taken by
kobject_add(), these references also exist for objects that are never
added to sysfs, and the release callbacks drop them unconditionally.
This guarantees the patch kobject is released only after all child
kobjects have been released, so patch->finish cannot be completed before
the static structures are safe to free.
Because kobj->parent is set only by kobject_add(), add explicit
back-pointers, obj->patch and func->obj, so the release callbacks can
find the parent. Dynamic objects and nop functions are freed by their
release callbacks; save the parent pointer before freeing and drop the
parent reference afterwards.
Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/all/20260809094046.50ED31F000E9@smtp.kernel.org/
Suggested-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Acked-by: Song Liu <song@kernel.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Tested-by: Petr Mladek <pmladek@suse.com>
---
include/linux/livepatch.h | 4 ++++
kernel/livepatch/core.c | 10 ++++++++++
2 files changed, 14 insertions(+)
diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
index ba9e3988c07c..5f74f79c22b4 100644
--- a/include/linux/livepatch.h
+++ b/include/linux/livepatch.h
@@ -33,6 +33,7 @@
* @kobj: kobject for sysfs resources
* @node: list node for klp_object func_list
* @stack_node: list node for klp_ops func_stack list
+ * @obj: back pointer to the owning object
* @old_size: size of the old function
* @new_size: size of the new function
* @nop: temporary patch to use the original code again; dyn. allocated
@@ -72,6 +73,7 @@ struct klp_func {
struct kobject kobj;
struct list_head node;
struct list_head stack_node;
+ struct klp_object *obj;
unsigned long old_size, new_size;
bool nop;
bool patched;
@@ -86,6 +88,7 @@ struct klp_func {
* @kobj: kobject for sysfs resources
* @func_list: dynamic list of the function entries
* @node: list node for klp_patch obj_list
+ * @patch: back pointer to the owning patch
* @mod: kernel module associated with the patched object
* (NULL for vmlinux)
* @dynamic: temporary object for nop functions; dynamically allocated
@@ -101,6 +104,7 @@ struct klp_object {
struct kobject kobj;
struct list_head func_list;
struct list_head node;
+ struct klp_patch *patch;
struct module *mod;
bool dynamic;
bool patched;
diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
index a240d1144e89..517fe427ff92 100644
--- a/kernel/livepatch/core.c
+++ b/kernel/livepatch/core.c
@@ -646,12 +646,15 @@ static const struct kobj_type klp_ktype_patch = {
static void klp_kobj_release_object(struct kobject *kobj)
{
+ struct klp_patch *patch;
struct klp_object *obj;
obj = container_of(kobj, struct klp_object, kobj);
+ patch = obj->patch;
if (obj->dynamic)
klp_free_object_dynamic(obj);
+ kobject_put(&patch->kobj);
}
static const struct kobj_type klp_ktype_object = {
@@ -662,12 +665,15 @@ static const struct kobj_type klp_ktype_object = {
static void klp_kobj_release_func(struct kobject *kobj)
{
+ struct klp_object *obj;
struct klp_func *func;
func = container_of(kobj, struct klp_func, kobj);
+ obj = func->obj;
if (func->nop)
klp_free_func_nop(func);
+ kobject_put(&obj->kobj);
}
static const struct kobj_type klp_ktype_func = {
@@ -943,7 +949,9 @@ static void klp_init_func_early(struct klp_object *obj,
struct klp_func *func)
{
kobject_init(&func->kobj, &klp_ktype_func);
+ kobject_get(&obj->kobj);
list_add_tail(&func->node, &obj->func_list);
+ func->obj = obj;
}
static void klp_init_object_early(struct klp_patch *patch,
@@ -951,7 +959,9 @@ static void klp_init_object_early(struct klp_patch *patch,
{
INIT_LIST_HEAD(&obj->func_list);
kobject_init(&obj->kobj, &klp_ktype_object);
+ kobject_get(&patch->kobj);
list_add_tail(&obj->node, &patch->obj_list);
+ obj->patch = patch;
}
static void klp_init_patch_early(struct klp_patch *patch)
--
2.52.0
^ permalink raw reply related [flat|nested] 4+ messages in thread* [PATCH v4 2/2] selftests/livepatch: filter debug messages in check_result()
2026-08-30 5:48 [PATCH v4 0/2] livepatch: Fix issues around CONFIG_DEBUG_KOBJECT_RELEASE Yafang Shao
2026-08-30 5:48 ` [PATCH v4 1/2] livepatch: Fix UAF of unregistered patch kobjects Yafang Shao
@ 2026-08-30 5:48 ` Yafang Shao
2026-09-04 10:26 ` [PATCH v4 0/2] livepatch: Fix issues around CONFIG_DEBUG_KOBJECT_RELEASE Petr Mladek
2 siblings, 0 replies; 4+ messages in thread
From: Yafang Shao @ 2026-08-30 5:48 UTC (permalink / raw)
To: jpoimboe, jikos, mbenes, pmladek, joe.lawrence, song
Cc: live-patching, Yafang Shao
CONFIG_DEBUG_KOBJECT makes kobject_add_internal(), kobject_uevent_env(),
fill_kobj_path() and friends emit pr_debug() messages, and
CONFIG_DEBUG_KOBJECT_RELEASE makes kobject_release() emit a pr_info()
for every delayed kobject free. All of these carry the "kobject:" prefix
via pr_fmt(), e.g.:
# --- expected
# +++ result
# @@ -1,7 +1,13 @@
# % insmod test_modules/test_klp_livepatch.ko
# +kobject: 'test_klp_livepatch' (000000009dbf565e): kobject_add_internal: parent: 'module', set: 'module'
# +kobject: 'holders' (000000002856f0ae): kobject_add_internal: parent: 'test_klp_livepatch', set: '<NULL>'
# +kobject: 'test_klp_livepatch' (00000000dcae1113): kobject_add_internal: parent: 'livepatch', set: '<NULL>'
# +kobject: 'vmlinux' (000000007b8837e6): kobject_add_internal: parent: 'test_klp_livepatch', set: '<NULL>'
# livepatch: enabling patch 'test_klp_livepatch'
# livepatch: 'test_klp_livepatch': initializing patching transition
# livepatch: 'test_klp_livepatch': starting patching transition
# +kobject: 'test_klp_livepatch' (000000009dbf565e): kobject_uevent_env
# +kobject: 'test_klp_livepatch' (000000009dbf565e): fill_kobj_path: path = '/module/test_klp_livepatch'
# livepatch: 'test_klp_livepatch': completing patching transition
# livepatch: 'test_klp_livepatch': patching complete
# % echo 0 > /sys/kernel/livepatch/test_klp_livepatch/enabled
# @@ -9,4 +15,17 @@ livepatch: 'test_klp_livepatch': initial
# livepatch: 'test_klp_livepatch': starting unpatching transition
# livepatch: 'test_klp_livepatch': completing unpatching transition
# livepatch: 'test_klp_livepatch': unpatching complete
# +kobject: 'test_klp_livepatch' (00000000dcae1113): kobject_release, parent 00000000f8785d63 (delayed 2000)
# +kobject: 'test_klp_livepatch' (00000000dcae1113): kobject_cleanup, parent 00000000f8785d63
# +kobject: 'test_klp_livepatch' (00000000dcae1113): auto cleanup kobject_del
# +kobject: 'test_klp_livepatch' (00000000dcae1113): calling ktype release
# +kobject: 'test_klp_livepatch': free name
# % rmmod test_klp_livepatch
# +kobject: 'test_klp_livepatch' (000000009dbf565e): kobject_release, parent 0000000052e5c022 (delayed 3000)
# +kobject: 'test_klp_livepatch' (000000009dbf565e): kobject_cleanup, parent 0000000052e5c022
# +kobject: 'test_klp_livepatch' (000000009dbf565e): auto cleanup kobject_del
# +kobject: 'test_klp_livepatch' (000000009dbf565e): auto cleanup 'remove' event
# +kobject: 'test_klp_livepatch' (000000009dbf565e): kobject_uevent_env
# +kobject: 'test_klp_livepatch' (000000009dbf565e): fill_kobj_path: path = '/module/test_klp_livepatch'
# +kobject: 'test_klp_livepatch' (000000009dbf565e): calling ktype release
# +kobject: 'test_klp_livepatch': free name
#
# ERROR: livepatch kselftest(s) failed
not ok 1 selftests: livepatch: test-livepatch.sh # exit=1
The livepatch test modules' kobjects are named "test_klp_*", so these
lines match the check_result() grep for "test_klp" and leak into the
result. The extra lines no longer match the expected output, so the
selftests fail when either debug config is enabled.
Filtering out every "kobject:" line also hides real WARN()s, e.g. the
one kobject_get() emits for an object whose refcount was not
initialized. Filtering with "dmesg --level=..." is not enough either,
since the tests enable livepatch pr_debug() through dynamic_debug/control
and expect its messages. Read the log with "dmesg --raw" and drop only
the debug-level messages without the "livepatch:" prefix, plus the
specific delayed-release info message.
Suggested-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Acked-by: Song Liu <song@kernel.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Tested-by: Petr Mladek <pmladek@suse.com>
---
tools/testing/selftests/livepatch/functions.sh | 12 +++++++++---
1 file changed, 9 insertions(+), 3 deletions(-)
diff --git a/tools/testing/selftests/livepatch/functions.sh b/tools/testing/selftests/livepatch/functions.sh
index 30dc677b2f45..8352c8d509a5 100644
--- a/tools/testing/selftests/livepatch/functions.sh
+++ b/tools/testing/selftests/livepatch/functions.sh
@@ -306,9 +306,9 @@ function start_test {
# find new kernel messages since the test started.
local last_dmesg_msg="livepatch kselftest timestamp: $(date --rfc-3339=ns)"
log "$last_dmesg_msg"
- loop_until 'dmesg | grep -q "$last_dmesg_msg"' ||
+ loop_until 'dmesg -r | grep -q "$last_dmesg_msg"' ||
die "buffer busy? can't find canary dmesg message: $last_dmesg_msg"
- LAST_DMESG=$(dmesg | grep "$last_dmesg_msg")
+ LAST_DMESG=$(dmesg -r | grep "$last_dmesg_msg")
echo -n "TEST: $test ... "
log "===== TEST: $test ====="
@@ -321,12 +321,18 @@ function check_result {
local result
# Test results include any new dmesg entry since LAST_DMESG, then:
+ # - exclude debug messages except with "livepatch:" prefix
# - include lines matching keywords
# - exclude lines matching keywords
# - filter out dmesg timestamp prefixes
- result=$(dmesg | awk -v last_dmesg="$LAST_DMESG" 'p; $0 == last_dmesg { p=1 }' | \
+ # - exclude the delayed kobject release messages when CONFIG_DEBUG_KOBJECT_RELEASE is on
+ result=$(dmesg -r | \
+ awk -v last_dmesg="$LAST_DMESG" \
+ 'p { if ($0 !~ /^<7>/ || $0 ~ /livepatch:/) print; next } $0 == last_dmesg { p = 1 }' | \
grep -e 'livepatch:' -e 'test_klp' | \
grep -v '\(tainting\|taints\) kernel' | \
+ grep -v 'kobject: .*parent.*(delayed' | \
+ sed 's/^<[0-9]*>//' | \
sed 's/^\[[ 0-9.]*\] //' | \
sed 's/^\[[ ]*[CT][0-9]*\] //')
--
2.52.0
^ permalink raw reply related [flat|nested] 4+ messages in thread