🇰🇷 한글 설명
Linux kernel의 pps(Pulse Per Second) subsystem에서 발생하는 use-after-free 취약점입니다. cdev_del() 호출 후 아직 열려있는 장치의 파일 연산(fops)이 해제된 메모리에 접근하여 커널 패닉을 유발합니다. ntpd/gpsd 실행 중 재부팅 시 일관되게 발생합니다.
💡 쉬운 설명
시스템이 메모리를 해제한 후에도 프로그램이 그 메모리에 접근하려고 하면 충돌합니다. PPS 시간 서비스를 사용하는 시스템의 재부팅 시 발생합니다.
🇺🇸 원문 (English)
In the Linux kernel, the following vulnerability has been resolved:
pps: Fix a use-after-free
On a board running ntpd and gpsd, I'm seeing a consistent use-after-free
in sys_exit() from gpsd when rebooting:
pps pps1: removed
------------[ cut here ]------------
kobject: '(null)' (00000000db4bec24): is not initialized, yet kobject_put() is being called.
WARNING: CPU: 2 PID: 440 at lib/kobject.c:734 kobject_put+0x120/0x150
CPU: 2 UID: 299 PID: 440 Comm: gpsd Not tainted 6.11.0-rc6-00308-gb31c44928842 #1
Hardware name: Raspberry Pi 4 Model B Rev 1.1 (DT)
pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : kobject_put+0x120/0x150
lr : kobject_put+0x120/0x150
sp : ffffffc0803d3ae0
x29: ffffffc0803d3ae0 x28: ffffff8042dc9738 x27: 0000000000000001
x26: 0000000000000000 x25: ffffff8042dc9040 x24: ffffff8042dc9440
x23: ffffff80402a4620 x22: ffffff8042ef4bd0 x21: ffffff80405cb600
x20: 000000000008001b x19: ffffff8040b3b6e0 x18: 0000000000000000
x17: 0000000000000000 x16: 0000000000000000 x15: 696e6920746f6e20
x14: 7369203a29343263 x13: 205d303434542020 x12: 0000000000000000
x11: 0000000000000000 x10: 0000000000000000 x9 : 0000000000000000
x8 : 0000000000000000 x7 : 0000000000000000 x6 : 0000000000000000
x5 : 0000000000000000 x4 : 0000000000000000 x3 : 0000000000000000
x2 : 0000000000000000 x1 : 0000000000000000 x0 : 0000000000000000
Call trace:
kobject_put+0x120/0x150
cdev_put+0x20/0x3c
__fput+0x2c4/0x2d8
____fput+0x1c/0x38
task_work_run+0x70/0xfc
do_exit+0x2a0/0x924
do_group_exit+0x34/0x90
get_signal+0x7fc/0x8c0
do_signal+0x128/0x13b4
do_notify_resume+0xdc/0x160
el0_svc+0xd4/0xf8
el0t_64_sync_handler+0x140/0x14c
el0t_64_sync+0x190/0x194
---[ end trace 0000000000000000 ]---
...followed by more symptoms of corruption, with similar stacks:
refcount_t: underflow; use-after-free.
kernel BUG at lib/list_debug.c:62!
Kernel panic - not syncing: Oops - BUG: Fatal exception
This happens because pps_device_destruct() frees the pps_device with the
embedded cdev immediately after calling cdev_del(), but, as the comment
above cdev_del() notes, fops for previously opened cdevs are still
callable even after cdev_del() returns. I think this bug has always
been there: I can't explain why it suddenly started happening every time
I reboot this particular board.
In commit d953e0e837e6 ("pps: Fix a use-after free bug when
unregistering a source."), George Spelvin suggested removing the
embedded cdev. That seems like the simplest way to fix this, so I've
implemented his suggestion, using __register_chrdev() with pps_idr
becoming the source of truth for which minor corresponds to which
device.
But now that pps_idr defines userspace visibility instead of cdev_add(),
we need to be sure the pps->dev refcount can't reach zero while
userspace can still find it again. So, the idr_remove() call moves to
pps_unregister_cdev(), and pps_idr now holds a reference to pps->dev.
pps_core: source serial1 got cdev (251:1)
<...>
pps pps1: removed
pps_core: unregistering pps1
pps_core: deallocating pps1
📍 영향받는 시스템
Linux kernel 6.11.0-rc 이전 버전의 pps subsystem 사용 시스템 (Raspberry Pi, 임베디드 보드 등)
⚔️ 공격 벡터
로컬: gpsd/ntpd 프로세스가 /dev/pps* 장치를 열었을 때 시스템 재부팅으로 유발 가능
🛡️ 대응 방법
Linux kernel을 최신 버전으로 업데이트하여 cdev 제거 및 pps_idr 기반 리팩토링 적용
🔧 검증 코드
PPS enabled가 출력되면 취약점 영향 가능성 있음. 커널 버전으로 패치 여부 확인 필요 (uname -r 로 확인)
grep -q 'pps' /boot/config-$(uname -r) && echo 'PPS enabled' || echo 'PPS disabled'