CVE-2024-57979

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