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, Im 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 cant 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 Ive 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 cant 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

Weakness

Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code.

Affected Software

Extended Description

The use of previously-freed memory can have any number of adverse consequences, ranging from the corruption of valid data to the execution of arbitrary code, depending on the instantiation and timing of the flaw. The simplest way data corruption may occur involves the system’s reuse of the freed memory. Use-after-free errors have two common and sometimes overlapping causes:

In this scenario, the memory in question is allocated to another pointer validly at some point after it has been freed. The original pointer to the freed memory is used again and points to somewhere within the new allocation. As the data is changed, it corrupts the validly used memory; this induces undefined behavior in the process. If the newly allocated data happens to hold a class, in C++ for example, various function pointers may be scattered within the heap data. If one of these function pointers is overwritten with an address to valid shellcode, execution of arbitrary code can be achieved.

Potential Mitigations

References

• https://git.kernel.org/stable/c/7e5ee3281dc09014367f5112b6d566ba36ea2d49
• https://git.kernel.org/stable/c/85241f7de216f8298f6e48540ea13d7dcd100870
• https://git.kernel.org/stable/c/91932db1d96b2952299ce30c1c693d834d10ace6
• https://git.kernel.org/stable/c/c79a39dc8d060b9e64e8b0fa9d245d44befeefbe
• https://git.kernel.org/stable/c/cd3bbcb6b3a7caa5ce67de76723b6d8531fb7f64