linux-cve-announce - CVE-2024-57979: pps: Fix a use-after-free

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Message-ID: <2025022634-CVE-2024-57979-aad0@gregkh>
Date: Wed, 26 Feb 2025 18:05:38 -0800
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: CVE-2024-57979: pps: Fix a use-after-free

Description
===========

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

The Linux kernel CVE team has assigned CVE-2024-57979 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 3.9 with commit d953e0e837e65ecc1ddaa4f9560f7925878a0de6 and fixed in 6.1.129 with commit 91932db1d96b2952299ce30c1c693d834d10ace6
	Issue introduced in 3.9 with commit d953e0e837e65ecc1ddaa4f9560f7925878a0de6 and fixed in 6.6.76 with commit cd3bbcb6b3a7caa5ce67de76723b6d8531fb7f64
	Issue introduced in 3.9 with commit d953e0e837e65ecc1ddaa4f9560f7925878a0de6 and fixed in 6.12.13 with commit 7e5ee3281dc09014367f5112b6d566ba36ea2d49
	Issue introduced in 3.9 with commit d953e0e837e65ecc1ddaa4f9560f7925878a0de6 and fixed in 6.13.2 with commit 85241f7de216f8298f6e48540ea13d7dcd100870
	Issue introduced in 3.9 with commit d953e0e837e65ecc1ddaa4f9560f7925878a0de6 and fixed in 6.14-rc1 with commit c79a39dc8d060b9e64e8b0fa9d245d44befeefbe
	Issue introduced in 3.2.40 with commit 77327a71f9841b7dfa708195d1cb133d4ef4a989
	Issue introduced in 3.4.87 with commit cd59fb14918a6b20c1ac8be121fa6397b97b00cb
	Issue introduced in 3.8.1 with commit 49626fbb0360332e40fd76a48cb2ba876d6134ad

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2024-57979
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	drivers/pps/clients/pps-gpio.c
	drivers/pps/clients/pps-ktimer.c
	drivers/pps/clients/pps-ldisc.c
	drivers/pps/clients/pps_parport.c
	drivers/pps/kapi.c
	drivers/pps/kc.c
	drivers/pps/pps.c
	drivers/ptp/ptp_ocp.c
	include/linux/pps_kernel.h


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/91932db1d96b2952299ce30c1c693d834d10ace6
	https://git.kernel.org/stable/c/cd3bbcb6b3a7caa5ce67de76723b6d8531fb7f64
	https://git.kernel.org/stable/c/7e5ee3281dc09014367f5112b6d566ba36ea2d49
	https://git.kernel.org/stable/c/85241f7de216f8298f6e48540ea13d7dcd100870
	https://git.kernel.org/stable/c/c79a39dc8d060b9e64e8b0fa9d245d44befeefbe