CVE-2024-36899

In the Linux kernel, the following vulnerability has been resolved:

gpiolib: cdev: Fix use after free in lineinfo_changed_notify

The use-after-free issue occurs as follows: when the GPIO chip device file

is being closed by invoking gpio_chrdev_release(), watched_lines is freed

by bitmap_free(), but the unregistration of lineinfo_changed_nb notifier

chain failed due to waiting write rwsem. Additionally, one of the GPIO

chip's lines is also in the release process and holds the notifier chain's

read rwsem. Consequently, a race condition leads to the use-after-free of

watched_lines.

Here is the typical stack when issue happened:

[free]

gpio_chrdev_release()

--> bitmap_free(cdev->watched_lines) <-- freed

--> blocking_notifier_chain_unregister()

--> down_write(&nh->rwsem) <-- waiting rwsem

--> __down_write_common()

--> rwsem_down_write_slowpath()

--> schedule_preempt_disabled()

--> schedule()

[use]

st54spi_gpio_dev_release()

--> gpio_free()

--> gpiod_free()

--> gpiod_free_commit()

--> gpiod_line_state_notify()

--> blocking_notifier_call_chain()

--> down_read(&nh->rwsem); <-- held rwsem

--> notifier_call_chain()

--> lineinfo_changed_notify()

--> test_bit(xxxx, cdev->watched_lines) <-- use after free

The side effect of the use-after-free issue is that a GPIO line event is

being generated for userspace where it shouldn't. However, since the chrdev

is being closed, userspace won't have the chance to read that event anyway.

To fix the issue, call the bitmap_free() function after the unregistration

of lineinfo_changed_nb notifier chain.