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 chips lines is also in the release process and holds the notifier chains 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 shouldnt. However, since the chrdev is being closed, userspace wont 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.

Weakness

The product contains a code sequence that can run concurrently with other code, and the code sequence requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence that is operating concurrently.

Affected Software

Extended Description

This can have security implications when the expected synchronization is in security-critical code, such as recording whether a user is authenticated or modifying important state information that should not be influenced by an outsider. A race condition occurs within concurrent environments, and is effectively a property of a code sequence. Depending on the context, a code sequence may be in the form of a function call, a small number of instructions, a series of program invocations, etc. A race condition violates these properties, which are closely related:

A race condition exists when an “interfering code sequence” can still access the shared resource, violating exclusivity. Programmers may assume that certain code sequences execute too quickly to be affected by an interfering code sequence; when they are not, this violates atomicity. For example, the single “x++” statement may appear atomic at the code layer, but it is actually non-atomic at the instruction layer, since it involves a read (the original value of x), followed by a computation (x+1), followed by a write (save the result to x). The interfering code sequence could be “trusted” or “untrusted.” A trusted interfering code sequence occurs within the product; it cannot be modified by the attacker, and it can only be invoked indirectly. An untrusted interfering code sequence can be authored directly by the attacker, and typically it is external to the vulnerable product.

Potential Mitigations

• Minimize the usage of shared resources in order to remove as much complexity as possible from the control flow and to reduce the likelihood of unexpected conditions occurring.
• Additionally, this will minimize the amount of synchronization necessary and may even help to reduce the likelihood of a denial of service where an attacker may be able to repeatedly trigger a critical section (CWE-400).

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/26.html
• https://cwe.mitre.org/data/definitions/29.html

References

• https://git.kernel.org/stable/c/02f6b0e1ec7e0e7d059dddc893645816552039da
• https://git.kernel.org/stable/c/95ca7c90eaf5ea8a8460536535101e3e81160e2a
• https://git.kernel.org/stable/c/ca710b5f40b8b16fdcad50bebd47f50e4c62d239
• https://git.kernel.org/stable/c/02f6b0e1ec7e0e7d059dddc893645816552039da
• https://git.kernel.org/stable/c/95ca7c90eaf5ea8a8460536535101e3e81160e2a
• https://git.kernel.org/stable/c/ca710b5f40b8b16fdcad50bebd47f50e4c62d239