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gpiolib: cdev: Fix use after free in lineinfo_changed_notify

Steve Best requested to merge sfbest/centos-stream-9:39849 into main

JIRA: https://issues.redhat.com/browse/RHEL-39849
CVE: CVE-2024-36899

Build Info: https://brewweb.engineering.redhat.com/brew/taskinfo?taskID=61800387
Tested: Did sanity boot testing Intel (intel-arrowlake-s-02) system.

commit 02f6b0e1ec7e0e7d059dddc893645816552039da
Author: Zhongqiu Han quic_zhonhan@quicinc.com
Date: Sun May 5 22:11:56 2024 +0800

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.  

Fixes: 51c1064e82e7 ("gpiolib: add new ioctl() for monitoring changes in line info")  
Signed-off-by: Zhongqiu Han <quic_zhonhan@quicinc.com>  
Link: https://lore.kernel.org/r/20240505141156.2944912-1-quic_zhonhan@quicinc.com  
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

Signed-off-by: Steve Best sbest@redhat.com

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