Medium [CVE-2026-72482] fix double decrement of descriptor_busy in command_ioctl

In the Linux kernel, the following vulnerability has been resolved: gpib: fix double decrement of descriptor_busy in command_ioctl() commit d1857f8296dc ("gpib: fix use-after-free in IO ioctl handlers") introduced a descriptor_busy reference counter to pin struct gpib_descriptor across IO ioctl operations.

In command_ioctl(), the error path inside the loop decrements descriptor_busy and breaks, but execution then falls through to the unconditional decrement after the loop, underflowing the counter to -1.

This re-enables the use-after-free that the original fix was meant to prevent: a concurrent close_dev_ioctl() sees descriptor_busy == 0 on an actively-used descriptor and frees it. The post-loop decrement already handles all exit paths, matching the correct pattern used in read_ioctl() and write_ioctl().

A double decrement of a descriptor counter within the `command_ioctl()` function can lead to a counter underflow. This re-enables a use-after-free vulnerability, where a concurrently active descriptor can be prematurely freed.

This could result in system instability, a denial of service, or potentially allow for arbitrary code execution. Red Hat severity: Moderate — CVSS 5.5 (CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H).

Weakness: CWE-911.