CVE-2021-47281 is a race condition vulnerability identified in the Linux kernel's ALSA (Advanced Linux Sound Architecture) sequencer subsystem, specifically in the snd_seq_timer_open() function. The ALSA sequencer manages MIDI and other sound-related timing events, where each queue has an exclusive timer instance. The vulnerability arises because snd_seq_timer_open() does not properly synchronize concurrent accesses to the timer instance. Although the function checks for an existing timer instance at the start, it lacks adequate protection mechanisms, allowing multiple concurrent calls to override the timer instance. This can lead to a use-after-free (UAF) condition where a timer instance continues running even after the associated queue has been closed. The issue was discovered through syzkaller, a kernel fuzzing tool, which highlighted the race condition. The fix involves adding a proper check during the assignment of the timer instance (tmr->timeri) and returning an -EBUSY error if a timer is already registered, thereby preventing concurrent overwrites and eliminating the race condition. This vulnerability affects Linux kernel versions identified by the commit hash 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and potentially other versions with similar code. No known exploits are currently reported in the wild, and no CVSS score has been assigned yet.

For European organizations, this vulnerability could have several implications. Systems running Linux kernels with vulnerable ALSA sequencer code—commonly found in servers, desktops, embedded devices, and IoT devices—may be susceptible to exploitation. An attacker with local access or the ability to trigger concurrent snd_seq_timer_open() calls could exploit the race condition to cause a use-after-free, potentially leading to kernel crashes (denial of service) or, in a worst-case scenario, privilege escalation if the UAF is leveraged to execute arbitrary code in kernel space. This could compromise system integrity and availability. Given the widespread use of Linux in European government, financial, industrial, and telecommunications sectors, the vulnerability could affect critical infrastructure and services. However, exploitation requires concurrent access to the ALSA sequencer timer, which may limit remote exploitation vectors. Nonetheless, systems that allow untrusted users or processes to interact with ALSA sequencer interfaces are at higher risk. The absence of known exploits reduces immediate threat but does not eliminate future risk, especially as attackers often weaponize such race conditions once patches are released.

European organizations should prioritize updating their Linux kernel versions to include the patch that adds proper synchronization and returns -EBUSY when a timer instance is already registered. Specifically, kernel maintainers and system administrators should verify that their distributions have incorporated the fix associated with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 or later. For environments where immediate patching is not feasible, restricting access to ALSA sequencer interfaces to trusted users only can reduce risk. Employing kernel hardening techniques such as Kernel Address Space Layout Randomization (KASLR), Kernel Page Table Isolation (KPTI), and enabling seccomp filters to limit access to sound-related syscalls can further mitigate exploitation potential. Continuous monitoring for unusual kernel crashes or suspicious activity related to ALSA interfaces is recommended. Additionally, organizations should audit and restrict local user permissions to prevent untrusted users from triggering concurrent snd_seq_timer_open() calls. Finally, maintaining a robust patch management process and staying informed about Linux kernel security advisories will help mitigate this and future vulnerabilities.