CVE-2025-21996 is a vulnerability identified in the Linux kernel's Radeon driver component, specifically within the function radeon_vce_cs_parse(). This function processes command streams passed from userspace via ioctl() calls. The vulnerability arises when a specially crafted command stream is sent, where the first command to execute is an encode operation (case 0x03000001). In this scenario, the function attempts to call radeon_vce_cs_reloc() with a 'size' argument that is uninitialized. The 'size' parameter points to a temporary variable ('tmp') that has not been assigned any value before use, leading to undefined behavior. The root cause is the lack of initialization of the 'tmp' variable before it is referenced. The fix involves initializing 'tmp' to zero, ensuring that radeon_vce_cs_reloc() detects an early error condition rather than proceeding with potentially corrupted or unpredictable data. This vulnerability was discovered by the Linux Verification Center using static analysis tools and has been addressed in the Linux kernel source code. No known exploits are reported in the wild at this time, and no CVSS score has been assigned yet.

The vulnerability could potentially allow a local attacker with the ability to send ioctl() commands to the Radeon driver to cause undefined behavior in the kernel, which might lead to system instability, crashes (denial of service), or potentially memory corruption. While the description does not explicitly mention privilege escalation or remote exploitation, kernel vulnerabilities involving uninitialized variables and improper input validation can sometimes be leveraged to execute arbitrary code or escalate privileges if combined with other vulnerabilities. For European organizations, especially those relying on Linux systems with Radeon graphics hardware—common in workstations, servers, and embedded devices—this vulnerability could disrupt critical operations if exploited. Systems running custom or older Linux kernels that have not been patched may be at risk. The absence of known exploits reduces immediate risk, but the vulnerability's presence in a core kernel component means that it should be addressed promptly to maintain system integrity and availability.

1. Apply the official Linux kernel patch that initializes the 'tmp' variable in radeon_vce_cs_parse() as soon as it becomes available in your distribution's kernel updates. 2. For organizations using custom or long-term support (LTS) kernels, backport the patch or upgrade to a kernel version that includes the fix. 3. Restrict access to the Radeon device nodes to trusted users only, minimizing the risk of unprivileged users sending crafted ioctl() commands. 4. Implement strict access controls and monitoring on systems with Radeon hardware to detect unusual ioctl() activity. 5. Regularly audit and update Linux kernel versions across infrastructure to ensure timely application of security patches. 6. Consider deploying kernel hardening features such as Kernel Address Space Layout Randomization (KASLR) and Kernel Page Table Isolation (KPTI) to reduce the impact of potential kernel exploits. 7. Maintain comprehensive logging and alerting for kernel errors or crashes that could indicate exploitation attempts.