CVE-2021-20261: 
Linux Kernel vulnerability analysis and mitigation

A race condition vulnerability (CVE-2021-20261) was discovered in the Linux kernel's floppy disk drive controller driver software. The vulnerability was identified in early 2021 and affects Linux kernel versions up to 4.5. The impact of this vulnerability is mitigated by default permissions on the floppy device (/dev/fd0) being restricted to root access (NVD, RedHat).

The vulnerability stems from improper error handling in the lock_fdc() function within the floppy driver. When multiple threads attempt to access /dev/fd0 simultaneously, the race condition can occur because floppy_revalidate() doesn't perform proper error handling on lock_fdc() results. This leads to a situation where critical section protection is compromised, as the lock_fdc() function might be interrupted by a signal while waiting for the fdc to become non-busy (Kernel Commit). The vulnerability has a CVSS v3.1 base score of 6.4 (Medium), with vector: CVSS:3.1/AV:L/AC:H/PR:H/UI:N/S:U/C:H/I:H/A:H (NVD).

The vulnerability can lead to serious memory corruptions when multiple threads attempt to open the floppy device simultaneously. At minimum, this can cause system crashes, and at worst, could potentially lead to memory corruption and possible privilege escalation. However, the impact is significantly limited by the fact that root (or equivalent) permissions are required to exploit this flaw under default configurations (RedHat).

Exploitation requires local access to the system and high privileges (root access by default). The attacker would need to be able to access the floppy device and create multiple concurrent access attempts. The complexity of exploitation is considered high due to the specific conditions required to trigger the race condition (NVD).

Several mitigation options are available: 1) Prevent the floppy module from loading by adding 'install floppy /bin/true' to /etc/modprobe.d/disable-floppy.conf, 2) Disable the floppy in BIOS or virtual machine configuration, 3) Apply the kernel update that fixes the vulnerability. The fix was implemented in Linux kernel 4.5-rc5 (RedHat).