CVE-2022-50506

A NULL pointer dereference was found in the Linux kernel Distributed Replicated Block Device driver's I/O request handling. A local user with privileges to perform I/O operations on a DRBD block device can trigger I/O requests when the DRBD device is configured in diskless mode (no local backing device), causing the driver to dereference a NULL pointer when attempting to access the backing device structure, which results in denial of service through kernel crash.

The issue arises because of a code refactoring that moved device structure access earlier in the I/O request path without adding proper NULL validation. When drbd_req_new() processes a new I/O request, it attempts to clone the bio and access device->ldev->backing_bdev to set up the backing device information. However, when DRBD is configured in diskless mode (a valid configuration where DRBD acts purely as a network replication layer without local storage), the ldev pointer is NULL. The code lacks a check for this condition before dereferencing the pointer chain, causing an immediate kernel NULL pointer dereference. This occurs reliably whenever I/O is attempted on a diskless DRBD device. While this causes complete system failure through kernel panic, NULL pointer dereferences on modern kernels with memory protection mechanisms cannot be exploited for code execution or information disclosure, only denial of service.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

To mitigate this issue, prevent the drbd module from loading. See https://access.redhat.com/solutions/41278 for instructions on blacklisting kernel modules.