CVE-2022-50307

In the Linux kernel, the following vulnerability has been resolved:

s390/cio: fix out-of-bounds access on cio_ignore free

The channel-subsystem-driver scans for newly available devices whenever device-IDs are removed from the cio_ignore list using a command such as:

echo free >/proc/cio_ignore

Since an I/O device scan might interfer with running I/Os, commit 172da89ed0ea ("s390/cio: avoid excessive path-verification requests") introduced an optimization to exclude online devices from the scan.

The newly added check for online devices incorrectly assumes that an I/O-subchannel's drvdata points to a struct io_subchannel_private. For devices that are bound to a non-default I/O subchannel driver, such as the vfio_ccw driver, this results in an out-of-bounds read access during each scan.

Fix this by changing the scan logic to rely on a driver-independent online indication. For this we can use struct subchannel->config.ena, which is the driver's requested subchannel-enabled state. Since I/Os can only be started on enabled subchannels, this matches the intent of the original optimization of not scanning devices where I/O might be running.

An out-of-bounds read could occur in the s390/cio subsystem when removing device IDs from the cio_ignore list (e.g. echo free > /proc/cio_ignore). The previous logic incorrectly assumed driver-specific data layout, which caused invalid memory accesses if non-default I/O subchannel drivers (e.g. vfio_ccw) were in use. The bug could lead to kernel crashes on s390 systems, but does not appear to enable privilege escalation or data leaks.

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.