CVE-2025-39726

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

s390/ism: fix concurrency management in ism_cmd()

The s390x ISM device data sheet clearly states that only one

request-response sequence is allowable per ISM function at any point in

time. Unfortunately as of today the s390/ism driver in Linux does not

honor that requirement. This patch aims to rectify that.

This problem was discovered based on Aliaksei's bug report which states

that for certain workloads the ISM functions end up entering error state

(with PEC 2 as seen from the logs) after a while and as a consequence

connections handled by the respective function break, and for future

connection requests the ISM device is not considered -- given it is in a

dysfunctional state. During further debugging PEC 3A was observed as

well.

A kernel message like

[ 1211.244319] zpci: 061a:00:00.0: Event 0x2 reports an error for PCI function 0x61a

is a reliable indicator of the stated function entering error state

with PEC 2. Let me also point out that a kernel message like

[ 1211.244325] zpci: 061a:00:00.0: The ism driver bound to the device does not support error recovery

is a reliable indicator that the ISM function won't be auto-recovered

because the ISM driver currently lacks support for it.

On a technical level, without this synchronization, commands (inputs to

the FW) may be partially or fully overwritten (corrupted) by another CPU

trying to issue commands on the same function. There is hard evidence that

this can lead to DMB token values being used as DMB IOVAs, leading to

PEC 2 PCI events indicating invalid DMA. But this is only one of the

failure modes imaginable. In theory even completely losing one command

and executing another one twice and then trying to interpret the outputs

as if the command we intended to execute was actually executed and not

the other one is also possible. Frankly, I don't feel confident about

providing an exhaustive list of possible consequences.