[SRU][J][PATCH 0/1] CVE-2025-39726

[Cengiz Can]

https://ubuntu.com/security/CVE-2025-39726

[ Impact ]

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.

[ Fix ]

jammy/linux: backported from 897e8601b9cf

The fix serializes command submission in ism_cmd() by introducing a per-device
cmd_lock spinlock so that only one request-response sequence is in flight for a
given ISM function at any time.

In this tree the include/linux/ism.h header does not exist; struct ism_dev
instead lives in drivers/s390/net/ism.h, so the new cmd_lock member was added
there and the automatically created include/linux/ism.h was dropped from the
backport.

[ Test Plan ]

Build and boot tested.

[ Where Problems Could Occur ]

A bad fix would primarily affect s390x systems that use ISM devices, that is,
IBM Z machines running SMC-D over Internal Shared Memory; incorrect locking
could serialize command submission too aggressively and hurt SMC-D connection
setup and throughput, or in the worst case reintroduce command corruption. All
other architectures are unaffected, as is s390x hardware that does not have ISM
functions configured or that does not use SMC-D.

[ Other Info ]

Kybele flow-v12-16-g3521c8c5. Reference: 3cd24883/v1