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CVE-2026-80554
s390/vfio_ccw: Limit the number of channel program segments
Published 2026-08-26 (2 months ago) by the Linux kernel CNA. Subsystem drivers/s390. Debian urgency: not yet assigned.
Debian does not build this code. The per-release build detail is under "Where it lives" below.
What to do, per Debian release
Debian's own status for CVE-2026-80554 in every release this site tracks, and what each one means in practice. Every status below is the one the Debian Security Team publishes for this CVE. source
| Release | Status | Version | What this means |
|---|---|---|---|
| Debian 12bookworm | vulnerable | 6.1.190-1 | No fix published anywhere yet, neither in Debian nor upstream for the series bookworm tracks. Watch the Debian tracker page for this CVE. Advisories for bookworm have historically landed a median of 23 days after publication. |
| Debian 13trixie | fixed | 6.12.105-1 | Update the kernel and reboot. Fixed in 6.12.105-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.1.10-1 | Update the kernel and reboot. Fixed in 7.1.10-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.1.10-1 | Update the kernel and reboot. Fixed in 7.1.10-1. A running kernel keeps the old code until the machine restarts. |
| Debian 12bookworm · linux-6.12 | fixedDLA-4788-1 · 2026-09-19 · text | 6.12.107-1~deb12u1 | Update the kernel and reboot. DLA-4788-1 shipped the fix on 2026-09-19, in 6.12.107-1~deb12u1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
The processing of channel programs, and the CCWs within them, is done recursively. As such, there is an arbitrary (but not architectural) limit to the number of CCWs that can exist in a single channel program. The vfio-ccw logic breaks these channel programs into segments whenever it encounters a Transfer-In-Channel (TIC) CCW, and the combined number of segments count towards the global limit. Impose an equivalent limit to the number of segments until such logic can be made non-recursive.
Where it lives
The fix touches these files. If the subsystem is one you do not use, the practical exposure is lower, though the package is still the vulnerable one.
drivers/s390/cio/vfio_ccw_cp.c
drivers/s390/cio/vfio_ccw_cp.h
- What this is
- S390 VFIO-CCW DRIVERMAINTAINERS section matched on drivers/s390/cio/vfio_ccw* · linux-s390@vger.kernel.org · Supported
Check it on the machine
grep -wE 'CONFIG_S390|CONFIG_VFIO_CCW' /boot/config-$(uname -r)Reads the booted kernel's own configuration, on the machine and nowhere else. =y is built in, =m is a module, and an is not set line means the code is not there.
Show whether Debian builds this, per release
Debian does not build this code. In 5 releases below, every kernel flavour Debian publishes has the switch off, so the vulnerable file is compiled into no kernel binary Debian ships.
- bookworm, trixie, forky, sid, bookworm · linux-6.12n
- n
- not enabled on any flavour - this release does not build the code
- Build switches
- CONFIG_S390, CONFIG_VFIO_CCW
- Reads as
- all on means the file is compiled; any one off means it is not.
From the configuration Debian publishes for every kernel flavour it builds. It covers only the architectures Debian ships a linux-config package for, and nothing about a kernel you built yourself. Debian's own status above stays authoritative about the source package.MAINTAINERS kbuild Makefiles Debian linux-config packages
Signals
- Published
- 2026-08-26 (2 months ago)source
- CVSS
- 9.3 critical
CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:Hsource recompute it - Attack vector
- local (AV:L)source
- EPSS
- 0.20%, higher than 8.6% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- drivers/s390
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 4.12 | 6.6.153 | 15fb4559a7fd |
| 4.12 | 6.12.105 | a1625f66eaa1 |
| 4.12 | 6.18.46 | 06f4d6e5a8af |
| 4.12 | 7.1.10 | 4ee94790490a |
| 4.12 | 7.2 | 5405c90d6a47 |
Debian advisories
| Advisory | Date | Shipped to | Debian |
|---|---|---|---|
| DLA-4788-1 | 2026-09-19 | bookworm 6.12.107-1~deb12u1 | tracker |
Why the CNA scored it this way
Quoted from the kernel CNA's own scoring file, word for word. A highlighted metric carries the most severe value CVSS v3.1 defines for it, which is what pushes the score up. source
- AV:LAttack vector, local
- Reached only via local VFIO-ccw mediated-device write()/ioctl on the I/O region (QEMU relaying guest SSCH/channel programs into fsm_io_request→cp_init); no network, adjacent-radio, or physical-bus handler processes guest channel programs.
- AC:LAttack complexity, low
- Attacker fully controls guest channel program layout and can chain many unique TIC targets so ccwchain_handle_ccw()/ccwchain_loop_tic() recurse without bound until host kernel stack exhaustion; no race, timing, or uncontrollable memory layout is required.
- PR:NPrivileges required, none
- A malicious VM guest with an assigned vfio-ccw mediated subchannel needs no host credentials or capabilities; it only issues guest I/O that QEMU forwards, requiring none of the administrator privileges used to create the mdev.
- UI:NUser interaction, none
- Exploitation requires only the attacker's own guest channel program submission through normal passthrough I/O; no separate victim mount, click, or user action is needed once the device is assigned.
- S:CScope, changed
- Guest-supplied channel programs are parsed recursively in host kernel cp_init(), so stack corruption crosses the guest-to-host VFIO device-passthrough boundary and can compromise the hypervisor and co-resident VMs.
- C:HConfidentiality impact, high
- Unbounded recursion corrupts the host kernel stack frame, enabling disclosure of adjacent stack/kernel memory and supporting further exploitation beyond a mere crash.
- I:HIntegrity impact, high
- Stack overflow in ccwchain_handle_ccw() yields attacker-influenced host kernel stack corruption plausibly enabling control-flow hijacking and arbitrary host kernel code execution.
- A:HAvailability impact, high
- Deep recursion reliably causes host kernel stack exhaustion, oops, or panic, crashing the hypervisor and all VMs on the system at guest will.