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CVE-2026-63940
KVM: SEV: Ignore Port I/O requests of length '0'
Published 2026-07-19 (3 months ago) by the Linux kernel CNA. Subsystem arch/x86. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-63940 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 | 6.12.95-1 | Update the kernel and reboot. Fixed in 6.12.95-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.0.12-1 | Update the kernel and reboot. Fixed in 7.0.12-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.0.12-1 | Update the kernel and reboot. Fixed in 7.0.12-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
Explicitly ignore Port I/O requests of length '0' (or count '0'), so that setting up the software scratch area (and other code) doesn't have to worry about underflowing the length, and to allow for WARNing on trying to configure the scratch area with len==0.
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.
arch/x86/kvm/svm/sev.c
- What this is
- KERNEL VIRTUAL MACHINE FOR X86 (KVM/x86)MAINTAINERS section matched on arch/x86/kvm/*/ · kvm@vger.kernel.org · Supported
Show whether Debian builds this, per release
Not established. This site could not find the build rule for the affected file in the kernel trees Debian ships, so it has no answer about whether Debian compiles this code. Read that as unknown, never as absent.
- bookworm, trixie, forky, sid?
- ?
- not established - no Makefile entry for the affected file
Where a release above reads "not established", no build rule for the affected file was found in that release's kernel tree. That is a gap in this lookup, not a finding that the code is absent.
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
Signals
- Published
- 2026-07-19 (3 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
- 0.18%, higher than 7.6% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- arch/x86
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 5.11 | 6.12.95 | 3b6035bc6bff |
| 5.11 | 6.18.35 | 2254972d4d69 |
| 5.11 | 7.0.12 | c30cde934c78 |
| 5.11 | 7.1 | 3988bd2723de |
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
- The bug is reached when a SEV-ES/SEV-SNP guest triggers a #VMGEXIT with a crafted GHCB Port I/O request; exploitation requires executing code inside a guest VM on the host, not remote network delivery.
- AC:LAttack complexity, low
- A guest can reliably set GHCB exit_info_1 size to 0 (or count to 0) and issue string Port I/O via VMGEXIT without races or special host state; success does not depend on conditions outside attacker control.
- PR:NPrivileges required, none
- The attacker needs no privileges on the host kernel—only code execution inside an SEV-ES guest VM, which is the normal capability of a cloud confidential-VM tenant with no elevated host access.
- UI:NUser interaction, none
- Exploitation is fully attacker-driven from guest code via VMGEXIT; no host administrator or other user action is required.
- S:CScope, changed
- The vulnerable component is the host KVM hypervisor, but exploitation is initiated from a guest VM and impacts host kernel integrity/availability, crossing the guest/host security boundary.
- C:HConfidentiality impact, high
- Allowing zero-length scratch setup sets ghcb_sa_len to 0, enabling unsigned length underflow in scratch-area size arithmetic; combined with ZERO_SIZE_PTR from kvzalloc(0), this is a memory-safety defect class that can yield out-of-bounds read primitives.
- I:HIntegrity impact, high
- Zero-length scratch configuration corrupts length tracking used by subsequent GHCB scratch handling, creating integer-underflow conditions that can lead to out-of-bounds writes; the divide-by-zero path also demonstrates uncontrolled host kernel failure from guest input.
- A:HAvailability impact, high
- A malicious guest can reliably panic the host kernel via divide-by-zero in kvm_sev_es_string_io() when size is 0 and count is non-zero, causing complete loss of hypervisor availability for all co-hosted workloads.