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CVE-2026-63938
KVM: SEV: Check PSC request indices against the actual size of the buffer
Published 2026-07-19 (3 months ago) by the Linux kernel CNA. Subsystem arch/x86. Debian urgency: unimportant.
What to do, per Debian release
Debian's own status for CVE-2026-63938 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 | not affected | 6.1.190-1 | Nothing to do. bookworm never shipped the vulnerable code. |
| Debian 13trixie | fixed | 6.12.94-1 | Update the kernel and reboot. Fixed in 6.12.94-1. 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
When processing Page State Change (PSC) requests, validate the PSC buffer against the effective size of the scratch area, which could be less than the maximum size if the guest provided a pointer that isn't exactly at the start of the GHCB shared buffer.
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
- 0.20%, higher than 8.6% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- unimportantsource
- 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 |
|---|---|---|
| 6.11 | 6.12.93 | 5198f70c09a5 |
| 6.11 | 6.18.35 | 75c8d1d72912 |
| 6.11 | 7.0.12 | 505a3b945355 |
| 6.11 | 7.1 | 121d88de56bc |
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 flaw is reached when malicious code inside an SEV-SNP guest issues a Page State Change VMGEXIT (SVM_VMGEXIT_PSC) through the GHCB, causing a VM exit handled locally by KVM on the host.
- AC:LAttack complexity, low
- The guest fully controls sw_scratch placement, PSC header indices, and entry contents; triggering the out-of-bounds access is deterministic and does not depend on races or conditions outside attacker control.
- PR:NPrivileges required, none
- Exploitation requires only the ability to run code in an SEV-SNP guest (e.g., a cloud confidential-VM tenant), not host root, CAP_SYS_ADMIN, or other elevated host privileges.
- UI:NUser interaction, none
- No victim user action is required beyond the attacker operating inside their own guest; the malicious VMGEXIT is self-contained.
- S:CScope, changed
- The vulnerability crosses the guest/host security boundary: a guest VM can corrupt host kernel memory via out-of-bounds access during PSC processing, enabling VM escape.
- C:HConfidentiality impact, high
- Out-of-bounds reads of guest_psc->entries[idx] via READ_ONCE() can read kernel memory beyond the effective scratch buffer, including data past the GHCB page mapping. Out-of-bounds writes in __snp_complete_one_psc() (guest_psc->entries[idx].cur_page) corrupt memory beyond the validated buffer, providing a memory corruption primitive exploitable for host code execution. Out-of-bounds reads of guest_psc->entries[idx] via READ_ONCE() can read kernel memory beyond the effective scratch buffer, including data past the GHCB page mapping.
- I:HIntegrity impact, high
- Out-of-bounds writes in __snp_complete_one_psc() (guest_psc->entries[idx].cur_page) corrupt memory beyond the validated buffer, providing a memory corruption primitive exploitable for host code execution.
- A:HAvailability impact, high
- Out-of-bounds access on the GHCB mapping can cause host kernel oops, panic, or BUG when indices extend past the mapped page boundary.