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CVE-2026-63937
KVM: SEV: Use READ_ONCE() when reading entries/indices from PSC 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-2026-63937 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
Use READ_ONCE() when reading entries/indices from the guest-accessible Page State Change buffer to defend against TOCTOU bugs. Don't bother with READ_ONCE()/WRITE_ONCE() for cases where KVM is writing (and not consuming the result!), as the guest isn't supposed to touch the buffer while it's being processed. I.e. using READ_ONCE() is all about protecting against misbehaving guests.
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
- 8.8 high
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:Hsource recompute it - Attack vector
- local (AV:L)source
- EPSS
- 0.18%, higher than 6.5% 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 | bd232801ef1d |
| 6.11 | 6.18.35 | b1dfaa6f7a95 |
| 6.11 | 7.0.12 | edbbe88f83b5 |
| 6.11 | 7.1 | c8cc238093ca |
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
- Exploitation requires code execution inside an SEV-SNP guest that issues VMGEXIT PSC events into the host KVM hypervisor; this is local access to the host kernel via a guest-controlled hypercall path, not a remote network protocol.
- AC:LAttack complexity, low
- The attacker fully controls the guest-accessible PSC buffer and can trigger concurrent VMGEXITs or use multiple vCPUs to modify indices/entries between KVM's validation read and later use, reliably winning the TOCTOU race without depending on uncontrollable host conditions.
- PR:LPrivileges required, low
- The attacker only needs the ability to run code in an SEV-SNP guest on a KVM host (e.g., a cloud confidential-computing tenant), not real root or capabilities in the host's init namespace.
- UI:NUser interaction, none
- No victim interaction is required beyond the attacker operating their own malicious guest workload; PSC VMGEXITs are triggered programmatically from guest code.
- S:CScope, changed
- The vulnerable code runs in the host hypervisor but is reached from an untrusted guest; successful exploitation crosses the VM security boundary and can alter host-managed SEV-SNP page-state (RMP) for guest physical addresses.
- C:HConfidentiality impact, high
- TOCTOU manipulation of PSC entries/indices can bypass validation and drive KVM_HC_MAP_GPA_RANGE on unvalidated GPA ranges, potentially flipping pages between private and shared and disclosing encrypted guest (or host-visible) memory beyond what was checked.
- I:HIntegrity impact, high
- The same TOCTOU allows inflating combined page ranges and operations passed to userspace, enabling unauthorized modification of page encryption attributes and corrupting hypervisor-managed memory state via misdirected MAP_GPA_RANGE processing.
- A:HAvailability impact, high
- Incorrect indices and out-of-sync buffer state can cause invalid hypervisor processing, kernel warnings/panics, or repeated disruption of the host's KVM/SEV-SNP path; memory-corruption-class TOCTOU bugs are scored at full availability impact when exploitability is uncertain.