[CVE-2026-53360][IMPORTANT] KVM: SEV: Require in-GHCB scratch area if GHCB v2+ is in use

[CVE-2026-53360][IMPORTANT] KVM: SEV: Require in-GHCB scratch area if GHCB v2+ is in use

[AL-KERNEL]

CVE: CVE-2026-53360
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: KVM: SEV: Require in-GHCB scratch area if GHCB v2+ is in use
Commit: bf9ba093fbb83c0c9a3dedd50efec29424eca2fc
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bf9ba093fbb83c0c9a3dedd50efec29424eca2fc
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53360
Analysis date: Sat, 04 Jul 2026 08:54:59 -0400
ActionableScore: 10
ActionableScore lower bound: 9
Actionable bucket: High-end Important / possible Critical candidate
Manual review required: YES

Summary:
A malicious SEV-SNP guest can abuse GHCB v2 scratch handling to force host KVM to process a guest-sized buffer as a larger PSC entry array, causing host kernel OOB reads, OOB writes, heap layout disclosure, heap corruption, and observed UAF conditions across the guest-to-host boundary.

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ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-53360 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53360

The original announcement does not normally provide a security severity
estimate, CVSS assessment, or enough information to determine whether the
reported kernel bug represents a practically relevant security issue.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin. It combines
an autonomous classifier with LLM-assisted technical analysis and a
separate ActionableScore mechanism.

The purpose of this report is to prioritize Linux kernel CVEs before
manual review, identify cases that require prompt investigation, and
support automatic closure of issues that are unlikely to have meaningful
security impact.

Published priority for this report: IMPORTANT
Manual review required: YES

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-53360	IMPORTANT	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H';*CWE-787;CWE-125;*CWE-416;CWE-122;BEST CVSS score: '8.8';DESCR 'A malicious SEV SNP guest can abuse GHCB v2 or later scratch handling so that KVM allocates a guest controlled scratch buffer outside the GHCB and later treats it as if it contained a full PSC entry array. By choosing a small exit_info_2 length and a larger PSC entry range, the guest can make the host read and write past the allocated kmalloc object. This provides OOB read, OOB write, host heap layout disclosure, heap corruption, and observed use after free reports in the host kernel. For the CVSS the PR:L is used because the attacker needs control of a guest VM or guest execution context, but does not need host privileges. The issue is not network reachable. Impact can include guest to host compromise risk, not only denial of service, because the primitive is host kernel memory corruption across the VM isolation boundary.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is High-end Important / possible Critical candidate (with actual score 10)	YES	READ OOB VIRT LEAK UAF HARDWARE  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=10
ActionableScoreLower=9

## 1. ActionableScore

* Conservative score: **9**
* Paranoid score: **10**
* Final recommended bucket: **High-end Important / possible Critical candidate**

## 2. Signal breakdown

* **Local unprivileged trigger: +1**. A malicious SEV-SNP guest can trigger the issue through VMGEXIT handling without host privileges.
* **Strong LPE / guest-to-host plausibility: +2**. The primitive crosses the guest-to-host isolation boundary and affects host kernel heap memory.
* **Memory corruption, strong primitive: +2**. The description explicitly shows OOB read, OOB write, heap corruption, and KASAN slab-out-of-bounds reports.
* **Real lifetime corruption: +1**. The report includes slab-use-after-free and use-after-free findings across VMGEXITs.
* **Reliable kernel crash / strong DoS: +1**. KASAN reports are reproducible from a PoC guest module, and host kernel corruption can plausibly crash the host.
* **Confidentiality impact plausible: +1**. OOB reads and error behavior can leak host heap layout information to the guest.
* **Integrity impact plausible: +1**. OOB writes into adjacent host slab objects are explicitly described.
* **Cross-boundary isolation bypass: +1**. The bug violates the SEV-SNP guest/host isolation model.
* **Practical exploit chain demonstrated or strongly implied: +1**. The input describes a concrete PoC pattern with controlled allocation length, controlled PSC range, repeated VMGEXITs, and KASAN evidence.
* **Rare/config-dependent subsystem: -1**. Exposure requires KVM SEV-SNP with GHCB v2+ support, which is not a default path on all systems.

Paranoid score reaches **10** because repeated attacker-controlled VMGEXITs, explicit OOB write, OOB read, and UAF reports make this more than a crash-only KVM bug.

Call-site confidence: **high**. The input includes the affected helper, the downstream PSC parsing behavior, concrete fields, and KASAN evidence.

## 3. Reachability analysis

The attacker is a malicious SEV-SNP guest or code running inside such a guest. No host root privileges are required once the attacker controls the guest execution context. This is not network reachable by itself, but it is highly relevant in multi-tenant virtualization where guests are controlled by less-trusted tenants.

Namespaces and containers do not materially reduce this issue, because the relevant boundary is VM guest to host, not ordinary Linux namespace isolation. The path is configuration-dependent and requires SEV-SNP/GHCB v2+, but in affected virtualization deployments the guest-controlled VMGEXIT path is realistic.

## 4. Severity interpretation

This behaves like an **Important to possible Critical** kernel vulnerability, not an ordinary Moderate issue. The realistic evidence already includes host heap OOB read/write and UAF reports, so the concern is not merely theoretical memory corruption. The main uncertainty is exploit reliability for full guest-to-host code execution, but the demonstrated primitives are strong enough that auto-closing as Moderate would be unsafe.

## 5. One-sentence report phrase

A malicious SEV-SNP guest can abuse GHCB v2 scratch handling to force host KVM to process a guest-sized buffer as a larger PSC entry array, causing host kernel OOB reads, OOB writes, heap layout disclosure, heap corruption, and observed UAF conditions across the guest-to-host boundary.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

This requires manual review because it is explicit host kernel memory corruption reachable from a guest boundary, with OOB read, OOB write, UAF evidence, and plausible guest-to-host compromise impact.

======================================================================
UPSTREAM PATCH SUMMARY
======================================================================

Patch: KVM: SEV: Require in-GHCB scratch area if GHCB v2+ is in use
Commit: bf9ba093fbb83c0c9a3dedd50efec29424eca2fc
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bf9ba093fbb83c0c9a3dedd50efec29424eca2fc

Commit description:

As per the GHCB spec, when using GHCB v2+ require the software scratch area
to reside in the GHCB's shared buffer.  Note, things like Page State Change
(PSC) requests _rely_ on this behavior, as the guest can't provide a length
when making the request, i.e. the size of the guest payload is bounded by
the size of the shared buffer.

Failure to force usage of the GHCB, and a slew of other flaws, lets a
malicious SNP guest corrupt host kernel heap memory, and leak host heap
layout information.

setup_vmgexit_scratch() allocates a buffer via kvzalloc(exit_info_2),
where exit_info_2 is guest-controlled. With exit_info_2=24, this yields
a 24-byte allocation in kmalloc-cg-32 (32-byte slab objects). The buffer
holds an 8-byte psc_hdr followed by 8-byte psc_entry structs, so only
entries[0] and entries[1] are in-bounds.

snp_begin_psc() validates end_entry against VMGEXIT_PSC_MAX_COUNT (253)
but NOT against the actual buffer size:

      idx_end = hdr->end_entry;

      if (idx_end >= VMGEXIT_PSC_MAX_COUNT) {   // checks 253, not buffer
          snp_complete_psc(svm, ...);
          return 1;
      }

      for (idx = idx_start; idx <= idx_end; idx++) {
          entry_start = entries[idx];           // OOB when idx >= 2

The guest sets end_entry=10+, causing the host to iterate entries[2+]
which are OOB into adjacent slab objects. For each OOB entry:

  - The host reads 8 bytes (OOB READ / info leak oracle)
  - If the data passes PSC validation, __snp_complete_one_psc() writes
    cur_page = 1 or 512 into the entry (OOB WRITE, sev.c:3806)
  - If validation fails, the error response reveals whether adjacent
    memory is zero vs non-zero (information disclosure to guest)

The guest controls allocation size (exit_info_2), entry range
(cur_entry/end_entry), and can fire unlimited VMGEXITs to repeatedly
hit different slab positions.

By exploiting the variety of bugs, a malicious SEV-SNP guest can:
    - OOB read adjacent kmalloc-cg-32 objects (heap layout disclosure)
    - OOB write cur_page bits into adjacent objects (heap corruption)
    - Trigger use-after-free conditions across VMGEXITs

E.g. with KASAN enabled, a single insmod of the PoC guest module
produces 73 KASAN reports:

    BUG: KASAN: slab-out-of-bounds in snp_begin_psc+0x126/0x890
    Read of size 8 at addr ffff888219ffb5e0 by task qemu-system-x86/2199

    BUG: KASAN: slab-out-of-bounds in snp_begin_psc+0x468/0x890
    Write of size 8 at addr ffff888351566648 by task qemu-system-x86/2199

    The buggy address belongs to the object at ffff888XXXXXXXXX
     which belongs to the cache kmalloc-cg-32 of size 32
    The buggy address is located N bytes to the right of
     allocated 32-byte region [ffff888XXXXXXXXX, ffff888XXXXXXXXX)

  Breakdown:
    62 slab-out-of-bounds (reads + writes past allocation)
     7 slab-use-after-free
     4 use-after-free

All credit to Stan for the wonderful description and reproducer!

Reported-by: Stan Shaw <shawstan96@gmail.com>
Cc: Michael Roth <michael.roth@amd.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Peter Gonda <pgonda@google.com>
Cc: Jacky Li <jackyli@google.com>
Fixes: 4af663c ("KVM: SEV: Allow per-guest configuration of GHCB protocol version")
Cc: stable@vger.kernel.org
Signed-off-by: Michael Roth <michael.roth@amd.com>
[sean: write changelog]
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20260501202250.2115252-2-seanjc@google.com>

[Commit description truncated; see the upstream URL below]

Changed files:
  arch/x86/kvm/svm/sev.c

Diff excerpt:

Not included in this email. See the upstream URL for the full patch.

Full patch:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bf9ba093fbb83c0c9a3dedd50efec29424eca2fc

======================================================================
DETAILED REPORT METHODOLOGY
======================================================================

The original Linux kernel CVE announcement for CVE-2026-53360 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53360

The original CVE announcement normally does not include a security-level
estimate. In particular, it may not contain a CVSS assessment, an impact
level, or enough information to determine whether the reported bug is a
practically relevant security issue. One purpose of this parallel CVE list
is to provide that missing technical and prioritization information.

The original goal of the AL-KERNEL project was to prioritize Linux kernel
CVE analysis automatically before manual review. The system can also help
identify non-security issues that may be suitable for automatic closure.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin.

The first analysis stage combines an autonomous classifier with additional
LLM-based analysis. The autonomous classifier runs locally on a CPU and is
based on a backpropagation neural network. Together, these mechanisms
produce a technical vulnerability description, identify likely weakness
types, estimate CVSS severity, and provide input for ActionableScore.

Two CVSS estimates are retained because incomplete kernel vulnerability
information often permits more than one defensible interpretation:

  Conservative CVSS vector: Not available
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H
  The Best / paranoid CVSS score: 8.8

The conservative vector represents a lower-impact interpretation.
The Best/paranoid vector intentionally represents a plausible upper-bound
interpretation and should not automatically be treated as demonstrated
real-world impact.

CVSS may also need to be adjusted for a particular Linux deployment,
because actual reachability, privileges, enabled kernel configuration,
hardware, namespaces, exposed device nodes, and other environmental
conditions can differ significantly between systems.

A separate ActionableScore mechanism evaluates practical remediation
urgency. Its analysis may include reachability, attack prerequisites,
subsystem exposure, memory-corruption characteristics, denial-of-service
reliability, and possible confidentiality, integrity, or
privilege-escalation impact.

  Conservative ActionableScore: 9
  Paranoid ActionableScore: 10

The final base severity is taken directly from the second tab-separated
field of the AL-KERNEL classification result. ActionableScore does not
replace or independently override that final AL-KERNEL decision, and
if ActionableScore adjusted impact level of ALKERNEL, then you would see
self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7.

For an AL-KERNEL result of MODERATE, this report uses the following
additional presentation split:

  ActionableScore below 5   -> MODERATE REGULAR
  ActionableScore 5 or more -> MODERATE 7.0

The distinction between MODERATE REGULAR and MODERATE 7.0 makes it
possible to identify Moderate issues that should receive manual analysis
and fixes before lower-priority MODERATE REGULAR issues. In many cases,
MODERATE REGULAR fixes may wait for a later rebase or routine update.

There is one override in which MODERATE REGULAR becomes MODERATE 7.0
even when the ActionableScore is below 5. When the AL-KERNEL result
contains the KPANIC flag, a MODERATE result is always presented as
MODERATE 7.0. The KPANIC flag selected with few regexps without
usage of AI at all, so it helps to detect cases when Kernel Crash happens
and similar (to filter False-Negative results from the LLM usage).

KPANIC indicates that a reliable kernel crash, kernel panic, or similarly
serious kernel availability impact was identified by the classification
workflow.

AL-KERNEL base severity for this report: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

These results are intended to support engineering triage. They are
machine-generated estimates, and cases marked for manual review should
be validated by a human security engineer before final disposition.
For more info read docs linked from here: https://kernelcve.org/
(and you can submit you own patch there to generate such a report
for non-existant CVE-id yet).

Note that in many cases this AI tool selects higher severity, than
real is (means you can expect Importants instead of Moderate 7.0 or
Moderates 7.0 instead of regular Moderates). If you see such cases,
please use reply email interface to add additional manual analyses
info to this particular CVE.
And please, please, let me know when you see Lows instead of Importants
or Important instead of Low (because particular for such cases I
need to tune this AI tool to make it better for this one and next similar).
My contact email for such notifications is alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).