[CVE-2026-63794][IMPORTANT] KVM: SVM: Fix page overflow in sev_dbg_crypt() for ENCRYPT path

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-63794
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: KVM: SVM: Fix page overflow in sev_dbg_crypt() for ENCRYPT path
Commit: f701ae476cb92a3a3d8844bb39bb63b4512684c8
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f701ae476cb92a3a3d8844bb39bb63b4512684c8
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63794
Analysis date: Sun, 19 Jul 2026 09:43:56 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A page-offset bounds bug in KVM AMD SEV `sev_dbg_crypt()` can let a local process with KVM/SEV ioctl access overflow a single-page kernel buffer in the ENCRYPT path, causing a host kernel crash and requiring manual review for possible local privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-63794 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63794

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-2026-63794	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-787;CWE-122;*CWE-20;BEST CVSS score: '7.8';DESCR 'A page overflow in KVM SVM SEV sev_dbg_crypt can occur in the ENCRYPT path because the per iteration length is bounded by the source page offset but not by the destination page offset. When d_off is greater than s_off, the single page dst_tpage buffer can be overrun by the PSP output and by the following copy into page_address(dst_tpage) plus the destination alignment offset. This is kernel memory corruption rather than a pure warning or resource leak, and KASAN reports a slab-use-after-free style bad access in sev_dbg_crypt. For the CVSS the PR:L is used because triggering requires local access to KVM/SEV ioctls such as a process that can open /dev/kvm and operate the affected VM. The issue is not network reachable. Impact is at least local denial of service via kernel crash and may allow privilege escalation due to an out of bounds kernel write. YES REQUIRES MANUAL CHECK.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	OOB DANGER VIRT UAF HARDWARE LINUS KPANIC MEMORY  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	NO	NO	checked

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

ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged or low-privileged trigger: +1. Triggering is through KVM VM ioctls, typically requiring access to `/dev/kvm`, not full host root.
* Constrained kernel buffer overwrite: +1. The overflow is into/around a single-page temporary buffer and is limited by page-offset arithmetic.
* Weak LPE concern: +1. The bug is a concrete kernel OOB write, but target selection and reliable object control are not demonstrated.
* Reliable kernel crash / strong DoS: +1. KASAN shows a concrete bad write in `sev_dbg_crypt()` from the ioctl path.
* Memory layout invariant restoration: +1. The patch restores consistency between transfer length and both source and destination page offsets.
* Privileged kernel/device-management corruption path: +1. The corruption occurs in the KVM AMD SEV debug encryption path, a highly trusted virtualization context.
* Corruption primitive highly constrained: -1. The overflow is small, up to roughly alignment-sized spillover, and the adjacent target is not shown to be attacker-selected.
* Rare/configuration-dependent exposure: -1. The path requires AMD SEV/KVM support and use of SEV debug encrypt/decrypt functionality.

Paranoid additional interpretation:

* Memory corruption, strong corruption primitive: +2 instead of constrained-only treatment. The patch and KASAN trace show an OOB kernel write, including a `copy_from_user()` component in the ENCRYPT path.
* Privilege escalation plausible: +2. Local kernel OOB writes in a KVM ioctl path are plausible LPE candidates even without a published reliable exploit.
* Confidentiality and integrity impact plausible: +1/+1 in the paranoid view due to possible kernel memory corruption consequences.

## 3. Reachability analysis

The bug is locally reachable through the KVM AMD SEV memory encryption ioctl path, specifically `sev_mem_enc_ioctl()` through `kvm_vm_ioctl()`. A realistic attacker needs the ability to create or control an affected KVM VM and issue SEV debug encrypt operations, which usually means access to `/dev/kvm` and suitable SEV hardware support. This is not network reachable. Namespaces do not automatically make this unprivileged, but container or service configurations that delegate `/dev/kvm` access can reduce the practical privilege barrier. The path is hardware and configuration dependent, so exposure is narrower than generic KVM, but it is still security-relevant in virtualization deployments.

Call-site confidence: high. The provided trace and patch show the relevant ioctl call path and the exact overflow location.

## 4. Severity interpretation

This is not an ordinary Moderate cleanup bug. The patch fixes a concrete page-boundary overflow in kernel memory, and KASAN demonstrates an actual bad write in the KVM AMD SEV path. Conservative handling is Actionable Moderate because the primitive is constrained and configuration-dependent. Paranoid handling is Strong Important candidate because the issue is a local kernel OOB write in a virtualization subsystem and may plausibly support privilege escalation with further exploit development.

## 5. One-sentence report phrase

A page-offset bounds bug in KVM AMD SEV `sev_dbg_crypt()` can let a local process with KVM/SEV ioctl access overflow a single-page kernel buffer in the ENCRYPT path, causing a host kernel crash and requiring manual review for possible local privilege escalation.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is concrete kernel memory corruption in a KVM ioctl path, with KASAN evidence and plausible LPE relevance despite constrained overwrite size and SEV-specific reachability.

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

Patch: KVM: SVM: Fix page overflow in sev_dbg_crypt() for ENCRYPT path
Commit: f701ae476cb92a3a3d8844bb39bb63b4512684c8
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f701ae476cb92a3a3d8844bb39bb63b4512684c8

Commit description:

In sev_dbg_crypt(), the per-iteration transfer length is bounded by
the source page offset (PAGE_SIZE - s_off) but not by the destination
page offset (PAGE_SIZE - d_off).  When d_off > s_off, the encrypt
path (__sev_dbg_encrypt_user) performs a read-modify-write using a
single-page intermediate buffer (dst_tpage):

  1. __sev_dbg_decrypt() expands the size to round_up(len + (d_off & 15), 16)
     before issuing the PSP command.  If len + (d_off & 15) > PAGE_SIZE,
     the PSP writes beyond the end of the 4096-byte dst_tpage allocation.

  2. The subsequent memcpy()/copy_from_user() into
     page_address(dst_tpage) + (d_off & 15) of 'len' bytes overflows
     by up to 15 bytes under the same condition.

Trigger example: s_off = 0, d_off = 1, debug.len = PAGE_SIZE -
the PSP is instructed to write round_up(4097, 16) = 4112 bytes to
a 4096-byte buffer.

Fix by also bounding len by (PAGE_SIZE - d_off), the same check that
sev_send_update_data() already performs for its single-page guest
region.

 ==================================================================
 BUG: KASAN: slab-use-after-free in sev_dbg_crypt+0x993/0xd10 [kvm_amd]
 Write of size 4095 at addr ff110062293bb009 by task sev_dbg_test/228214

 CPU: 96 UID: 0 PID: 228214 Comm: sev_dbg_test Tainted: G     U  W           7.0.0-smp--5ce9b0c48211-dbg #156 PREEMPTLAZY
 Tainted: [U]=USER, [W]=WARN
 Hardware name: Google Astoria/astoria, BIOS 0.20250817.1-0 08/25/2025
 Call Trace:
  <TASK>
  dump_stack_lvl+0x54/0x70
  print_report+0xbc/0x260
  kasan_report+0xa2/0xd0
  kasan_check_range+0x25f/0x2c0
  __asan_memcpy+0x40/0x70
  sev_dbg_crypt+0x993/0xd10 [kvm_amd]
  sev_mem_enc_ioctl+0x33c/0x450 [kvm_amd]
  kvm_vm_ioctl+0x65d/0x6d0 [kvm]
  __se_sys_ioctl+0xb2/0x100
  do_syscall_64+0xe8/0x870
  entry_SYSCALL_64_after_hwframe+0x4b/0x53
  </TASK>

 The buggy address belongs to the physical page:
 page: refcount:1 mapcount:0 mapping:0000000000000000 index:0x7fe72b6a0 pfn:0x62293bb
 memcg:ff11000112827d82
 flags: 0x1400000000000000(node=1|zone=1)
 raw: 1400000000000000 0000000000000000 dead000000000122 0000000000000000
 raw: 00000007fe72b6a0 0000000000000000 00000001ffffffff ff11000112827d82
 page dumped because: kasan: bad access detected

 Memory state around the buggy address:
  ff110062293bbf00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  ff110062293bbf80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 >ff110062293bc000: fa fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
                    ^
  ff110062293bc080: fa fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
  ff110062293bc100: fa fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
 ==================================================================
 Disabling lock debugging due to kernel taint

Fixes: 24f41fb ("KVM: SVM: Add support for SEV DEBUG_DECRYPT command")
Fixes: 7d1594f ("KVM: SVM: Add support for SEV DEBUG_ENCRYPT command")
Cc: stable@vger.kernel.org
Signed-off-by: Ashutosh Desai <ashutoshdesai993@gmail.com>
[sean: add sample KASAN splat, Fixes, and stable@]
Link: https://patch.msgid.link/20260501203537.2120074-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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=f701ae476cb92a3a3d8844bb39bb63b4512684c8

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

The original Linux kernel CVE announcement for CVE-2026-63794 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63794

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:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.8

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: 5
  Paranoid ActionableScore: 7

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).