[CVE-2025-21756][IMPORTANT] vsock: Keep the binding until socket destruction

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

CVE: CVE-2025-21756
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: vsock: Keep the binding until socket destruction
Commit: 3f43540166128951cc1be7ab1ce6b7f05c670d8b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3f43540166128951cc1be7ab1ce6b7f05c670d8b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2025-21756
Analysis date: Mon, 12 Oct 2026 22:04:07 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2025-21756 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2025-21756

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2025-21756	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2025-21756 CHECK WITH IMPACT FROM ORIG NN IMPORTANT | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 LEGACY=IMPORTANT RESULT=DIFF INDEPENDENT_LEGACY=IMPORTANT TEMP_KPANIC_MERGE=IMPORTANT_WINS_DISAGREEMENT PRE_MERGE_LEGACY=IMPORTANT (independent Legacy and Aegis differ; FINAL: IMPORTANT, Aegis: MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-911;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A VSOCK socket lifetime error can remove a binding during transport reassignment even though the socket remains active. A later bind operation can drop the same reference again, reduce the socket reference count to zero, free the struct sock object inside __vsock_bind, and then continue accessing the freed object. KASAN confirms a slab use after free read, and the reported release path also produces a reference count underflow. For the CVSS the PR:L value is used because an unprivileged local process can normally create and operate AF_VSOCK sockets, while no administrative capability is shown as necessary for the socket operations themselves. AC:H is used because reliable triggering depends on a suitable multi transport configuration and a transport reassignment sequence. The issue is not directly reachable through ordinary IP network traffic, and the available code does not establish a remote guest to host trigger without local socket operations on the affected endpoint. Impact is at least local denial of service through memory corruption and kernel failure. In the worst defensible case, reuse of the freed socket object may permit confidentiality and integrity impact or local privilege escalation, so manual exploitability review is required.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)	MAYBE	READ OOB DANGER UAF HARDWARE LINUS KPANIC  TEMP_KPANIC_MERGE_IMPORTANT_WINS	-	-	checked

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

ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw conservative score: 5
* Conservative score: 5
* Raw paranoid score: 6
* Paranoid score: 6
* REAL UAF FLOOR 5 applies but does not modify the conservative score.
* No cap applies.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Conservative positive signals:

* +2 Real UAF and lifetime violation. The second `sock_put()` reduces the socket reference count to zero inside `__vsock_bind()`, frees the active `struct sock`, and execution then continues using that object. KASAN directly confirms the slab use-after-free.
* +1 Local unprivileged trigger. The trace and call path use ordinary `socket()`, `bind()`, and VSOCK operations without an identified capability or host-administrator check.
* +1 Reliable kernel crash or strong DoS. Continued access to a freed socket and later refcount underflow can produce an Oops or fatal kernel failure.
* +1 Concrete repeated release/refcount mutation. After `__vsock_remove_bound()` drops the final reference and frees the socket, the same socket reference remains reachable. Final release processing later reaches `vsock_remove_bound()` and performs another refcount-affecting operation, observed as refcount underflow.

Paranoid-only signal:

* +1 Repeated-refcount paranoid leverage. The later stale operation is not merely a read that immediately faults. It attempts a write-like refcount mutation through storage whose original socket lifetime has ended. This leaves unresolved post-free lifetime corruption leverage, although controlled reclaim is not demonstrated.

Repeated-release checks:

* The initial transport reassignment removal changes the refcount from 2 to 1 and does not itself free the socket in the documented trace.
* The subsequent removal in `__vsock_bind()` acts on the same binding reference, changes the refcount from 1 to 0, and destroys the socket.
* The socket reference survives that destruction and is used again by the bind path and later release processing.
* The later release path performs a real refcount mutation and reports underflow.
* Concrete repeated release/refcount +1 was awarded.
* Paranoid repeated-refcount leverage +1 was awarded because the stale operation itself mutates post-lifetime refcount storage.
* FLOOR 5 applies because the exact reference survives a lifetime-ending put and reaches another lifetime mutation.

Material signals not awarded:

* No controlled reclaim, attacker-shaped replacement object, refcount takeover, arbitrary write, callback control, or demonstrated LPE is established.
* Structured stale-object leverage was not separately awarded. The demonstrated additional leverage is already represented by the concrete repeated refcount mutation.
* Weak LPE concern was not separately awarded for the same refcount fact.
* Attacker-influenced lifetime manipulation was not separately awarded because it would overlap substantially with the bind, reassignment, and repeated-release sequence already scored.
* No extra Availability point was awarded because it would double-count the same kernel fault.
* Confidentiality and integrity points were not awarded merely because reclaimed memory could theoretically be corrupted.
* Remote reachability and guest-to-host exploitation are not established.
* B5-R does not apply because there is no retry-accumulated heap OOB write.
* B3-PENDING does not apply because there is no shared-backing write.
* B4-PENDING does not apply because no authorization decision is bypassed.

3. Reachability analysis

The supported attacker is an ordinary local process on an affected endpoint. The vulnerable sequence is reached through AF_VSOCK socket creation, transport selection or reassignment, and a later bind or release operation. These socket operations do not normally require UID 0 or a host-administrator capability, and the supplied trace shows a userspace process reaching the vulnerable system-call path.

An available multi-transport VSOCK environment is a prerequisite, but the evidence does not show that the attacking process must configure those transports or perform a privileged management operation. This is an environmental prerequisite rather than proof of host-admin privilege.

No namespace-scoped capability is required or relevant to the demonstrated path. There is no supported ordinary IP network trigger. The evidence also does not establish that a remote VSOCK peer or malicious guest can independently perform the complete sequence against the host kernel.

This is an ordered lifetime sequence, not a timing race. No hard-race penalty applies.

4. Primitive and severity interpretation

The exact primitive is an active structured socket-object UAF caused by incorrect binding ownership across transport reassignment. `__vsock_remove_bound()` drops the final socket reference inside `__vsock_bind()`, but the bind path continues accessing the freed `struct sock`. Subsequent final-release processing can perform another refcount operation through the stale socket and produce underflow.

This is category B, concrete repeated lifetime corruption, rather than an ordinary cleanup-only stale dereference. The lifetime-ending `sock_put()` is followed by continued structured socket use and a later lifetime/refcount mutation on the same stale object.

It remains CLASS_B rather than CLASS_A because controlled reclaim, directed corruption, arbitrary kernel read/write, callback influence, and privilege escalation are not demonstrated. The practical established consequence is local kernel DoS. The speculative consequence is stronger exploitation through reuse of the freed socket allocation.

ImportantCandidate is YES because the concrete post-free refcount mutation leaves meaningful Important-vs-Moderate exploitation leverage unresolved. AutoDowngradeAllowed is NO because automatically reducing this to an ordinary crash-only UAF would discard that concrete repeated lifetime mutation.

5. One-sentence report phrase

A local unprivileged process can trigger a VSOCK socket lifetime error that frees an active socket during bind, continues using it, and later mutates its stale refcount, causing kernel failure and requiring manual exploitation review.

6. Manual review recommendation

MANUAL CHECK REQUIRED

The confirmed active-object UAF and concrete post-free refcount mutation justify manual review even though controlled reclaim, arbitrary read/write, and privilege escalation have not been demonstrated.

7. Call-site confidence

Call-site confidence: high

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

Patch: vsock: Keep the binding until socket destruction
Commit: 3f43540166128951cc1be7ab1ce6b7f05c670d8b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3f43540166128951cc1be7ab1ce6b7f05c670d8b

Commit description:

Preserve sockets bindings; this includes both resulting from an explicit
bind() and those implicitly bound through autobind during connect().

Prevents socket unbinding during a transport reassignment, which fixes a
use-after-free:

    1. vsock_create() (refcnt=1) calls vsock_insert_unbound() (refcnt=2)
    2. transport->release() calls vsock_remove_bound() without checking if
       sk was bound and moved to bound list (refcnt=1)
    3. vsock_bind() assumes sk is in unbound list and before
       __vsock_insert_bound(vsock_bound_sockets()) calls
       __vsock_remove_bound() which does:
           list_del_init(&vsk->bound_table); // nop
           sock_put(&vsk->sk);               // refcnt=0

BUG: KASAN: slab-use-after-free in __vsock_bind+0x62e/0x730
Read of size 4 at addr ffff88816b46a74c by task a.out/2057
 dump_stack_lvl+0x68/0x90
 print_report+0x174/0x4f6
 kasan_report+0xb9/0x190
 __vsock_bind+0x62e/0x730
 vsock_bind+0x97/0xe0
 __sys_bind+0x154/0x1f0
 __x64_sys_bind+0x6e/0xb0
 do_syscall_64+0x93/0x1b0
 entry_SYSCALL_64_after_hwframe+0x76/0x7e

Allocated by task 2057:
 kasan_save_stack+0x1e/0x40
 kasan_save_track+0x10/0x30
 __kasan_slab_alloc+0x85/0x90
 kmem_cache_alloc_noprof+0x131/0x450
 sk_prot_alloc+0x5b/0x220
 sk_alloc+0x2c/0x870
 __vsock_create.constprop.0+0x2e/0xb60
 vsock_create+0xe4/0x420
 __sock_create+0x241/0x650
 __sys_socket+0xf2/0x1a0
 __x64_sys_socket+0x6e/0xb0
 do_syscall_64+0x93/0x1b0
 entry_SYSCALL_64_after_hwframe+0x76/0x7e

Freed by task 2057:
 kasan_save_stack+0x1e/0x40
 kasan_save_track+0x10/0x30
 kasan_save_free_info+0x37/0x60
 __kasan_slab_free+0x4b/0x70
 kmem_cache_free+0x1a1/0x590
 __sk_destruct+0x388/0x5a0
 __vsock_bind+0x5e1/0x730
 vsock_bind+0x97/0xe0
 __sys_bind+0x154/0x1f0
 __x64_sys_bind+0x6e/0xb0
 do_syscall_64+0x93/0x1b0
 entry_SYSCALL_64_after_hwframe+0x76/0x7e

refcount_t: addition on 0; use-after-free.
WARNING: CPU: 7 PID: 2057 at lib/refcount.c:25 refcount_warn_saturate+0xce/0x150
RIP: 0010:refcount_warn_saturate+0xce/0x150
 __vsock_bind+0x66d/0x730
 vsock_bind+0x97/0xe0
 __sys_bind+0x154/0x1f0
 __x64_sys_bind+0x6e/0xb0
 do_syscall_64+0x93/0x1b0
 entry_SYSCALL_64_after_hwframe+0x76/0x7e

refcount_t: underflow; use-after-free.
WARNING: CPU: 7 PID: 2057 at lib/refcount.c:28 refcount_warn_saturate+0xee/0x150
RIP: 0010:refcount_warn_saturate+0xee/0x150
 vsock_remove_bound+0x187/0x1e0
 __vsock_release+0x383/0x4a0
 vsock_release+0x90/0x120
 __sock_release+0xa3/0x250
 sock_close+0x14/0x20
 __fput+0x359/0xa80
 task_work_run+0x107/0x1d0
 do_exit+0x847/0x2560
 do_group_exit+0xb8/0x250
 __x64_sys_exit_group+0x3a/0x50
 x64_sys_call+0xfec/0x14f0

[Commit description truncated; see the upstream URL below]

Changed files:
  lib/refcount.c
  net/vmw_vsock/af_vsock.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=3f43540166128951cc1be7ab1ce6b7f05c670d8b

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

The original Linux kernel CVE announcement for CVE-2025-21756 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2025-21756

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: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7

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: 6

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).