* [CVE-2026-98776][MODERATE 7.0] binderfs: fix UAF write in binder_add_device
@ 2026-10-19 10:36 21% AL-KERNEL
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From: AL-KERNEL @ 2026-10-19 10:36 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-98776
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: binderfs: fix UAF write in binder_add_device
Commit: 885bbbe52d31856768de439e9720f1ffa474a93a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=885bbbe52d31856768de439e9720f1ffa474a93a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98776
Analysis date: Mon, 19 Oct 2026 06:36:37 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES
Summary:
A repeatable binderfs creation and unlink race causes an 8-byte use-after-free list write into a freed device object and corrupts active global Binder device state, requiring manual review for unresolved slab-reuse and delegated-interface impact.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98776 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98776
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: MODERATE 7.0
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-2026-98776 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-98776 CHECK WITH IMPACT FROM ORIG NN IMPORTANT | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 LEGACY=MODERATE_KPANIC RESULT=SAME (final reconciliation agrees; DECISION: both 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-362;*CWE-672;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in binderfs device creation can cause a use-after-free write because simple_done_creating() releases the parent directory lock and the creator dentry reference before binder_add_device() registers the device. A concurrent unlinkat() can remove the new entry and trigger binderfs_evict_inode(), freeing the device before hlist_add_head writes list pointers into it. The demonstrated primitive is an 8 byte write into a freed kmalloc-512 object rather than a NULL pointer dereference or a warning only condition. For the CVSS the PR:L reflects that a local process needs access to the binderfs control interface and permission to create and unlink binderfs device entries, but full host administrator privileges may not be required when binderfs is delegated to a container or service. AC:H applies because reliable triggering requires winning a race window. The issue is not network reachable. Impact is at least a local denial of service through kernel memory corruption and a crash. In the worst still defensible case, allocator reuse could turn the stale list write into corruption of a replacement kernel object and potentially allow privilege escalation. Automated impact decision path. The original SimpleNet classifier selected IMPORTANT. The primary LLM estimated paranoid CVSS 7 and the production ActionableScore returned 5 with recommendation Actionable Moderate / Strong Important candidate requiring manual review. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage IMPORTANT to MODERATE_KPANIC because the CVSS score, vector or bug class threshold rule was applied. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were LOWERED_FROM_HIGH_BASED_ON_NULLPTR. The independent weaker Aegis shadow evaluated CVSS 4.2 and ActionableScore 4 with primitive class CLASS_B and manual review RECOMMENDED. Its independently reconciled impact was MODERATE_KPANIC. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5) MAYBE OOB DANGER UAF KERNEL_PANIC_PLUS_UAF HARDWARE LINUS KPANIC KPANIC LOWERED_FROM_HIGH_BASED_ON_NULLPTR - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
## 1. ActionableScore
* Raw conservative score: 4
* Conservative score: 4
* Raw paranoid score: 5
* Paranoid score: 5
* No floor or cap changed either score.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
## 2. Signal breakdown
Conservative signals:
* +2 Real UAF and lifetime violation. `binderfs_evict_inode()` frees the device before `binder_add_device()` uses the same pointer.
* +1 Reliable kernel crash or strong DoS. KASAN confirms an 8-byte post-free write into a `kmalloc-512` object. Without KASAN, this can produce slab or list corruption and a kernel fault.
* +1 Attacker-influenced lifetime manipulation. The triggering actor drives device creation and concurrent `unlinkat()`, controlling both sides of a repeatable create-versus-remove race.
* Total: 4.
Additional paranoid signal:
* +1 Structured stale-object leverage. `hlist_add_head()` writes linkage into the freed object and connects that address with the active global `binder_devices` list. This is more than a stale read or immediate invalid dereference, although controlled reclaim is not demonstrated.
* Total: 5.
Important non-awarded signals:
* No +2 strong exploitation leverage. Controlled reclaim, attacker-selected replacement contents, arbitrary write, callback control, and refcount takeover are not established.
* No confidentiality or integrity consequence points. Corruption is confirmed, but disclosure, protected-state modification, and privilege escalation are unresolved.
* No separate availability point. The same potential kernel crash is counted only once.
* No local-unprivileged bonus. The reproducer runs as UID 0, and the supplied evidence does not establish that an ordinary user can complete the binderfs mount, device creation, and unlink sequence.
* No privilege penalty. Host-admin-equivalent privilege is also not positively established.
* No race penalty. The attacker can repeatedly invoke both operations, and the evidence does not establish a one-shot or externally uncontrollable race.
* No repeated-release or refcount signal. The stale operation is a list insertion write, not a second `put`, `free`, or refcount mutation.
* B5-R does not apply because this is not a retry-accumulated OOB write.
* B3-PENDING and B4-PENDING do not apply because no shared-backing write or authorization decision bypass exists.
## 3. Reachability analysis
The trigger uses `binder_ctl_ioctl()` to create a binderfs device while another execution path removes the new entry with `unlinkat()`. The supplied trace proves UID 0 reachability, but numeric UID 0 alone does not resolve whether this was host root, user-namespace root, or a delegated service.
Binderfs can appear in containerized and delegated Android environments, but the supplied material does not establish the complete capability checks, mount ownership, directory permissions, or whether an attacker-created user namespace is sufficient for every stage. Therefore, both host-admin and namespace-scoped privilege requirements remain unknown.
No special hardware is required. The issue is local and not network-triggerable. Repeated race attempts appear practical because the actor can repeatedly create and remove binderfs device entries, so no hard-race deduction is justified.
## 4. Primitive and severity interpretation
The exact primitive is an 8-byte heap use-after-free write at offset zero of a freed `kmalloc-512` object. The object is active structured state rather than fixed stale storage. `hlist_add_head()` writes list pointers through the dangling device pointer and publishes that stale address into the global `binder_devices` list.
This is not cleanup-only behavior that merely dereferences a poisoned pointer. It mutates both freed storage and active list state. However, the supplied evidence does not show that the attacker can reclaim the slot with chosen contents, direct the write to an arbitrary address, or control a later callback. It is therefore CLASS_B rather than CLASS_A.
The demonstrated consequence is kernel memory corruption with realistic crash or DoS potential. Corruption of a replacement slab object and subsequent privileged list consumption are technically credible but unresolved exploitation mechanisms. The binderfs or container boundary is also credible but not proven below host admin.
Under the V33 active-state rule, the independent elements are:
1. Write: list-link fields are written through a freed binder device pointer.
2. Privileged consumer: kernel Binder code maintains and later consumes the global device list.
3. Boundary: binderfs may be exposed through a container or delegated control interface, but the complete privilege boundary is unresolved.
This supports `ImportantCandidate=YES`, mandatory review, and no automatic downgrade. It does not prove LPE, arbitrary kernel write, host escape, or Important severity.
## 5. One-sentence report phrase
A repeatable binderfs creation and unlink race causes an 8-byte use-after-free list write into a freed device object and corrupts active global Binder device state, requiring manual review for unresolved slab-reuse and delegated-interface impact.
## 6. Manual review recommendation
MANUAL CHECK REQUIRED
The patch establishes a real post-free heap write into active structured list state, while reclaim leverage and the binderfs privilege boundary remain unresolved.
## 7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: binderfs: fix UAF write in binder_add_device
Commit: 885bbbe52d31856768de439e9720f1ffa474a93a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=885bbbe52d31856768de439e9720f1ffa474a93a
Commit description:
binderfs_binder_device_create() publishes the new dentry with
d_make_persistent() and then calls simple_done_creating(), which drops
the parent directory lock and the creator's dentry reference. It then
calls binder_add_device() to register the device in the global
binder_devices list.
After simple_done_creating() releases the parent directory lock, a
concurrent unlinkat() can remove the new device entry. Dropping the
creator's dentry reference can then trigger binderfs_evict_inode(),
freeing the device. binder_add_device() later accesses the freed
object, causing UAF write.
Found by a modified Syzkaller:
BUG: KASAN: slab-use-after-free in hlist_add_head include/linux/list.h:1073 [inline]
BUG: KASAN: slab-use-after-free in binder_add_device+0xa9/0xc0 drivers/android/binder.c:7068
Write of size 8 at addr ffff88805b340c00 by task syz.1.532/11389
CPU: 0 UID: 0 PID: 11389 Comm: syz.1.532 Not tainted 7.2.0 #4 PREEMPT(full)
Hardware name: QEMU Ubuntu 24.04 PC v2 (i440FX + PIIX, arch_caps fix, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:94 [inline]
dump_stack_lvl+0x10e/0x1f0 lib/dump_stack.c:120
print_address_description mm/kasan/report.c:378 [inline]
print_report+0xf7/0x600 mm/kasan/report.c:482
kasan_report+0xe4/0x120 mm/kasan/report.c:595
hlist_add_head include/linux/list.h:1073 [inline]
binder_add_device+0xa9/0xc0 drivers/android/binder.c:7068
binderfs_binder_device_create.isra.0+0x724/0x990 drivers/android/binderfs.c:196
binder_ctl_ioctl+0x186/0x1b0 drivers/android/binderfs.c:241
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:597 [inline]
__se_sys_ioctl fs/ioctl.c:583 [inline]
__x64_sys_ioctl+0x18e/0x210 fs/ioctl.c:583
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x116/0x800 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7ff9027a833d
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b0 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ff903674018 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007ff902a35fa0 RCX: 00007ff9027a833d
RDX: 0000200000000500 RSI: 00000000c1086201 RDI: 0000000000000004
RBP: 00007ff902850733 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007ff902a36038 R14: 00007ff902a35fa0 R15: 00007ffd7166bfa0
</TASK>
Allocated by task 11389:
kasan_save_stack+0x33/0x60 mm/kasan/common.c:57
kasan_save_track+0x14/0x30 mm/kasan/common.c:78
poison_kmalloc_redzone mm/kasan/common.c:398 [inline]
__kasan_kmalloc+0xaa/0xb0 mm/kasan/common.c:415
kasan_kmalloc include/linux/kasan.h:263 [inline]
__kmalloc_cache_noprof+0x2e4/0x6f0 mm/slub.c:5489
_kmalloc_noprof include/linux/slab.h:988 [inline]
_kzalloc_noprof include/linux/slab.h:1309 [inline]
binderfs_binder_device_create.isra.0+0x17a/0x990 drivers/android/binderfs.c:148
binder_ctl_ioctl+0x186/0x1b0 drivers/android/binderfs.c:241
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:597 [inline]
__se_sys_ioctl fs/ioctl.c:583 [inline]
__x64_sys_ioctl+0x18e/0x210 fs/ioctl.c:583
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x116/0x800 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
Freed by task 11389:
kasan_save_stack+0x33/0x60 mm/kasan/common.c:57
kasan_save_track+0x14/0x30 mm/kasan/common.c:78
kasan_save_free_info+0x3b/0x60 mm/kasan/generic.c:584
poison_slab_object mm/kasan/common.c:253 [inline]
__kasan_slab_free+0x5f/0x80 mm/kasan/common.c:285
kasan_slab_free include/linux/kasan.h:235 [inline]
slab_free_hook mm/slub.c:2677 [inline]
slab_free mm/slub.c:6377 [inline]
kfree+0x2fc/0x6e0 mm/slub.c:6692
binderfs_evict_inode+0x1e8/0x260 drivers/android/binderfs.c:268
evict+0x3c2/0xad0 fs/inode.c:825
iput_final fs/inode.c:2019 [inline]
[Commit description truncated; see the upstream URL below]
Changed files:
include/linux/list.h
drivers/android/binder.c
lib/dump_stack.c
mm/kasan/report.c
drivers/android/binderfs.c
fs/ioctl.c
arch/x86/entry/syscall_64.c
mm/kasan/common.c
include/linux/kasan.h
mm/slub.c
include/linux/slab.h
mm/kasan/generic.c
fs/inode.c
fs/dcache.c
fs/namei.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=885bbbe52d31856768de439e9720f1ffa474a93a
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98776 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98776
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: 4
Paranoid ActionableScore: 5
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: MODERATE
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): MODERATE 7.0
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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