* [CVE-2026-89793][MODERATE 7.0] ublk: clear VM_MAYWRITE on read-only ublk char device mmap
@ 2026-09-16 17:53 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-16 17:53 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89793
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ublk: clear VM_MAYWRITE on read-only ublk char device mmap
Commit: be41733c24be58e2a1ef718c80fafdfb98a40d5e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=be41733c24be58e2a1ef718c80fafdfb98a40d5e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89793
Analysis date: Wed, 16 Sep 2026 13:53:04 -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-2026-89793 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89793
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-2026-89793 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89793 CHECK WITH IMPACT FROM ORIG NN LOW 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-732;CWE-284;CWE-269;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.8';DESCR 'A protection flaw in ublk_ch_mmap leaves VM_MAYWRITE set on a command buffer that is intended to be mapped read only. A local ublk daemon can map the buffer with PROT_READ and then use mprotect to upgrade the mapping to PROT_WRITE, gaining a constrained write primitive over kernel-written ublksrv_io_desc entries. The attacker can modify fields such as addr, op_flags, nr_sectors, and start_sector while the buffer participates in block I/O processing. This can corrupt request semantics, disrupt the affected ublk device, damage data, or cause a denial of service. A direct arbitrary kernel write or reliable privilege escalation is not established, but the writable kernel-allocated ABI buffer creates a plausible memory corruption boundary that requires review of all later kernel consumers. For the CVSS the PR:L is selected because triggering requires a local process with access to the affected ublk character device, but it does not necessarily require full root privileges. The issue is not directly network reachable. The paranoid score uses CIA:HHH because controlled corruption of kernel-owned I/O descriptors may have broader confidentiality, integrity, and availability consequences if any later kernel path trusts the modified fields.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6) SKIP CVE-2026-89793 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 3fee8d7599e1 YES NO NO unknown MAYBE READ KASAN INIT SIMPLEFIX HARDWARE IO_URING KPANIC INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4 KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHERTHAN5 DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
1. ActionableScore
* Raw conservative score: 5
* Conservative score: 5. No floor or cap applied.
* Raw paranoid score: 6
* Paranoid score: 6. No floor or cap applied.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
2. Signal breakdown
Conservative signals:
* +2 Strong corruption primitive. An unprivileged daemon obtains a writable mapping of a kernel-owned command buffer and can directly modify active `ublksrv_io_desc` fields, including addresses, flags, lengths, and sectors.
* +1 Local unprivileged trigger. The supplied evidence explicitly identifies an unprivileged daemon and provides a working `mmap` plus `mprotect` sequence.
* +1 Integrity plausible. The attacker can make unauthorized modifications to kernel-written I/O descriptors that define block request semantics.
* +1 Important storage path. The corrupted descriptors participate in the ublk block-I/O path.
* Total: 5.
Paranoid addition:
* +1 Availability realistic. Corruption of active address, operation, size, and sector fields can plausibly invalidate requests or disrupt the affected block device. This is not counted as a demonstrated host-wide kernel crash.
* Total: 6.
Material non-awards:
* No UAF, stale lifetime, or repeated refcount operation is established.
* No arbitrary or directed write to an attacker-selected kernel address is established.
* No controlled reclaim, callback control, credential modification, or demonstrated LPE is established.
* Confidentiality is not awarded because no read-back or protected-data disclosure path is shown.
* Remote reachability is not established.
* The write-protection bypass is not scored as a second independent +2 authorization signal because it is the mechanism enabling the already-counted descriptor corruption.
* No reliable-kernel-crash point is awarded because the evidence demonstrates descriptor corruption, not a confirmed panic.
* No high-control API bonus is added separately because attacker influence over the descriptors is already intrinsic to the +2 corruption primitive.
3. Reachability analysis
The trigger is local and requires a process with access to the affected ublk character-device mapping. The supplied evidence positively states that an unprivileged daemon can map the buffer read-only and upgrade it with `mprotect`, so host-admin-equivalent privilege is not required for the vulnerable operation itself.
Access to the relevant ublk device or file descriptor remains a prerequisite. Device creation or delegation may involve prior privileged setup, but that does not make the daemon performing the corruption host administrator. The supplied context does not establish that an arbitrary account without ublk access can trigger the flaw.
No namespace capability check is shown, so complete namespace-scoped reachability is unresolved. No special hardware, network access, or timing race is required. The PoC indicates a deterministic and repeatable `mmap` followed by `mprotect` sequence.
4. Primitive and severity interpretation
The exact primitive is an attacker-controlled writable mapping of a kernel-allocated, kernel-written command buffer that was intended to remain read-only. This permits constrained but direct modification of active structured I/O descriptors.
This is broader than passive metadata corruption because the affected fields influence addresses, operation flags, transfer sizes, and block-sector selection. It is nevertheless not an arbitrary kernel write because the destination is confined to the mapped command-buffer region.
There is no lifetime violation or stale storage. The issue is classified as CLASS_A because attacker-controlled corruption of active kernel descriptors is directly established. The demonstrated consequence is unauthorized descriptor modification. Device disruption and data-integrity damage are plausible consequences. Host-wide memory corruption, information disclosure, and privilege escalation remain unproven.
ImportantCandidate is YES because the descriptor consumers must be inspected to determine whether corrupted fields can reach unsafe memory operations or cross a stronger storage boundary. AutoDowngradeAllowed is NO because automatically reducing this to an ordinary configuration defect would discard a concrete controlled kernel-state corruption primitive.
5. One-sentence report phrase
An unprivileged ublk daemon can use mprotect to convert an intended read-only command-buffer mapping into a writable mapping and corrupt active kernel-written I/O descriptors, creating a concrete storage-integrity primitive with unresolved memory-safety and availability consequences.
6. Manual review recommendation
MANUAL CHECK REQUIRED
Trace every kernel consumer of `addr`, `op_flags`, `nr_sectors`, and `start_sector` to determine whether concurrent descriptor modification can produce an out-of-bounds access, attacker-directed memory operation, cross-request data corruption, or host-wide crash.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: ublk: clear VM_MAYWRITE on read-only ublk char device mmap
Commit: be41733c24be58e2a1ef718c80fafdfb98a40d5e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=be41733c24be58e2a1ef718c80fafdfb98a40d5e
Commit description:
ublk_ch_mmap() rejects mmap requests with VM_WRITE set, but never
clears VM_MAYWRITE on the resulting read-only mapping. This allows
a userspace daemon to mmap the per-queue command buffer PROT_READ,
then upgrade it to PROT_WRITE via mprotect(), since VM_MAYWRITE was
never cleared.
The command buffer holds struct ublksrv_io_desc entries that are
kernel-written ABI; a writable mapping lets an unprivileged daemon
process corrupt fields such as addr, op_flags, nr_sectors, and
start_sector.
Same bug class as the drm/panthor and drm/vc4 VM_MAYWRITE fixes, and
the 2026-08-13 ptp/vmclock fix ( a5edadb ).
Verified via mprotect() PoC: before the fix, a PROT_READ mapping can
be upgraded to PROT_READ|PROT_WRITE and a write into the command
buffer corrupts io_desc fields (confirmed under KASAN). After the
fix, mprotect() returns -EACCES.
Fixes: 3fee8d7599e1 ("ublk_drv: add io_uring based userspace block driver")
Cc: [email protected]
Signed-off-by: Kanishka De Silva <[email protected]>
Reviewed-by: Ming Lei <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jens Axboe <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
drivers/block/ublk_drv.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=be41733c24be58e2a1ef718c80fafdfb98a40d5e
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89793 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89793
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:L/PR:L/UI:N/S:U/C:L/I:L/A:H
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: 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: 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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