[CVE-2026-46079][MODERATE REGULAR] rbd: fix null-ptr-deref when device_add_disk() fails

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

CVE: CVE-2026-46079
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: rbd: fix null-ptr-deref when device_add_disk() fails
Commit: 2f4809a879f0750c7790bbeeae86c9505797a06f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2f4809a879f0750c7790bbeeae86c9505797a06f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46079
Analysis date: Wed, 27 May 2026 11:39:00 -0400
ActionableScore: 3
ActionableScore lower bound: 1
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
An RBD add error path can double-teardown disk state after `device_add_disk()` failure, causing a NULL pointer dereference in blk-mq cleanup and local kernel DoS under storage-management error conditions.

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

The original Linux kernel CVE announcement for CVE-2026-46079 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46079

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 REGULAR
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-2026-46079	MODERATE	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';**CWE-476;CWE-703;*CWE-672;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '4.7';DESCR 'A NULL pointer dereference can occur in the RBD add path when device_add() succeeds but device_add_disk() later fails. The old error path could call rbd_free_disk() directly and then reach rbd_dev_device_release(), causing inconsistent double teardown of disk and blk mq state before __blk_mq_free_map_and_rqs(). For the CVSS the PR:L is used only in the paranoid score for environments where a reduced privilege storage service, delegated container, or local management role can create RBD mappings. The typical case is closer to PR:H because mapping an RBD device through sysfs usually requires administrative storage control. The issue is not network reachable by itself and does not involve a demonstrated UAF, out of bounds write, arbitrary write, information disclosure, or privilege escalation. Impact is denial of service via kernel crash during an error path, usually requiring allocation failure or another device_add_disk() failure condition.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 2)	YES	KASAN OOB NULLPTR INIT ERRORPATH KERNEL_PANIC_PLUS_UAF HARDWARE LINUS  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	YES	NO	checked

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

ActionableScore=3
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 3
* Final recommended bucket: **Borderline, manual review recommended**::

## 2. Signal breakdown

Conservative signals:

* Reliable kernel crash / strong DoS: +1
  The reproduced failure reaches a NULL pointer dereference in `__blk_mq_free_map_and_rqs()` during blk-mq cleanup.

* Availability impact realistic: +1
  Successful triggering can panic the kernel.

* Important filesystem/storage path: +1
  RBD is a block storage driver and the fault occurs in block device add / teardown handling.

Subtracting signals:

* Requires admin/storage-management control in typical deployments: -2
  Creating RBD mappings through sysfs normally requires privileged storage administration.

* Hard error-path condition: -1
  Triggering requires `device_add_disk()` failure after `device_add()` succeeds, often needing allocation failure injection or unusual resource failure.

Paranoid additions:

* Reduced-privilege storage-management concern: +1
  Some environments may delegate RBD mapping to a storage service, container, or local management role.

* Weak lifetime/teardown concern: +1
  The double teardown is not demonstrated as exploitable UAF, but cleanup ordering bugs in block device teardown deserve limited manual validation.

No signals apply for remote reachability, UAF as demonstrated primitive, OOB write, arbitrary write, information disclosure, or privilege escalation.

## 3. Reachability analysis

The bug is local and occurs when adding an RBD device. The expected trigger is a privileged user or storage management service writing to the RBD sysfs add interface while `device_add_disk()` fails in a specific window.

Namespaces and containers matter only if RBD mapping is delegated. The Ceph backend is networked, but the vulnerability is not network-triggerable by a remote Ceph peer based on the patch. Practical exploitation is constrained because the reliable reproducer uses KASAN and `CONFIG_FAILSLAB` to force allocation failure.

## 4. Severity interpretation

This behaves like a borderline local storage DoS. The observed primitive is a NULL pointer dereference caused by inconsistent double teardown, not a confirmed UAF or controllable memory corruption.

Theoretical concern exists because double-cleanup bugs sometimes hide stronger lifetime issues, but the patch and reproducer show a crash in blk-mq cleanup only. No evidence supports privilege escalation.

## 5. One-sentence report phrase

An RBD add error path can double-teardown disk state after `device_add_disk()` failure, causing a NULL pointer dereference in blk-mq cleanup and local kernel DoS under storage-management error conditions.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: this is primarily a privileged error-path NULL dereference, but it involves double teardown in block-device cleanup, so limited manual review is useful to rule out stronger lifetime corruption.

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

Patch: rbd: fix null-ptr-deref when device_add_disk() fails
Commit: 2f4809a879f0750c7790bbeeae86c9505797a06f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2f4809a879f0750c7790bbeeae86c9505797a06f

Commit description:

do_rbd_add() publishes the device with device_add() before calling
device_add_disk(). If device_add_disk() fails after device_add()
succeeds, the error path calls rbd_free_disk() directly and then later
falls through to rbd_dev_device_release(), which calls rbd_free_disk()
again. This double teardown can leave blk-mq cleanup operating on
invalid state and trigger a null-ptr-deref in
__blk_mq_free_map_and_rqs(), reached from blk_mq_free_tag_set().

Fix this by following the normal remove ordering: call device_del()
before rbd_dev_device_release() when device_add_disk() fails after
device_add(). That keeps the teardown sequence consistent and avoids
re-entering disk cleanup through the wrong path.

The bug was first flagged by an experimental analysis tool we are
developing for kernel memory-management bugs while analyzing
v6.13-rc1. The tool is still under development and is not yet publicly
available.

We reproduced the bug on v7.0 with a real Ceph backend and a QEMU x86_64
guest booted with KASAN and CONFIG_FAILSLAB enabled. The reproducer
confines failslab injections to the __add_disk() range and injects
fail-nth while mapping an RBD image through
/sys/bus/rbd/add_single_major.

On the unpatched kernel, fail-nth=4 reliably triggered the fault:

	Oops: general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] SMP KASAN NOPTI
	KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
	CPU: 0 UID: 0 PID: 273 Comm: bash Not tainted 7.0.0-01247-gd60bc1401583 #6 PREEMPT(lazy)
	Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.15.0-1 04/01/2014
	RIP: 0010:__blk_mq_free_map_and_rqs+0x8c/0x240
	Code: 00 00 48 8b 6b 60 41 89 f4 49 c1 e4 03 4c 01 e5 45 85 ed 0f 85 0a 01 00 00 48 b8 00 00 00 00 00 fc ff df 48 89 e9 48 c1 e9 03 <80> 3c 01 00 0f 85 31 01 00 00 4c 8b 6d 00 4d 85 ed 0f 84 e2 00 00
	RSP: 0018:ff1100000ab0fac8 EFLAGS: 00000246
	RAX: dffffc0000000000 RBX: ff1100000c4806a0 RCX: 0000000000000000
	RDX: 0000000000000002 RSI: 0000000000000000 RDI: ff1100000c4806f4
	RBP: 0000000000000000 R08: 0000000000000001 R09: ffe21c000189001b
	R10: ff1100000c4800df R11: ff1100006cf37be0 R12: 0000000000000000
	R13: 0000000000000000 R14: ff1100000c480700 R15: ff1100000c480004
	FS:  00007f0fbe8fe740(0000) GS:ff110000e5851000(0000) knlGS:0000000000000000
	CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
	CR2: 00007fe53473b2e0 CR3: 0000000012eef000 CR4: 00000000007516f0
	PKRU: 55555554
	Call Trace:
	 <TASK>
	 blk_mq_free_tag_set+0x77/0x460
	 do_rbd_add+0x1446/0x2b80
	 ? __pfx_do_rbd_add+0x10/0x10
	 ? lock_acquire+0x18c/0x300
	 ? find_held_lock+0x2b/0x80
	 ? sysfs_file_kobj+0xb6/0x1b0
	 ? __pfx_sysfs_kf_write+0x10/0x10
	 kernfs_fop_write_iter+0x2f4/0x4a0
	 vfs_write+0x98e/0x1000
	 ? expand_files+0x51f/0x850
	 ? __pfx_vfs_write+0x10/0x10
	 ksys_write+0xf2/0x1d0
	 ? __pfx_ksys_write+0x10/0x10
	 do_syscall_64+0x115/0x690
	 entry_SYSCALL_64_after_hwframe+0x77/0x7f
	RIP: 0033:0x7f0fbea15907
	Code: 10 00 f7 d8 64 89 02 48 c7 c0 ff ff ff ff eb b7 0f 1f 00 f3 0f 1e fa 64 8b 04 25 18 00 00 00 85 c0 75 10 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 51 c3 48 83 ec 28 48 89 54 24 18 48 89 74 24
	RSP: 002b:00007ffe22346ea8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
	RAX: ffffffffffffffda RBX: 0000000000000058 RCX: 00007f0fbea15907
	RDX: 0000000000000058 RSI: 0000563ace6c0ef0 RDI: 0000000000000001
	RBP: 0000563ace6c0ef0 R08: 0000563ace6c0ef0 R09: 6b6435726d694141
	R10: 5250337279762f78 R11: 0000000000000246 R12: 0000000000000058
	R13: 00007f0fbeb1c780 R14: ff1100000c480700 R15: ff1100000c480004
	 </TASK>

With this fix applied, rerunning the reproducer over fail-nth=1..256
yields no KASAN reports.

[ idryomov: rename err_out_device_del -> err_out_device ]

Cc: stable@vger.kernel.org
Fixes: 27c97ab ("rbd: add add_disk() error handling")
Signed-off-by: Zilin Guan <zilin@seu.edu.cn>
Signed-off-by: Dawei Feng <dawei.feng@seu.edu.cn>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

[Commit description truncated; see the upstream URL below]

Changed files:
  drivers/block/rbd.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=2f4809a879f0750c7790bbeeae86c9505797a06f

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

The original Linux kernel CVE announcement for CVE-2026-46079 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46079

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:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: 1
  Paranoid ActionableScore: 3

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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).