From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-63810][LOW] block: Avoid mounting the bdev pseudo-filesystem in userspace Date: Sun, 19 Jul 2026 10:03:27 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-63810 X-AL-KERNEL-Priority: LOW X-AL-KERNEL-Severity: LOW X-AL-KERNEL-Base-Severity: LOW X-AL-KERNEL-KPANIC: NO X-AL-KERNEL-ActionableScore: 3 X-AL-KERNEL-ActionableScore-Lower: 1 X-AL-KERNEL-Commit: 197971e6ffc0a6356b2ba2b22beb42bc0f7e412d List-Id: CVE: CVE-2026-63810 Priority: LOW AL-KERNEL base severity: LOW KPANIC flag: NO Patch: block: Avoid mounting the bdev pseudo-filesystem in userspace Commit: 197971e6ffc0a6356b2ba2b22beb42bc0f7e412d Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=197971e6ffc0a6356b2ba2b22beb42bc0f7e412d Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63810 Analysis date: Sun, 19 Jul 2026 10:03:27 -0400 ActionableScore: 3 ActionableScore lower bound: 1 Actionable bucket: Borderline Ordinary Moderate. Not an Important candidate Manual review required: NO Summary: Userspace could mount the internal bdev pseudo filesystem and trigger a NULL function pointer dereference through VFS file access paths, causing a local kernel crash, typically requiring CAP_SYS_ADMIN or delegated mount privileges. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-63810 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63810 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: LOW Manual review required: NO A detailed explanation of the methodology and priority rules is included at the end of this message. ====================================================================== AL-KERNEL CLASSIFICATION RESULT ====================================================================== CVE-2026-63810 LOW 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:N/I:N/A:H';**CWE-476;CWE-284;CWE-668;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'The bdev pseudo filesystem is an internal kernel filesystem that could be registered for userspace mounting. If a local actor mounts it and then performs file access operations, VFS paths such as lookup_open can reach inode operations that are empty for this filesystem and can result in a NULL function pointer dereference at address 0. For the CVSS the PR:L is used in the paranoid interpretation for environments where mount capability is delegated to a reduced capability root, container root, or service account. The base case is PR:H because normal mounting requires administrative privileges. The issue is not network reachable and is triggered through local mount and file access operations. Impact is denial of service via kernel crash, with no supported evidence of information disclosure or privilege escalation on modern systems.';NO MANUAL CHECK; ,and ActionableScore result is Borderline Ordinary Moderate. Not an Important candidate (with actual score 2) YES KASAN NULLPTR INIT HARDWARE DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 NO NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=3 ActionableScoreLower=1 ## 1. ActionableScore * Conservative score: 1 * Paranoid score: 3 * Final recommended bucket: **Borderline Ordinary Moderate. Not an Important candidate** ## 2. Signal breakdown Conservative signals: * Reliable kernel crash / strong DoS: +1. The trace shows a NULL function pointer dereference at RIP 0x0 through VFS open path after userspace mounts the internal bdev pseudo filesystem. * Broad/default kernel code exposure: +1. The bdev pseudo filesystem exists as part of the core block layer, but the vulnerable operation is not normally reachable without mount privileges. * Requires admin/root/CAP_SYS_ADMIN in typical deployments: -2. Mounting this filesystem normally requires CAP_SYS_ADMIN or equivalent mount privileges. Conservative total: 1. Paranoid signals: * Reliable kernel crash / strong DoS: +1. * Broad/default kernel code exposure: +1. * Important filesystem/storage path: +1. The issue is in the block layer pseudo filesystem and VFS mount/open interaction. * No memory corruption escalation bonus. This is a NULL function pointer dereference, not UAF, OOB write, double free, stale pointer reuse, refcount takeover, or page-cache corruption. * No confidentiality or integrity bonus. The patch and trace support crash only. * No full CAP_SYS_ADMIN penalty in paranoid interpretation. A reduced-capability root, container root, or delegated mount-management service may make this practically less than full host-root in some environments. Paranoid total: 3. ## 3. Reachability analysis The bug is locally triggerable by mounting the bdev pseudo filesystem and then accessing files through paths that reach empty inode operations. In normal deployments this requires CAP_SYS_ADMIN, so a fully unprivileged local user should not be able to trigger it directly. Namespaces or containers may matter only if mount capability is delegated to a reduced-privilege context. That supports the paranoid PR:L-style interpretation, but not true PR:N or ordinary unprivileged reachability. The path is not network reachable. There is no packet-processing, remote protocol, firmware-mediated input, or external storage protocol trigger shown. ## 4. Severity interpretation This behaves like an ordinary Moderate or Low-like kernel DoS in the conservative case, and a borderline manual-review Moderate in the paranoid case if delegated mount capabilities are relevant. The theoretical risk is limited because the primitive is a NULL function pointer dereference at address 0. Modern kernels normally prevent useful exploitation of NULL calls, and the patch does not indicate attacker-controlled code pointers, object reuse, memory corruption, information disclosure, or privilege escalation. ## 5. One-sentence report phrase Userspace could mount the internal bdev pseudo filesystem and trigger a NULL function pointer dereference through VFS file access paths, causing a local kernel crash, typically requiring CAP_SYS_ADMIN or delegated mount privileges. ## 6. Manual review recommendation NO MANUAL CHECK NEEDED This is a local mount-triggered NULL dereference DoS with no demonstrated memory corruption primitive, no network exposure, and normal CAP_SYS_ADMIN gating. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: block: Avoid mounting the bdev pseudo-filesystem in userspace Commit: 197971e6ffc0a6356b2ba2b22beb42bc0f7e412d Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=197971e6ffc0a6356b2ba2b22beb42bc0f7e412d Commit description: The bdev pseudo-filesystem is an internal kernel filesystem with which userspace should not interfere. Unregister it so that userspace cannot even attempt to mount it. This fixes a bug [1] that occurs when attempting to access files, because the system call move_mount() uses pointers declared in the inode_operations structure, which for the bdev pseudo-filesystem are always equal to 0. `inode->i_op = &empty_iops;` [1] BUG: kernel NULL pointer dereference, address: 0000000000000000 #PF: supervisor instruction fetch in kernel mode #PF: error_code(0x0010) - not-present page PGD 23380067 P4D 23380067 PUD 23381067 PMD 0 Oops: 0010 [#1] PREEMPT SMP KASAN NOPTI CPU: 2 PID: 17125 Comm: syz-executor.0 Not tainted 6.1.155-syzkaller-00350-g84221fde2681 #0 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014 RIP: 0010:0x0 Call Trace: lookup_open.isra.0+0x700/0x1180 fs/namei.c:3460 open_last_lookups fs/namei.c:3550 [inline] path_openat+0x953/0x2700 fs/namei.c:3780 do_filp_open+0x1c5/0x410 fs/namei.c:3810 do_sys_openat2+0x171/0x4d0 fs/open.c:1318 do_sys_open fs/open.c:1334 [inline] __do_sys_openat fs/open.c:1350 [inline] __se_sys_openat fs/open.c:1345 [inline] __x64_sys_openat+0x13c/0x1f0 fs/open.c:1345 do_syscall_x64 arch/x86/entry/common.c:51 [inline] do_syscall_64+0x35/0x80 arch/x86/entry/common.c:81 entry_SYSCALL_64_after_hwframe+0x6e/0xd8 Found by Linux Verification Center (linuxtesting.org) with Syzkaller. Fixes: 1da177e ("Linux-2.6.12-rc2") Link: https://lore.kernel.org/all/20131010004732.GJ13318@ZenIV.linux.org.uk/T/# Cc: stable@vger.kernel.org Signed-off-by: Denis Arefev Reviewed-by: Christoph Hellwig Link: https://patch.msgid.link/20260521072857.5078-1-arefev@swemel.ru Signed-off-by: Jens Axboe Signed-off-by: Greg Kroah-Hartman Changed files: fs/namei.c fs/open.c arch/x86/entry/common.c block/bdev.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=197971e6ffc0a6356b2ba2b22beb42bc0f7e412d ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-63810 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63810 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:H/UI:N/S:U/C:N/I:N/A:H The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H The Best / paranoid CVSS score: 5.5 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: LOW KPANIC detected for this report: NO Published priority for this report (same as in Subject): LOW 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).