From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-74487][LOW] binfmt_misc: restore write access when removing an entry Date: Sat, 15 Aug 2026 12:47:17 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-74487 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: 2 X-AL-KERNEL-ActionableScore-Lower: 0 X-AL-KERNEL-Commit: fdc1d702bf3001586221fa07e598e876a0a854c5 List-Id: CVE: CVE-2026-74487 Priority: LOW AL-KERNEL base severity: LOW KPANIC flag: NO Patch: binfmt_misc: restore write access when removing an entry Commit: fdc1d702bf3001586221fa07e598e876a0a854c5 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fdc1d702bf3001586221fa07e598e876a0a854c5 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74487 Analysis date: Sat, 15 Aug 2026 12:47:17 -0400 ActionableScore: 2 ActionableScore lower bound: 0 Actionable bucket: Ordinary Moderate / Low-like Manual review required: NO Summary: A `binfmt_misc` cleanup bug leaves the interpreter inode write-denied after removing a `MISC_FMT_OPEN_FILE` entry, causing persistent `ETXTBSY` write failures until inode eviction, with local administrative or delegated management access required. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-74487 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74487 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-2026-74487 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-772;CWE-404;CWE-459;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'binfmt_misc entries created with MISC_FMT_OPEN_FILE keep the interpreter opened through open_exec, which denies write access to the interpreter inode while the entry exists. On entry removal the file was closed without restoring write access, leaving i_writecount unbalanced and causing later write opens of the interpreter to fail with ETXTBSY until inode eviction. For the CVSS the PR:L is used for the paranoid interpretation because some deployments may delegate binfmt_misc management to reduced root, container root, or service accounts rather than full host administration. The issue is local and is not network reachable. Impact is denial of service against file modification or update workflows, with no evidence of memory corruption, information disclosure, or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 1) YES WRITE OOB LOCK DISK SIMPLEFIX LEAK ERRORPATH LINUS INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 NO NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=2 ActionableScoreLower=0 ## 1. ActionableScore * Conservative score: 0 * Paranoid score: 2 * Final recommended bucket: **Ordinary Moderate / Low-like**:: ## 2. Signal breakdown Conservative signals: * Availability impact realistic: +1. Removing a `binfmt_misc` entry can leave the interpreter inode with permanently denied write access, causing later write opens to fail with `ETXTBSY` until inode eviction. * Reliable operational DoS: +1. The failure mode is deterministic once the affected entry lifecycle occurs, but it is limited to write denial on the interpreter file rather than a kernel crash. * Requires admin/root/CAP_SYS_ADMIN in typical deployments: -2. Creating and removing `binfmt_misc` entries is normally an administrative operation. * Legacy / non-default execution mode: -1. The issue depends on `MISC_FMT_OPEN_FILE`, which is a specific persistent-open handler mode rather than the default path. Conservative total floors at 0. Paranoid signals: * Local reduced-privilege trigger: +1. Some container or service-account deployments may delegate `binfmt_misc` management to reduced root rather than full host root. * Availability impact realistic: +1. The write-denial state can persist after entry removal until inode eviction. * Reliable operational DoS: +1. The affected interpreter remains unwritable with `ETXTBSY`. * Non-default `MISC_FMT_OPEN_FILE` mode: -1. Paranoid total: 2. Not awarded: * No generic memory corruption. The patch restores a missing `exe_file_allow_write_access()` call before `filp_close()`. * No UAF, double-free, OOB write, refcount takeover, or type confusion. * No confidentiality or integrity impact is supported. * No privilege escalation primitive is visible. * No remote or network trigger. Call-site confidence: high. The affected cleanup path and the missing balancing call are directly visible in the provided patch. ## 3. Reachability analysis The bug is triggered by registering a `binfmt_misc` entry with `MISC_FMT_OPEN_FILE` and then removing it. In normal deployments this requires administrative control over `binfmt_misc`, usually root or equivalent capability. In some container-oriented setups, reduced root or a service account may be allowed to manage entries, which supports the paranoid PR:L-style interpretation, but not a remote or unauthenticated one. The path is local and configuration-dependent. It is not packet-triggerable and does not depend on attacker-controlled kernel memory layout. Realistic exploitation is an operational denial of service against updates or writes to the interpreter inode, not kernel compromise. ## 4. Severity interpretation This behaves like a low-end Moderate or Low-like operational DoS. Theoretical impact is persistent write denial on a specific interpreter file until inode eviction. Realistic exploitation evidence does not support memory corruption, data disclosure, privilege escalation, or arbitrary modification of protected data. The issue should not be treated as Important-class. It is mainly a resource-accounting imbalance in the executable write-denial mechanism. ## 5. One-sentence report phrase A `binfmt_misc` cleanup bug leaves the interpreter inode write-denied after removing a `MISC_FMT_OPEN_FILE` entry, causing persistent `ETXTBSY` write failures until inode eviction, with local administrative or delegated management access required. ## 6. Manual review recommendation NO MANUAL CHECK NEEDED The patch shows a straightforward missing cleanup balance for write-access denial, with no memory corruption or privilege-escalation primitive. This can be handled as an ordinary Moderate or Low-like local DoS unless the product specifically delegates `binfmt_misc` management to untrusted container users. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: binfmt_misc: restore write access when removing an entry Commit: fdc1d702bf3001586221fa07e598e876a0a854c5 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fdc1d702bf3001586221fa07e598e876a0a854c5 Commit description: Registering an entry with the MISC_FMT_OPEN_FILE flag opens the interpreter via open_exec() which denies write access to it for as long as the entry exists. Removing the entry closes the interpreter file via filp_close() but never restores write access, leaving the inode's i_writecount permanently negative. Opening the interpreter for writing keeps failing with ETXTBSY long after the entry is gone until the inode is evicted from the inode cache. Commit 90f601b ("binfmt_misc: restore write access before closing files opened by open_exec()") fixed the same imbalance in the error path of bm_register_write() but the actual removal path has been leaking the write denial since the introduction of the flag. Restore write access in put_binfmt_handler() before closing the interpreter file. Link: https://patch.msgid.link/20260710-work-binfmt_misc-locking-v3-1-a162f7cb58d6@kernel.org Fixes: 948b701 ("binfmt_misc: add persistent opened binary handler for containers") Cc: stable@vger.kernel.org Signed-off-by: Christian Brauner (Amutable) Signed-off-by: Greg Kroah-Hartman Changed files: fs/binfmt_misc.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=fdc1d702bf3001586221fa07e598e876a0a854c5 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-74487 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74487 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:L 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: 0 Paranoid ActionableScore: 2 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).