From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-53341][MODERATE 7.0] fhandle: fix UAF due to unlocked ->mnt_ns read in may_decode_fh() Date: Wed, 01 Jul 2026 10:03:03 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-53341 X-AL-KERNEL-Priority: MODERATE 7.0 X-AL-KERNEL-Severity: MODERATE 7.0 X-AL-KERNEL-Base-Severity: MODERATE X-AL-KERNEL-KPANIC: YES X-AL-KERNEL-ActionableScore: 5 X-AL-KERNEL-ActionableScore-Lower: 4 X-AL-KERNEL-Commit: 32138633e51e6db59e474765cf93268c92b42888 List-Id: CVE: CVE-2026-53341 Priority: MODERATE 7.0 AL-KERNEL base severity: MODERATE KPANIC flag: YES Patch: fhandle: fix UAF due to unlocked ->mnt_ns read in may_decode_fh() Commit: 32138633e51e6db59e474765cf93268c92b42888 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=32138633e51e6db59e474765cf93268c92b42888 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53341 Analysis date: Wed, 01 Jul 2026 10:03:03 -0400 ActionableScore: 5 ActionableScore lower bound: 4 Actionable bucket: Strong Important candidate / Actionable Moderate at minimum Manual review required: YES Summary: A local race in the fhandle permission path can allow `may_decode_fh()` to dereference a freed `mnt_namespace` through an unlocked `mount::mnt_ns` read, causing a UAF with realistic DoS impact and limited but review-worthy permission-path risk. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-53341 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53341 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-2026-53341 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:H/I:H/A:H';*CWE-416;CWE-362;CWE-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A use after free can occur in the fhandle permission path because may_decode_fh reads mount::mnt_ns without RCU protection while another thread can unmount and free the related mnt_namespace after an RCU grace period. A local attacker that can reach open_by_handle_at() and race it with mount teardown may cause ns_capable() to dereference freed namespace memory through mnt_namespace::user_ns. For the CVSS the PR:L is used for the paranoid score because triggering requires local code execution and mount or file handle operations, but it does not necessarily require full host administrator privileges in all user namespace or reduced capability environments. The issue is not network reachable and requires a timing window, so AC:H is appropriate. Impact is at least local denial of service via kernel crash or hang, and in the worst case may allow confidentiality or integrity impact due to UAF in a permission check path.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 5) YES DISK RACE LEAK UAF KERNEL_PANIC_PLUS_UAF LINUS KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN7 YES NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=5 ActionableScoreLower=4 ## 1. ActionableScore * Conservative score: **4** * Paranoid score: **5** * Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**:: ## 2. Signal breakdown Conservative scoring: * **Local low-privilege or namespace-assisted trigger: +1** The path is local via `open_by_handle_at()` and mount teardown. Full host root is not clearly required in all namespace or reduced-capability setups. * **Memory corruption, weak primitive: +1** This is a real UAF, but the patch only shows stale pointer dereference of `mnt_namespace`, not attacker-controlled reclaim, overwrite, callback dispatch, or type confusion. * **Real lifetime corruption: +1** `mount::mnt_ns` can outlive its namespace lifetime incorrectly because it is read without RCU protection while the namespace can be freed after an RCU grace period. * **Kernel crash or hang plausible: +1** The commit explicitly mentions invalid dereference, endless loop, or crash as worst-case outcomes. * **Broad VFS / namespace permission path exposure: +1** The affected code is in VFS file-handle permission logic and mount namespace lifetime handling, not a narrow hardware-only driver path. * **Hard race / special timing: -1** Triggering requires a race between `open_by_handle_at()` and mount teardown plus an RCU grace period during the syscall. This keeps the conservative score at 4. Paranoid difference: * The paranoid score does **not** subtract the race point, because `CONFIG_PREEMPTION` is common and the bug is a real RCU lifetime violation in a permission path. This gives **5**. No extra points were added for strong LPE, confidentiality, or integrity because the patch does not show controlled reclaim, write-after-free, object replacement, type confusion, arbitrary write, or a reliable permission-bypass primitive. ## 3. Reachability analysis A local process must be able to reach the file-handle open path and race it against teardown of a cloned mount created with `open_tree(..., OPEN_TREE_CLONE)`. The issue is not network reachable. Typical triggering may require mount/file-handle capabilities, but user namespaces, container-root cases, or reduced-capability service contexts can make this more relevant than full host-root-only bugs. The bug is unreachable unless `CONFIG_PREEMPTION` or `CONFIG_RCU_STRICT_GRACE_PERIOD` is enabled. `CONFIG_PREEMPTION` is common enough that this should not be treated as a purely theoretical configuration. Call-site confidence: **high**, because the patch shows the affected call path from `may_decode_fh()` to `ns_capable(mnt_ns->user_ns, CAP_SYS_ADMIN)` and the lifetime fix directly adds RCU protection. ## 4. Severity interpretation This behaves like an **Actionable Moderate** issue and deserves manual review. It is not an ordinary low-risk cleanup because it is a real UAF in a VFS permission path. Realistic impact is most likely local DoS through crash or hang. Theoretical privilege or policy impact is harder to prove from the patch because the stale object access is read-oriented and no reclaim or overwrite primitive is shown. Therefore it should not be auto-escalated to high-end Important solely on bug class, but it should also not be auto-closed. ## 5. One-sentence report phrase A local race in the fhandle permission path can allow `may_decode_fh()` to dereference a freed `mnt_namespace` through an unlocked `mount::mnt_ns` read, causing a UAF with realistic DoS impact and limited but review-worthy permission-path risk. ## 6. Manual review recommendation **MANUAL CHECK REQUIRED** Reason: this is a real race-dependent UAF in VFS namespace permission handling. Even though the demonstrated primitive looks weak and DoS-oriented, permission-path UAFs should be manually reviewed for namespace, capability, and object-reuse edge cases. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: fhandle: fix UAF due to unlocked ->mnt_ns read in may_decode_fh() Commit: 32138633e51e6db59e474765cf93268c92b42888 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=32138633e51e6db59e474765cf93268c92b42888 Commit description: may_decode_fh() accesses mount::mnt_ns without holding any locks; that means the mount can concurrently be unmounted, and the mnt_namespace can concurrently be freed after an RCU grace period. This race can happens as follows, assuming that the mount point was created by open_tree(..., OPEN_TREE_CLONE): thread 1 thread 2 RCU __do_sys_open_by_handle_at do_handle_open handle_to_path may_decode_fh is_mounted [mount::mnt_ns access] [mount::mnt_ns access] __do_sys_close fput_close_sync __fput dissolve_on_fput umount_tree class_namespace_excl_destructor namespace_unlock free_mnt_ns mnt_ns_tree_remove call_rcu(mnt_ns_release_rcu) mnt_ns_release_rcu mnt_ns_release kfree [mnt_namespace::user_ns access] **UAF** Fix it by taking rcu_read_lock() around the mount::mnt_ns access, like in __prepend_path(). Additionally, document the semantics of mount::mnt_ns, and use WRITE_ONCE() for writers that can race with lockless readers. This bug is unreachable unless one of the following is set: - CONFIG_PREEMPTION - CONFIG_RCU_STRICT_GRACE_PERIOD because it requires an RCU grace period to happen during a syscall without an explicit preemption. This doesn't seem to have interesting security impact; worst-case, it could leak the result of an integer comparison to userspace (from the level check in cap_capable()), cause an endless loop, or crash the kernel by dereferencing an invalid address. Fixes: 620c266 ("fhandle: relax open_by_handle_at() permission checks") Cc: stable@vger.kernel.org Signed-off-by: Jann Horn Link: https://patch.msgid.link/20260603-vfs-fhandle-uaf-fix-v2-1-d05db76a5084@google.com Signed-off-by: Christian Brauner (Amutable) Signed-off-by: Greg Kroah-Hartman Changed files: fs/fhandle.c fs/mount.h fs/namespace.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=32138633e51e6db59e474765cf93268c92b42888 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-53341 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53341 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 alexanjelausa@gmail.com (and both send reply to CVE record itself too and see "reply" button below for howto reply).