From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-89551][MODERATE 7.0] SUNRPC: xdr_buf_trim: clamp buf->len to avoid underflow Date: Sat, 12 Sep 2026 03:07:50 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-89551 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: 6 X-AL-KERNEL-ActionableScore-Lower: 5 X-AL-KERNEL-Commit: e6267cccd7b05cc514e57f2160aa8db85f5c2701 List-Id: CVE: CVE-2026-89551 Priority: MODERATE 7.0 AL-KERNEL base severity: MODERATE KPANIC flag: YES Patch: SUNRPC: xdr_buf_trim: clamp buf->len to avoid underflow Commit: e6267cccd7b05cc514e57f2160aa8db85f5c2701 Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e6267cccd7b05cc514e57f2160aa8db85f5c2701 Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89551 Analysis date: Sat, 12 Sep 2026 03:07:50 -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-2026-89551 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89551 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-2026-89551 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89551 CHECK WITH IMPACT FROM ORIG NN MODERATE Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';CWE-191;CWE-682;*CWE-20;CWE-125;Other CVSS 'AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.5';DESCR 'SUNRPC xdr_buf_trim can underflow buf len when the recorded XDR buffer length is smaller than the amount consumed from the tail pages and head iovecs. In the gss_krb5_unwrap_v2 path this can be reached from a crafted RPCSEC_GSS Kerberos protected buffer where buf len is a small wire derived value while the backing iovec lengths are larger. The wrapped length then becomes an oversized authoritative stream bound for downstream XDR decoders, which may allow parsing or reads beyond the intended authenticated payload and can lead to denial of service or stronger confidentiality and integrity impact. For the CVSS the PR:L is used because reliable triggering normally requires a valid RPCSEC_GSS security context or control of a trusted RPC peer rather than only sending unauthenticated traffic. The issue is network reachable in deployments using SUNRPC with Kerberos security, commonly NFS environments on internal storage or data center networks. Impact is at least network triggered denial of service and in worst case may allow confidentiality or integrity impact due to incorrect XDR bounds handling.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6) YES REMOTE OOB NETWORK KPANIC KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH NO NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=6 ActionableScoreLower=5 PrimitiveClass=CLASS_B RealUAF=NO HostAdminRequired=NO NamespaceScopedPrivilege=NO ImportantCandidate=YES ManualReview=REQUIRED AutoDowngradeAllowed=NO 1. ActionableScore * Raw conservative score: 5 * Conservative score: 5. No floor or cap changed the score. * Raw paranoid score: 6 * Paranoid score: 6. No floor or cap changed the score. * Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**:: 2. Signal breakdown Conservative signals: * +2 Remote reachable / network-triggerable: the affected path is in SUNRPC RPCSEC_GSS / Kerberos unwrap handling and can be reached through crafted RPC data in deployments using Kerberos-protected SUNRPC, commonly NFS. * +1 Weak / indirect corruption candidate: unsigned underflow turns a small recorded `xdr_buf.len` into a near-`UINT_MAX` authoritative decoder bound. * +1 Memory layout invariant restoration: the patch restores consistency between `buf->len` and the amount consumed from head/pages/tail iovecs. * +1 Reliable kernel crash / strong DoS: downstream XDR decoders trusting the wrapped length can parse beyond the intended buffer boundary and realistically fault or otherwise cause denial of service. Paranoid additional signal: * +1 Unresolved semantic decoder leverage: the wrapped length is not just local bookkeeping. It becomes the authoritative stream bound for later XDR decoding, so stronger parser confusion or C/I impact is plausible enough for manual review, though not proven. Non-awarded signals: * RealUAF not awarded: no stale post-lifetime object use is established. * Controlled reclaim not demonstrated. * Arbitrary write not demonstrated. * Concrete information disclosure not demonstrated. * Concrete integrity modification not demonstrated. * Privilege escalation not demonstrated. * Extra Availability not awarded because the same likely crash/decoder failure is already counted once. * Repeated release/refcount signal not applicable. * Race/timing penalty not applied because this is an arithmetic/bounds bug, not a race. * Host-admin penalty not applied because the attacker model is a remote authenticated or trusted RPC peer, not host administrator. * Namespace-scoped privilege not applicable. 3. Reachability analysis The reachable path is `gss_krb5_unwrap_v2()` calling `xdr_buf_trim()` after deriving a small `buf->len` from wire-controlled or peer-controlled RPCSEC_GSS data while the backing iovec lengths may remain page-scale. A remote unauthenticated Internet attacker is not established. Reliable triggering appears to require a valid RPCSEC_GSS / Kerberos context, a malicious or compromised RPC peer, or another position capable of sending crafted protected SUNRPC traffic. Host administrator privilege on the victim is not required for the attack model once the Kerberos-protected SUNRPC service is exposed. The service configuration itself is prior administrative setup, but that is normal deployment exposure rather than an attacker privilege requirement. Namespace capability does not appear relevant. No special hardware is required. No race repeatability issue is involved. 4. Primitive and severity interpretation The exact primitive is an unsigned integer underflow in `xdr_buf_trim()`: `buf->len -= (len - trim)` When `(len - trim)` is larger than `buf->len`, the unsigned subtraction wraps and makes `buf->len` very large. The resulting length is then propagated as the authoritative stream bound into downstream XDR decoders. This is not a UAF and not a concrete arbitrary read/write primitive. It is an active parser-bound / semantic bounds corruption issue. The demonstrated consequence is incorrect downstream decoding with realistic DoS potential. The speculative consequence is stronger confidentiality or integrity impact if later XDR decoders consume unintended memory or unauthenticated payload area in a security-relevant way. PrimitiveClass is CLASS_B because a meaningful kernel parser/bounds corruption primitive is established, but controlled memory corruption, disclosure, write primitive, or LPE is not directly shown. ImportantCandidate is YES because the issue is network reachable in Kerberized SUNRPC deployments and affects a central decoder bound after authentication unwrap. AutoDowngradeAllowed is NO because automatically treating this as ordinary DoS would discard unresolved parser-bound exploitation leverage. Likely operational severity: actionable Moderate at minimum, with Strong Important review warranted if downstream decoder behavior confirms disclosure, request semantic confusion, or memory corruption beyond crash. 5. One-sentence report phrase SUNRPC `xdr_buf_trim()` can underflow `buf->len` after RPCSEC_GSS Kerberos unwrap, causing downstream XDR decoders to trust a near-`UINT_MAX` stream bound and potentially enabling network-triggered DoS or stronger parser-bound impact. 6. Manual review recommendation MANUAL CHECK REQUIRED Reason: the bug is network reachable in Kerberized SUNRPC/NFS deployments and corrupts an authoritative XDR decoder length, so downstream decoder behavior must be reviewed before safely downgrading it to crash-only DoS. 7. Call-site confidence Call-site confidence: medium ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: SUNRPC: xdr_buf_trim: clamp buf->len to avoid underflow Commit: e6267cccd7b05cc514e57f2160aa8db85f5c2701 Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e6267cccd7b05cc514e57f2160aa8db85f5c2701 Commit description: xdr_buf_trim() trims `len` bytes from the tail of an xdr_buf by walking the tail, pages, and head iovecs. Each per-section step uses min_t() so it never removes more bytes than that section holds, but the final accounting at the fix_len label subtracts the total bytes actually consumed from buf->len without any clamp: fix_len: buf->len -= (len - trim); When the caller has set buf->len to a value smaller than the sum of the iov_lens, (len - trim) can exceed buf->len and the unsigned subtraction wraps to near UINT_MAX. gss_krb5_unwrap_v2() reaches xdr_buf_trim() in exactly that state: buf->head[0].iov_len -= GSS_KRB5_TOK_HDR_LEN + headskip; buf->len = len - (GSS_KRB5_TOK_HDR_LEN + headskip); xdr_buf_trim(buf, ec + GSS_KRB5_TOK_HDR_LEN + tailskip); buf->len is a small wire-derived value while the iov_lens are at page scale, so the per-section loops legitimately consume far more bytes than buf->len records. The wrapped buf->len then propagates as the authoritative stream bound into every downstream XDR decoder. Fix by clamping the decrement so buf->len bottoms out at zero: buf->len -= min_t(unsigned int, buf->len, len - trim); On the normal path where the iov_lens sum to buf->len, (len - trim) is always <= buf->len and the result is identical to before. No callers change behavior outside the underflow case. Fixes: 4c190e2 ("sunrpc: trim off trailing checksum before returning decrypted or integrity authenticated buffer") Cc: stable@vger.kernel.org Assisted-by: kres (claude-opus-4-7) Signed-off-by: Chris Mason Reviewed-by: Jeff Layton Link: https://patch.msgid.link/20260524010213.557424-4-cel@kernel.org Signed-off-by: Chuck Lever Signed-off-by: Greg Kroah-Hartman Changed files: net/sunrpc/xdr.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=e6267cccd7b05cc514e57f2160aa8db85f5c2701 ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-89551 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89551 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:N/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H The Best / paranoid CVSS vector: AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H The Best / paranoid CVSS score: 7.5 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: 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 alexanjelausa@gmail.com (and both send reply to CVE record itself too and see "reply" button below for howto reply).