CVE-2026-31709smb: client: validate the whole DACL before rewriting it in cifsacl

Official description

In the Linux kernel, the following vulnerability has been resolved: smb: client: validate the whole DACL before rewriting it in cifsacl build_sec_desc() and id_mode_to_cifs_acl() derive a DACL pointer from a server-supplied dacloffset and then use the incoming ACL to rebuild the chmod/chown security descriptor. The original fix only checked that the struct smb_acl header fits before reading dacl_ptr->size or dacl_ptr->num_aces. That avoids the immediate header-field OOB read, but the rewrite helpers still walk ACEs based on pdacl->num_aces with no structural validation of the incoming DACL body. A malicious server can return a truncated DACL that still contains a header, claims one or more ACEs, and then drive replace_sids_and_copy_aces() or set_chmod_dacl() past the validated extent while they compare or copy attacker-controlled ACEs. Factor the DACL structural checks into validate_dacl(), extend them to validate each ACE against the DACL bounds, and use the shared validator before the chmod/chown rebuild paths. parse_dacl() reuses the same validator so the read-side parser and write-side rewrite paths agree on what constitutes a well-formed incoming DACL.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/b8603d9ae6c9087662b098619996bc4a8064319d
• https://git.kernel.org/stable/c/c2abdebf72000a64603ced84d36ccbd164f11391
• https://git.kernel.org/stable/c/8e47d297e7cf9a6029a0d38e7b22faba7d7aaf12
• https://git.kernel.org/stable/c/d92f3f0b22414e7515696a02224d0af55e3004a3
• https://git.kernel.org/stable/c/ff0ca46b13b9ef6edbcd238a3b6caacfef8ba0e5
• https://git.kernel.org/stable/c/b78db9bddc84136f6a0bb49e8883cf200dfb87a8
• https://git.kernel.org/stable/c/0a8cf165566ba55a39fd0f4de172119dd646d39a
• https://access.redhat.com/security/cve/CVE-2026-31709 vdb-entry, x_refsource_REDHAT
• RHBZ#2464476 issue-tracking, x_refsource_REDHAT
• https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-31709.json x_sadp-csaf-vex
• https://access.redhat.com/errata/RHSA-2026:24343 vendor-advisory, x_refsource_REDHAT
• https://access.redhat.com/errata/RHSA-2026:23329 vendor-advisory, x_refsource_REDHAT
• https://access.redhat.com/errata/RHSA-2026:22940 vendor-advisory, x_refsource_REDHAT
• https://access.redhat.com/errata/RHSA-2026:23237 vendor-advisory, x_refsource_REDHAT
• https://access.redhat.com/errata/RHSA-2026:23224 vendor-advisory, x_refsource_REDHAT
• https://access.redhat.com/errata/RHSA-2026:21556 vendor-advisory, x_refsource_REDHAT
• https://access.redhat.com/errata/RHSA-2026:21706 vendor-advisory, x_refsource_REDHAT
• https://access.redhat.com/errata/RHSA-2026:21745 vendor-advisory, x_refsource_REDHAT
• https://access.redhat.com/errata/RHSA-2026:22900 vendor-advisory, x_refsource_REDHAT
• https://cert-portal.siemens.com/productcert/html/ssa-784472.html