CVE-2026-31709

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.