The Linux kernel's ksmbd smb2_ioctl() function's FSCTL_DUPLICATE_EXTENTS_TO_FILE operation previously permitted overwriting destination file data via vfs_clone_file_range() without enforcing share-level writable flags or per-handle write access checks. This omission allowed clients to overwrite files on read-only shares or with handles opened with only FILE_WRITE_ATTRIBUTES, which should not permit data writes. The fix introduces both share-level and per-handle permission checks, aligning with the FSCTL_SET_SPARSE permission model, and mandates FILE_WRITE_DATA access for the clone operation.

Clients could overwrite file data on read-only SMB shares or via handles that should not have write permissions, potentially leading to unauthorized data modification. This could compromise data integrity on affected systems running vulnerable Linux kernel versions with ksmbd enabled.

A fix has been implemented in the Linux kernel to add the missing permission checks for FSCTL_DUPLICATE_EXTENTS_TO_FILE in ksmbd. Users should apply the official kernel updates that include this patch. Patch status is not explicitly confirmed in the provided data; therefore, check the Linux kernel vendor advisory for the current remediation guidance and apply updates accordingly.