CVE-2026-52944

In the Linux kernel, the following vulnerability has been resolved:

ksmbd: fix FSCTL permission bypass by adding a permission check for FSCTLSETSPARSE

FSCTLSETSPARSE in fsctlsetsparse() modifies the file's sparse attribute and saves it through xattr without any permission checks.

This exposes two issues:

1) A client on a read-only share can change the sparse attribute on files it opened, even though the share is read-only. Other FSCTL write operations already check testtreeconnflag(work->tcon, KSMBDTREECONNFLAGWRITABLE), but FSCTLSET_SPARSE does not.

2) Even on writable shares, clients without FILEWRITEDATA or FILEWRITEATTRIBUTES access should not modify the sparse attribute. Similar handle-level checks exist in other functions but are missing here.

Add both share-level writable check and per-handle access check. Use goto out on error to avoid leaking file references.