CVE-2026-52944

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

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

FSCTL_SET_SPARSE in fsctl_set_sparse() 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

test_tree_conn_flag(work->tcon, KSMBD_TREE_CONN_FLAG_WRITABLE),

but FSCTL_SET_SPARSE does not.

2) Even on writable shares, clients without FILE_WRITE_DATA or

FILE_WRITE_ATTRIBUTES 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.