ksmbd: use opener credentials for delete-on-close

commit 52e2f21 upstream.

Delete-on-close can be completed by deferred or durable handle teardown,
where no request work is available. Both the base-file unlink and the ADS
xattr removal consequently run with the ksmbd worker credentials and can
bypass filesystem permission checks.

Run both operations with the credentials captured in struct file when the
handle was opened. This preserves the authenticated user's fsuid, fsgid,
supplementary groups and capability restrictions at final close.

Cc: stable@vger.kernel.org
Reported-by: Musaab Khan <musaab.khan@protonmail.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: namjaejeon

fs/smb/server/vfs.c

@@ -1084,13 +1084,15 @@ int ksmbd_vfs_remove_xattr(struct mnt_idmap *idmap,
 
 int ksmbd_vfs_unlink(struct file *filp)
 {
+	const struct cred *saved_cred;
 	int err = 0;
 	struct dentry *dir, *dentry = filp->f_path.dentry;
 	struct mnt_idmap *idmap = file_mnt_idmap(filp);
 
+	saved_cred = override_creds(filp->f_cred);
 	err = mnt_want_write(filp->f_path.mnt);
 	if (err)
-		return err;
+		goto out_revert;
 
 	dir = dget_parent(dentry);
 	err = ksmbd_vfs_lock_parent(dir, dentry);
@@ -1110,7 +1112,8 @@ int ksmbd_vfs_unlink(struct file *filp)
 out:
 	dput(dir);
 	mnt_drop_write(filp->f_path.mnt);
-
+out_revert:
+	revert_creds(saved_cred);
 	return err;
 }
 

fs/smb/server/vfs_cache.c

@@ -292,10 +292,14 @@ static void __ksmbd_inode_close(struct ksmbd_file *fp)
 		up_write(&ci->m_lock);
 
 		if (remove_stream_xattr) {
+			const struct cred *saved_cred;
+
+			saved_cred = override_creds(filp->f_cred);
 			err = ksmbd_vfs_remove_xattr(file_mnt_idmap(filp),
 						     &filp->f_path,
 						     fp->stream.name,
 						     true);
+			revert_creds(saved_cred);
 			if (err)
 				pr_err("remove xattr failed : %s\n",
 				       fp->stream.name);