s390/hypfs: Enable limited access during lockdown

When kernel lockdown is active, debugfs_locked_down() blocks access to
hypfs files that register ioctl callbacks, even if the ioctl interface
is not required for a function. This unnecessarily breaks userspace
tools that only rely on read operations.

Resolve this by registering a minimal set of file operations during
lockdown, avoiding ioctl registration and preserving access for affected
tooling.

Note that this change restores hypfs functionality when lockdown is
active from early boot (e.g. via lockdown=integrity kernel parameter),
but does not apply to scenarios where lockdown is enabled dynamically
while Linux is running.

Tested-by: Mete Durlu <[email protected]>
Reviewed-by: Vasily Gorbik <[email protected]>
Fixes: 5496197 ("debugfs: Restrict debugfs when the kernel is locked down")
Signed-off-by: Peter Oberparleiter <[email protected]>
Signed-off-by: Alexander Gordeev <[email protected]>

Author: oberpar

arch/s390/hypfs/hypfs_dbfs.c

@@ -6,6 +6,7 @@
  * Author(s): Michael Holzheu <[email protected]>
  */
 
+#include <linux/security.h>
 #include <linux/slab.h>
 #include "hypfs.h"
 
@@ -84,7 +85,7 @@ void hypfs_dbfs_create_file(struct hypfs_dbfs_file *df)
 {
 	const struct file_operations *fops = &dbfs_ops;
 
-	if (df->unlocked_ioctl)
+	if (df->unlocked_ioctl && !security_locked_down(LOCKDOWN_DEBUGFS))
 		fops = &dbfs_ops_ioctl;
 	df->dentry = debugfs_create_file(df->name, 0400, dbfs_dir, df, fops);
 	mutex_init(&df->lock);