firmware: qemu_fw_cfg: fix sysfs information leak

Make sure to always NUL-terminate file names retrieved from the firmware
to avoid accessing data beyond the entry slab buffer and exposing it
through sysfs in case the firmware data is corrupt.

Fixes: 75f3e8e4 ("firmware: introduce sysfs driver for QEMU's fw_cfg device")
Cc: stable@vger.kernel.org      # 4.6
Cc: Gabriel Somlo <somlo@cmu.edu>
Signed-off-by: NJohan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20211201132528.30025-4-johan@kernel.orgSigned-off-by: NMichael S. Tsirkin <mst@redhat.com>

drivers/firmware/qemu_fw_cfg.c

@@ -598,7 +598,7 @@ static int fw_cfg_register_file(const struct fw_cfg_file *f)
 	/* set file entry information */
 	entry->size = be32_to_cpu(f->size);
 	entry->select = be16_to_cpu(f->select);
-	memcpy(entry->name, f->name, FW_CFG_MAX_FILE_PATH);
+	strscpy(entry->name, f->name, FW_CFG_MAX_FILE_PATH);
 
 	/* register entry under "/sys/firmware/qemu_fw_cfg/by_key/" */
 	err = kobject_init_and_add(&entry->kobj, &fw_cfg_sysfs_entry_ktype,