scsi: target: Fix SELinux error when systemd-modules loads the target module

If the systemd-modules service loads the target module, the credentials of
that userspace process will be used to validate the access to the target db
directory. SELinux will prevent it, reporting an error like the following:

kernel: audit: type=1400 audit(1676301082.205:4): avc: denied { read }
for pid=1020 comm="systemd-modules" name="target" dev="dm-3"
ino=4657583 scontext=system_u:system_r:systemd_modules_load_t:s0
tcontext=system_u:object_r:targetd_etc_rw_t:s0 tclass=dir permissive=0

Fix the error by using the kernel credentials to access the db directory

Signed-off-by: Maurizio Lombardi <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Reviewed-by: Mike Christie <[e-mail]>
Signed-off-by: Martin K. Petersen <[e-mail]>

drivers/target/target_core_configfs.c

···
 {
 	struct configfs_subsystem *subsys = &target_core_fabrics;
 	struct t10_alua_lu_gp *lu_gp;
+	struct cred *kern_cred;
+	const struct cred *old_cred;
 	int ret;
 
 	pr_debug("TARGET_CORE[0]: Loading Generic Kernel Storage"
···
 	if (ret < 0)
 		goto out;
 
+	/* We use the kernel credentials to access the target directory */
+	kern_cred = prepare_kernel_cred(&init_task);
+	if (!kern_cred) {
+		ret = -ENOMEM;
+		goto out;
+	}
+	old_cred = override_creds(kern_cred);
 	target_init_dbroot();
+	revert_creds(old_cred);
+	put_cred(kern_cred);
 
 	return 0;
 
 out:
+	target_xcopy_release_pt();
 	configfs_unregister_subsystem(subsys);
 	core_dev_release_virtual_lun0();
 	rd_module_exit();