LoadPin: Allow filesystem switch when not enforcing

For LoadPin to be used at all in a classic distro environment, it needs
to allow for switching filesystems (from the initramfs to the "real"
root filesystem). To allow for this, if the "enforce" mode is not set at
boot, reset the pinned filesystem tracking when the pinned filesystem
gets unmounted instead of invalidating further loads. Once enforcement
is set, it cannot be unset, and the pinning will stick.

This means that distros can build with CONFIG_SECURITY_LOADPIN=y, but with
CONFIG_SECURITY_LOADPIN_ENFORCE disabled, but after boot is running,
the system can enable enforcement:

  $ sysctl -w kernel.loadpin.enforced=1

Cc: Paul Moore <paul@paul-moore.com>
Cc: James Morris <jmorris@namei.org>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: linux-security-module@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Serge Hallyn <serge@hallyn.com>
Link: https://lore.kernel.org/r/20221209195746.1366607-4-keescook@chromium.org

security/loadpin/loadpin.c

@@ -119,11 +119,16 @@ static void loadpin_sb_free_security(struct super_block *mnt_sb)
 	/*
 	 * When unmounting the filesystem we were using for load
 	 * pinning, we acknowledge the superblock release, but make sure
-	 * no other modules or firmware can be loaded.
+	 * no other modules or firmware can be loaded when we are in
+	 * enforcing mode. Otherwise, allow the root to be reestablished.
 	 */
 	if (!IS_ERR_OR_NULL(pinned_root) && mnt_sb == pinned_root) {
-		pinned_root = ERR_PTR(-EIO);
-		pr_info("umount pinned fs: refusing further loads\n");
+		if (enforce) {
+			pinned_root = ERR_PTR(-EIO);
+			pr_info("umount pinned fs: refusing further loads\n");
+		} else {
+			pinned_root = NULL;
+		}
 	}
 }
 
@@ -158,8 +163,9 @@ static int loadpin_check(struct file *file, enum kernel_read_file_id id)
 	/* First loaded module/firmware defines the root for all others. */
 	spin_lock(&pinned_root_spinlock);
 	/*
-	 * pinned_root is only NULL at startup. Otherwise, it is either
-	 * a valid reference, or an ERR_PTR.
+	 * pinned_root is only NULL at startup or when the pinned root has
+	 * been unmounted while we are not in enforcing mode. Otherwise, it
+	 * is either a valid reference, or an ERR_PTR.
 	 */
 	if (!pinned_root) {
 		pinned_root = load_root;