linux-kernel - [PATCH -next v2 2/6] landlock: abstract walk_to_visible

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Date:   Sat, 27 Aug 2022 19:12:11 +0800
From:   Xiu Jianfeng <xiujianfeng@...wei.com>
To:     <mic@...ikod.net>, <paul@...l-moore.com>, <jmorris@...ei.org>,
        <serge@...lyn.com>, <shuah@...nel.org>, <corbet@....net>
CC:     <linux-security-module@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <linux-kselftest@...r.kernel.org>,
        <linux-doc@...r.kernel.org>
Subject: [PATCH -next v2 2/6] landlock: abstract walk_to_visible_parent() helper

This helper will be used in the next commit which supports chmod and
chown access rights restriction.

Signed-off-by: Xiu Jianfeng <xiujianfeng@...wei.com>
---
 security/landlock/fs.c | 67 ++++++++++++++++++++++++++++++------------
 1 file changed, 49 insertions(+), 18 deletions(-)

diff --git a/security/landlock/fs.c b/security/landlock/fs.c
index c57f581a9cd5..4ef614a4ea22 100644
--- a/security/landlock/fs.c
+++ b/security/landlock/fs.c
@@ -38,6 +38,44 @@
 #include "ruleset.h"
 #include "setup.h"
 
+enum walk_result {
+	WALK_CONTINUE,
+	WALK_TO_REAL_ROOT,
+	WALK_TO_DISCONN_ROOT,
+};
+
+/*
+ * walk to the visible parent, caller should call path_get()/path_put()
+ * before/after this helpler.
+ *
+ * Returns:
+ * - WALK_TO_REAL_ROOT if walk to the real root;
+ * - WALK_TO_DISCONN_ROOT if walk to disconnected root;
+ * - WALK_CONTINUE otherwise.
+ */
+static enum walk_result walk_to_visible_parent(struct path *path)
+{
+	struct dentry *parent_dentry;
+jump_up:
+	if (path->dentry == path->mnt->mnt_root) {
+		if (follow_up(path)) {
+			/* Ignores hidden mount points. */
+			goto jump_up;
+		} else {
+			/* Stop at the real root. */
+			return WALK_TO_REAL_ROOT;
+		}
+	}
+	/* Stops at disconnected root directories. */
+	if (unlikely(IS_ROOT(path->dentry)))
+		return WALK_TO_DISCONN_ROOT;
+	parent_dentry = dget_parent(path->dentry);
+	dput(path->dentry);
+	path->dentry = parent_dentry;
+
+	return WALK_CONTINUE;
+}
+
 /* Underlying object management */
 
 static void release_inode(struct landlock_object *const object)
@@ -539,8 +577,8 @@ static int check_access_path_dual(
 	 * restriction.
 	 */
 	while (true) {
-		struct dentry *parent_dentry;
 		const struct landlock_rule *rule;
+		enum walk_result wr;
 
 		/*
 		 * If at least all accesses allowed on the destination are
@@ -588,20 +626,12 @@ static int check_access_path_dual(
 		if (allowed_parent1 && allowed_parent2)
 			break;
 
-jump_up:
-		if (walker_path.dentry == walker_path.mnt->mnt_root) {
-			if (follow_up(&walker_path)) {
-				/* Ignores hidden mount points. */
-				goto jump_up;
-			} else {
-				/*
-				 * Stops at the real root.  Denies access
-				 * because not all layers have granted access.
-				 */
-				break;
-			}
-		}
-		if (unlikely(IS_ROOT(walker_path.dentry))) {
+		wr = walk_to_visible_parent(&walker_path);
+		switch (wr) {
+		case WALK_TO_REAL_ROOT:
+			/* Stop at the real root. */
+			goto out;
+		case WALK_TO_DISCONN_ROOT:
 			/*
 			 * Stops at disconnected root directories.  Only allows
 			 * access to internal filesystems (e.g. nsfs, which is
@@ -609,12 +639,13 @@ static int check_access_path_dual(
 			 */
 			allowed_parent1 = allowed_parent2 =
 				!!(walker_path.mnt->mnt_flags & MNT_INTERNAL);
+			goto out;
+		case WALK_CONTINUE:
+		default:
 			break;
 		}
-		parent_dentry = dget_parent(walker_path.dentry);
-		dput(walker_path.dentry);
-		walker_path.dentry = parent_dentry;
 	}
+out:
 	path_put(&walker_path);
 
 	if (allowed_parent1 && allowed_parent2)
-- 
2.17.1