Landlock fix for v6.10-rc2

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Merge tag 'landlock-6.10-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/mic/linux

Pull landlock fix from Mickaël Salaün:
 "This fixes a wrong path walk triggered by syzkaller"

* tag 'landlock-6.10-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/mic/linux:
  selftests/landlock: Add layout1.refer_mount_root
  landlock: Fix d_parent walk

security/landlock/fs.c

@@ -1110,6 +1110,7 @@ static int current_check_refer_path(struct dentry *const old_dentry,
 	bool allow_parent1, allow_parent2;
 	access_mask_t access_request_parent1, access_request_parent2;
 	struct path mnt_dir;
+	struct dentry *old_parent;
 	layer_mask_t layer_masks_parent1[LANDLOCK_NUM_ACCESS_FS] = {},
 		     layer_masks_parent2[LANDLOCK_NUM_ACCESS_FS] = {};
 
@@ -1157,9 +1158,17 @@ static int current_check_refer_path(struct dentry *const old_dentry,
 	mnt_dir.mnt = new_dir->mnt;
 	mnt_dir.dentry = new_dir->mnt->mnt_root;
 
+	/*
+	 * old_dentry may be the root of the common mount point and
+	 * !IS_ROOT(old_dentry) at the same time (e.g. with open_tree() and
+	 * OPEN_TREE_CLONE).  We do not need to call dget(old_parent) because
+	 * we keep a reference to old_dentry.
+	 */
+	old_parent = (old_dentry == mnt_dir.dentry) ? old_dentry :
+						      old_dentry->d_parent;
+
 	/* new_dir->dentry is equal to new_dentry->d_parent */
-	allow_parent1 = collect_domain_accesses(dom, mnt_dir.dentry,
+	allow_parent1 = collect_domain_accesses(dom, mnt_dir.dentry, old_parent,
-						old_dentry->d_parent,
 						&layer_masks_parent1);
 	allow_parent2 = collect_domain_accesses(
 		dom, mnt_dir.dentry, new_dir->dentry, &layer_masks_parent2);

tools/testing/selftests/landlock/fs_test.c

@@ -35,6 +35,7 @@
  * See https://sourceware.org/glibc/wiki/Synchronizing_Headers.
  */
 #include <linux/fs.h>
+#include <linux/mount.h>
 
 #include "common.h"
 
@@ -47,6 +48,13 @@ int renameat2(int olddirfd, const char *oldpath, int newdirfd,
 }
 #endif
 
+#ifndef open_tree
+int open_tree(int dfd, const char *filename, unsigned int flags)
+{
+	return syscall(__NR_open_tree, dfd, filename, flags);
+}
+#endif
+
 #ifndef RENAME_EXCHANGE
 #define RENAME_EXCHANGE (1 << 1)
 #endif
@@ -2400,6 +2408,43 @@ TEST_F_FORK(layout1, refer_denied_by_default4)
 				layer_dir_s1d1_refer);
 }
 
+/*
+ * Tests walking through a denied root mount.
+ */
+TEST_F_FORK(layout1, refer_mount_root_deny)
+{
+	const struct landlock_ruleset_attr ruleset_attr = {
+		.handled_access_fs = LANDLOCK_ACCESS_FS_MAKE_DIR,
+	};
+	int root_fd, ruleset_fd;
+
+	/* Creates a mount object from a non-mount point. */
+	set_cap(_metadata, CAP_SYS_ADMIN);
+	root_fd =
+		open_tree(AT_FDCWD, dir_s1d1,
+			  AT_EMPTY_PATH | OPEN_TREE_CLONE | OPEN_TREE_CLOEXEC);
+	clear_cap(_metadata, CAP_SYS_ADMIN);
+	ASSERT_LE(0, root_fd);
+
+	ruleset_fd =
+		landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);
+	ASSERT_LE(0, ruleset_fd);
+
+	ASSERT_EQ(0, prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0));
+	ASSERT_EQ(0, landlock_restrict_self(ruleset_fd, 0));
+	EXPECT_EQ(0, close(ruleset_fd));
+
+	/* Link denied by Landlock: EACCES. */
+	EXPECT_EQ(-1, linkat(root_fd, ".", root_fd, "does_not_exist", 0));
+	EXPECT_EQ(EACCES, errno);
+
+	/* renameat2() always returns EBUSY. */
+	EXPECT_EQ(-1, renameat2(root_fd, ".", root_fd, "does_not_exist", 0));
+	EXPECT_EQ(EBUSY, errno);
+
+	EXPECT_EQ(0, close(root_fd));
+}
+
 TEST_F_FORK(layout1, reparent_link)
 {
 	const struct rule layer1[] = {