[PATCH v4 03/14] digest_cache: Add securityfs interface

[Roberto Sassu]

From: Roberto Sassu <roberto.sassu at huawei.com>

Add the digest_cache_path file in securityfs, to let root change/read the
default path (file or directory) from where digest lists are looked up.

An RW semaphore prevents the default path from changing while
digest_list_new() and read_default_path() are executed, so that those read
a stable value. Multiple digest_list_new() and read_default_path() calls,
instead, can be done in parallel, since they are the readers.

Changing the default path does not affect digest caches created with the
old path.

Signed-off-by: Roberto Sassu <roberto.sassu at huawei.com>
---
 security/digest_cache/Kconfig    |  4 ++
 security/digest_cache/Makefile   |  2 +-
 security/digest_cache/internal.h |  1 +
 security/digest_cache/main.c     | 10 +++-
 security/digest_cache/secfs.c    | 87 ++++++++++++++++++++++++++++++++
 5 files changed, 102 insertions(+), 2 deletions(-)
 create mode 100644 security/digest_cache/secfs.c

diff --git a/security/digest_cache/Kconfig b/security/digest_cache/Kconfig
index e53fbf0779d6..dfabe5d6e3ca 100644
--- a/security/digest_cache/Kconfig
+++ b/security/digest_cache/Kconfig
@@ -14,3 +14,7 @@ config DIGEST_LIST_DEFAULT_PATH
 	default "/etc/digest_lists"
 	help
 	  Default directory where digest_cache LSM expects to find digest lists.
+
+	  It can be changed at run-time, by writing the new path to the
+	  securityfs interface. Digest caches created with the old path are
+	  not affected by the change.
diff --git a/security/digest_cache/Makefile b/security/digest_cache/Makefile
index 48848c41253e..1330655e33b1 100644
--- a/security/digest_cache/Makefile
+++ b/security/digest_cache/Makefile
@@ -4,4 +4,4 @@
 
 obj-$(CONFIG_SECURITY_DIGEST_CACHE) += digest_cache.o
 
-digest_cache-y := main.o
+digest_cache-y := main.o secfs.o
diff --git a/security/digest_cache/internal.h b/security/digest_cache/internal.h
index 5f04844af3a5..bbf5eefe5c82 100644
--- a/security/digest_cache/internal.h
+++ b/security/digest_cache/internal.h
@@ -49,6 +49,7 @@ struct digest_cache_security {
 
 extern struct lsm_blob_sizes digest_cache_blob_sizes;
 extern char *default_path_str;
+extern struct rw_semaphore default_path_sem;
 
 static inline struct digest_cache_security *
 digest_cache_get_security(const struct inode *inode)
diff --git a/security/digest_cache/main.c b/security/digest_cache/main.c
index 14dba8915e99..661c8d106791 100644
--- a/security/digest_cache/main.c
+++ b/security/digest_cache/main.c
@@ -18,6 +18,9 @@ static struct kmem_cache *digest_cache_cache __read_mostly;
 
 char *default_path_str = CONFIG_DIGEST_LIST_DEFAULT_PATH;
 
+/* Protects default_path_str. */
+struct rw_semaphore default_path_sem;
+
 /**
  * digest_cache_alloc_init - Allocate and initialize a new digest cache
  * @path_str: Path string of the digest list
@@ -274,9 +277,12 @@ struct digest_cache *digest_cache_get(struct dentry *dentry)
 
 	/* Serialize accesses to inode for which the digest cache is used. */
 	mutex_lock(&dig_sec->dig_user_mutex);
-	if (!dig_sec->dig_user)
+	if (!dig_sec->dig_user) {
+		down_read(&default_path_sem);
 		/* Consume extra reference from digest_cache_create(). */
 		dig_sec->dig_user = digest_cache_new(dentry);
+		up_read(&default_path_sem);
+	}
 
 	if (dig_sec->dig_user)
 		/* Increment ref. count for reference returned to the caller. */
@@ -386,6 +392,8 @@ static const struct lsm_id digest_cache_lsmid = {
  */
 static int __init digest_cache_init(void)
 {
+	init_rwsem(&default_path_sem);
+
 	digest_cache_cache = kmem_cache_create("digest_cache_cache",
 					       sizeof(struct digest_cache),
 					       0, SLAB_PANIC,
diff --git a/security/digest_cache/secfs.c b/security/digest_cache/secfs.c
new file mode 100644
index 000000000000..d3a37bf3588e
--- /dev/null
+++ b/security/digest_cache/secfs.c
@@ -0,0 +1,87 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2023-2024 Huawei Technologies Duesseldorf GmbH
+ *
+ * Author: Roberto Sassu <roberto.sassu at huawei.com>
+ *
+ * Implement the securityfs interface of the digest_cache LSM.
+ */
+
+#define pr_fmt(fmt) "DIGEST CACHE: "fmt
+#include <linux/security.h>
+
+#include "internal.h"
+
+static struct dentry *default_path_dentry;
+
+/**
+ * write_default_path - Write default path
+ * @file: File descriptor of the securityfs file
+ * @buf: User space buffer
+ * @datalen: Amount of data to write
+ * @ppos: Current position in the file
+ *
+ * This function sets the new default path where digest lists can be found.
+ * Can be either a regular file or a directory.
+ *
+ * Return: Length of path written on success, a POSIX error code otherwise.
+ */
+static ssize_t write_default_path(struct file *file, const char __user *buf,
+				  size_t datalen, loff_t *ppos)
+{
+	char *new_default_path_str;
+
+	new_default_path_str = memdup_user_nul(buf, datalen);
+	if (IS_ERR(new_default_path_str))
+		return PTR_ERR(new_default_path_str);
+
+	down_write(&default_path_sem);
+	kfree_const(default_path_str);
+	default_path_str = new_default_path_str;
+	up_write(&default_path_sem);
+	return datalen;
+}
+
+/**
+ * read_default_path - Read default path
+ * @file: File descriptor of the securityfs file
+ * @buf: User space buffer
+ * @datalen: Amount of data to read
+ * @ppos: Current position in the file
+ *
+ * This function returns the current default path where digest lists can be
+ * found. Can be either a regular file or a directory.
+ *
+ * Return: Length of path read on success, a POSIX error code otherwise.
+ */
+static ssize_t read_default_path(struct file *file, char __user *buf,
+				 size_t datalen, loff_t *ppos)
+{
+	int ret;
+
+	down_read(&default_path_sem);
+	ret = simple_read_from_buffer(buf, datalen, ppos, default_path_str,
+				      strlen(default_path_str) + 1);
+	up_read(&default_path_sem);
+	return ret;
+}
+
+static const struct file_operations default_path_ops = {
+	.open = generic_file_open,
+	.write = write_default_path,
+	.read = read_default_path,
+	.llseek = generic_file_llseek,
+};
+
+static int __init digest_cache_path_init(void)
+{
+	default_path_dentry = securityfs_create_file("digest_cache_path", 0660,
+						     NULL, NULL,
+						     &default_path_ops);
+	if (IS_ERR(default_path_dentry))
+		return -EFAULT;
+
+	return 0;
+}
+
+late_initcall(digest_cache_path_init);
-- 
2.34.1