[PATCH] ext2 xattr handler for security modules

From: Stephen Smalley <sds@epoch.ncsc.mil>

This patch against 2.5.68 implements an xattr handler for ext2 to support the
use of extended attributes by security modules for storing file security
labels.  As per the earlier discussion of extended attributes for security
modules, this handler uses a "security." prefix and allows for per-module
attribute names.  Security checking on userspace access to these attributes
can be performed by the security module using the LSM hooks in fs/xattr.c,
and the security module is free to internally use the inode operations
without restriction for managing its security labels.  Unlike the trusted
namespace, these labels are used internally for access control purposes by
the security module, and controls over userspace access to them require finer
granularity than capable() supports.

fs/Kconfig

@@ -73,6 +73,18 @@ config EXT2_FS_POSIX_ACL
 
 	  If you don't know what Access Control Lists are, say N
 
+config EXT2_FS_SECURITY
+	bool "Ext2 Security Labels"
+	depends on EXT2_FS_XATTR
+	help
+	  Security labels support alternative access control models
+	  implemented by security modules like SELinux.  This option
+	  enables an extended attribute handler for file security
+	  labels in the ext2 filesystem.
+
+	  If you are not using a security module that requires using
+	  extended attributes for file security labels, say N.
+
 config EXT3_FS
 	tristate "Ext3 journalling file system support"
 	help

fs/ext2/Makefile

@@ -14,3 +14,7 @@ endif
 ifeq ($(CONFIG_EXT2_FS_POSIX_ACL),y)
 ext2-objs += acl.o
 endif
+
+ifeq ($(CONFIG_EXT2_FS_SECURITY),y)
+ext2-objs += xattr_security.o
+endif

fs/ext2/xattr.c

@@ -1102,22 +1102,33 @@ init_ext2_xattr(void)
 				  &ext2_xattr_trusted_handler);
 	if (err)
 		goto out;
+#ifdef CONFIG_EXT2_FS_SECURITY
+	err = ext2_xattr_register(EXT2_XATTR_INDEX_SECURITY,
+				  &ext2_xattr_security_handler);
+	if (err)
+		goto out1;
+#endif
 #ifdef CONFIG_EXT2_FS_POSIX_ACL
 	err = init_ext2_acl();
 	if (err)
-		goto out1;
+		goto out2;
 #endif
 	ext2_xattr_cache = mb_cache_create("ext2_xattr", NULL,
 		sizeof(struct mb_cache_entry) +
 		sizeof(struct mb_cache_entry_index), 1, 6);
 	if (!ext2_xattr_cache) {
 		err = -ENOMEM;
-		goto out2;
+		goto out3;
 	}
 	return 0;
-out2:
+out3:
 #ifdef CONFIG_EXT2_FS_POSIX_ACL
 	exit_ext2_acl();
+out2:
+#endif
+#ifdef CONFIG_EXT2_FS_SECURITY
+	ext2_xattr_unregister(EXT2_XATTR_INDEX_SECURITY,
+			      &ext2_xattr_security_handler);
 out1:
 #endif
 	ext2_xattr_unregister(EXT2_XATTR_INDEX_TRUSTED,
@@ -1134,6 +1145,10 @@ exit_ext2_xattr(void)
 	mb_cache_destroy(ext2_xattr_cache);
 #ifdef CONFIG_EXT2_FS_POSIX_ACL
 	exit_ext2_acl();
+#endif
+#ifdef CONFIG_EXT2_FS_SECURITY
+	ext2_xattr_unregister(EXT2_XATTR_INDEX_SECURITY,
+			      &ext2_xattr_security_handler);
 #endif
 	ext2_xattr_unregister(EXT2_XATTR_INDEX_TRUSTED,
 			      &ext2_xattr_trusted_handler);

fs/ext2/xattr.h

@@ -22,6 +22,7 @@
 #define EXT2_XATTR_INDEX_POSIX_ACL_ACCESS	2
 #define EXT2_XATTR_INDEX_POSIX_ACL_DEFAULT	3
 #define EXT2_XATTR_INDEX_TRUSTED		4
+#define EXT2_XATTR_INDEX_SECURITY	        6
 
 struct ext2_xattr_header {
 	__u32	h_magic;	/* magic number for identification */
@@ -134,4 +135,5 @@ exit_ext2_xattr(void)
 
 extern struct ext2_xattr_handler ext2_xattr_user_handler;
 extern struct ext2_xattr_handler ext2_xattr_trusted_handler;
+extern struct ext2_xattr_handler ext2_xattr_security_handler;
 

fs/ext2/xattr_security.c

+/*
+ * linux/fs/ext2/xattr_security.c
+ * Handler for storing security labels as extended attributes.
+ */
+
+#include <linux/module.h>
+#include <linux/string.h>
+#include <linux/fs.h>
+#include <linux/smp_lock.h>
+#include <linux/ext2_fs.h>
+#include "xattr.h"
+
+#define XATTR_SECURITY_PREFIX "security."
+
+static size_t
+ext2_xattr_security_list(char *list, struct inode *inode,
+			const char *name, int name_len)
+{
+	const int prefix_len = sizeof(XATTR_SECURITY_PREFIX)-1;
+
+	if (list) {
+		memcpy(list, XATTR_SECURITY_PREFIX, prefix_len);
+		memcpy(list+prefix_len, name, name_len);
+		list[prefix_len + name_len] = '\0';
+	}
+	return prefix_len + name_len + 1;
+}
+
+static int
+ext2_xattr_security_get(struct inode *inode, const char *name,
+		       void *buffer, size_t size)
+{
+	if (strcmp(name, "") == 0)
+		return -EINVAL;
+	return ext2_xattr_get(inode, EXT2_XATTR_INDEX_SECURITY, name,
+			      buffer, size);
+}
+
+static int
+ext2_xattr_security_set(struct inode *inode, const char *name,
+		       const void *value, size_t size, int flags)
+{
+	if (strcmp(name, "") == 0)
+		return -EINVAL;
+	return ext2_xattr_set(inode, EXT2_XATTR_INDEX_SECURITY, name,
+			      value, size, flags);
+}
+
+struct ext2_xattr_handler ext2_xattr_security_handler = {
+	.prefix	= XATTR_SECURITY_PREFIX,
+	.list	= ext2_xattr_security_list,
+	.get	= ext2_xattr_security_get,
+	.set	= ext2_xattr_security_set,
+};