evm: posix acls modify i_mode

The posix xattr acls are 'system' prefixed, which normally would not
affect security.evm. An interesting side affect of writing posix xattr
acls is their modifying of the i_mode, which is included in security.evm.

This patch updates security.evm when posix xattr acls are written.

Signed-off-by: Mimi Zohar <[e-mail]>

include/linux/evm.h

···
 extern int evm_inode_init_security(struct inode *inode,
 				   const struct xattr *xattr_array,
 				   struct xattr *evm);
+#ifdef CONFIG_FS_POSIX_ACL
+extern int posix_xattr_acl(const char *xattrname);
+#else
+static inline int posix_xattr_acl(const char *xattrname)
+{
+	return 0;
+}
+#endif
 #else
 #ifdef CONFIG_INTEGRITY
 static inline enum integrity_status evm_verifyxattr(struct dentry *dentry,

include/linux/xattr.h

···
 #define XATTR_CAPS_SUFFIX "capability"
 #define XATTR_NAME_CAPS XATTR_SECURITY_PREFIX XATTR_CAPS_SUFFIX
 
+#define XATTR_POSIX_ACL_ACCESS  "posix_acl_access"
+#define XATTR_NAME_POSIX_ACL_ACCESS XATTR_SYSTEM_PREFIX XATTR_POSIX_ACL_ACCESS
+#define XATTR_POSIX_ACL_DEFAULT  "posix_acl_default"
+#define XATTR_NAME_POSIX_ACL_DEFAULT XATTR_SYSTEM_PREFIX XATTR_POSIX_ACL_DEFAULT
+
 #ifdef  __KERNEL__
 
 #include <linux/types.h>

security/integrity/evm/Makefile

···
 obj-$(CONFIG_EVM) += evm.o
 
 evm-y := evm_main.o evm_crypto.o evm_secfs.o
+evm-$(CONFIG_FS_POSIX_ACL) += evm_posix_acl.o

security/integrity/evm/evm_main.c

···
 /*
  * evm_protect_xattr - protect the EVM extended attribute
  *
- * Prevent security.evm from being modified or removed.
+ * Prevent security.evm from being modified or removed without the
+ * necessary permissions or when the existing value is invalid.
+ *
+ * The posix xattr acls are 'system' prefixed, which normally would not
+ * affect security.evm.  An interesting side affect of writing posix xattr
+ * acls is their modifying of the i_mode, which is included in security.evm.
+ * For posix xattr acls only, permit security.evm, even if it currently
+ * doesn't exist, to be updated.
  */
 static int evm_protect_xattr(struct dentry *dentry, const char *xattr_name,
 			     const void *xattr_value, size_t xattr_value_len)
···
 	if (strcmp(xattr_name, XATTR_NAME_EVM) == 0) {
 		if (!capable(CAP_SYS_ADMIN))
 			return -EPERM;
-	} else if (!evm_protected_xattr(xattr_name))
-		return 0;
-
+	} else if (!evm_protected_xattr(xattr_name)) {
+		if (!posix_xattr_acl(xattr_name))
+			return 0;
+		evm_status = evm_verify_current_integrity(dentry);
+		if ((evm_status == INTEGRITY_PASS) ||
+			(evm_status == INTEGRITY_NOLABEL))
+			return 0;
+		return -EPERM;
+	}
 	evm_status = evm_verify_current_integrity(dentry);
 	return evm_status == INTEGRITY_PASS ? 0 : -EPERM;
 }
···
 void evm_inode_post_setxattr(struct dentry *dentry, const char *xattr_name,
 			     const void *xattr_value, size_t xattr_value_len)
 {
-	if (!evm_initialized || !evm_protected_xattr(xattr_name))
+	if (!evm_initialized || (!evm_protected_xattr(xattr_name)
+				 && !posix_xattr_acl(xattr_name)))
 		return;
 
 	evm_update_evmxattr(dentry, xattr_name, xattr_value, xattr_value_len);

security/integrity/evm/evm_posix_acl.c

···
+/*
+ * Copyright (C) 2011 IBM Corporation
+ *
+ * Author:
+ * Mimi Zohar <[e-mail]>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, version 2 of the License.
+ */
+
+#include <linux/module.h>
+#include <linux/xattr.h>
+
+int posix_xattr_acl(char *xattr)
+{
+	int xattr_len = strlen(xattr);
+
+	if ((strlen(XATTR_NAME_POSIX_ACL_ACCESS) == xattr_len)
+	     && (strncmp(XATTR_NAME_POSIX_ACL_ACCESS, xattr, xattr_len) == 0))
+		return 1;
+	if ((strlen(XATTR_NAME_POSIX_ACL_DEFAULT) == xattr_len)
+	     && (strncmp(XATTR_NAME_POSIX_ACL_DEFAULT, xattr, xattr_len) == 0))
+		return 1;
+	return 0;
+}