evm: add evm_inode_setattr to prevent updating an invalid security.evm

Permit changing of security.evm only when valid, unless in fixmode.
Reported-by: Roberto Sassu <roberto.sassu@polito.it>
Signed-off-by: Mimi Zohar <zohar@us.ibm.com>

include/linux/evm.h

@@ -19,6 +19,7 @@ extern enum integrity_status evm_verifyxattr(struct dentry *dentry,
 					     void *xattr_value,
 					     size_t xattr_value_len,
 					     struct integrity_iint_cache *iint);
+extern int evm_inode_setattr(struct dentry *dentry, struct iattr *attr);
 extern void evm_inode_post_setattr(struct dentry *dentry, int ia_valid);
 extern int evm_inode_setxattr(struct dentry *dentry, const char *name,
 			      const void *value, size_t size);
@@ -44,6 +45,11 @@ static inline enum integrity_status evm_verifyxattr(struct dentry *dentry,
 }
 #endif
 
+static int evm_inode_setattr(struct dentry *dentry, struct iattr *attr)
+{
+	return 0;
+}
+
 static inline void evm_inode_post_setattr(struct dentry *dentry, int ia_valid)
 {
 	return;

security/integrity/evm/evm_main.c

@@ -277,6 +277,21 @@ void evm_inode_post_removexattr(struct dentry *dentry, const char *xattr_name)
 	return;
 }
 
+/**
+ * evm_inode_setattr - prevent updating an invalid EVM extended attribute
+ * @dentry: pointer to the affected dentry
+ */
+int evm_inode_setattr(struct dentry *dentry, struct iattr *attr)
+{
+	unsigned int ia_valid = attr->ia_valid;
+	enum integrity_status evm_status;
+
+	if (ia_valid & ~(ATTR_MODE | ATTR_UID | ATTR_GID))
+		return 0;
+	evm_status = evm_verify_current_integrity(dentry);
+	return evm_status == INTEGRITY_PASS ? 0 : -EPERM;
+}
+
 /**
  * evm_inode_post_setattr - update 'security.evm' after modifying metadata
  * @dentry: pointer to the affected dentry

security/security.c

@@ -571,9 +571,14 @@ int security_inode_exec_permission(struct inode *inode, unsigned int flags)
 
 int security_inode_setattr(struct dentry *dentry, struct iattr *attr)
 {
+	int ret;
+
 	if (unlikely(IS_PRIVATE(dentry->d_inode)))
 		return 0;
-	return security_ops->inode_setattr(dentry, attr);
+	ret = security_ops->inode_setattr(dentry, attr);
+	if (ret)
+		return ret;
+	return evm_inode_setattr(dentry, attr);
 }
 EXPORT_SYMBOL_GPL(security_inode_setattr);