evm: prohibit userspace writing 'security.evm' HMAC value

[pandora-kernel.git] / security / integrity / evm / evm_main.c

diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
index 92d3d99..3439872 100644 (file)
--- a/security/integrity/evm/evm_main.c
+++ b/security/integrity/evm/evm_main.c
@@ -207,12 +207,20 @@ static int evm_protect_xattr(struct dentry *dentry, const char *xattr_name,
  * @xattr_value: pointer to the new extended attribute value
  * @xattr_value_len: pointer to the new extended attribute value length
  *
- * Updating 'security.evm' requires CAP_SYS_ADMIN privileges and that
- * the current value is valid.
+ * Before allowing the 'security.evm' protected xattr to be updated,
+ * verify the existing value is valid.  As only the kernel should have
+ * access to the EVM encrypted key needed to calculate the HMAC, prevent
+ * userspace from writing HMAC value.  Writing 'security.evm' requires
+ * requires CAP_SYS_ADMIN privileges.
  */
 int evm_inode_setxattr(struct dentry *dentry, const char *xattr_name,
                       const void *xattr_value, size_t xattr_value_len)
 {
+       const struct evm_ima_xattr_data *xattr_data = xattr_value;
+
+       if ((strcmp(xattr_name, XATTR_NAME_EVM) == 0)
+           && (xattr_data->type == EVM_XATTR_HMAC))
+               return -EPERM;
        return evm_protect_xattr(dentry, xattr_name, xattr_value,
                                 xattr_value_len);
 }