Detect unsigned integer overflow in mbedtls_ecp_check_budget()
This commit modifies a bounds check in `mbedtls_ecp_check_budget()` to
be correct even if the requested number of ECC operations would overflow
the operation counter.
@@ -248,9 +248,16 @@ int mbedtls_ecp_check_budget( const mbedtls_ecp_group *grp,
else if( grp->pbits >= 384 )
ops *= 2;
- /* avoid infinite loops: always allow first step */
- if( rs_ctx->ops_done != 0 && rs_ctx->ops_done + ops > ecp_max_ops )
+ /* Avoid infinite loops: always allow first step.
+ * Because of that, however, it's not generally true
+ * that ops_done <= ecp_max_ops, so the check
+ * ops_done > ecp_max_ops below is mandatory. */
+ if( ( rs_ctx->ops_done != 0 ) &&
+ ( rs_ctx->ops_done > ecp_max_ops ||
+ ops > ecp_max_ops - rs_ctx->ops_done ) )
+ {
return( MBEDTLS_ERR_ECP_IN_PROGRESS );
+ }
/* update running count */
rs_ctx->ops_done += ops;
