docs/limitations.rst


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Known security limitations
--------------------------

Lack of secure memory wiping
============================

`Memory wiping`_ is used to protect secret data or key material from attackers
with access to uninitialized memory. This can be either because the attacker
has some kind of local user access or because of how other software uses
uninitialized memory.

Python exposes no API for us to implement this reliably and as such almost all
software in Python is potentially vulnerable to this attack. However the
`CERT secure coding guidelines`_ consider this issue as "low severity,
unlikely, expensive to repair" and we do not consider this a high risk for most
users.

.. _`Memory wiping`:  http://blogs.msdn.com/b/oldnewthing/archive/2013/05/29/10421912.aspx
.. _`CERT secure coding guidelines`: https://www.securecoding.cert.org/confluence/display/seccode/MEM03-C.+Clear+sensitive+information+stored+in+reusable+resources
/*
 *  nextpnr -- Next Generation Place and Route
 *
 *  Copyright (C) 2018-19  gatecat <gatecat@ds0.me>
 *
 *  Permission to use, copy, modify, and/or distribute this software for any
 *  purpose with or without fee is hereby granted, provided that the above
 *  copyright notice and this permission notice appear in all copies.
 *
 *  THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 *  WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 *  MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 *  ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 *  WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 *  ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 *  OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 *
 */

#include <algorithm>
#include <iterator>
#include "cells.h"
#include "design_utils.h"
#include "log.h"
#include "util.h"

NEXTPNR_NAMESPACE_BEGIN

// Pack LUTs and LUT-FF pairs
static void pack_lut_lutffs(Context *ctx)
{
    log_info("Packing LUT-FFs..\n");

    pool<IdString> packed_cells;
    std::vector<std::unique_ptr<CellInfo>> new_cells;
    for (auto &cell : ctx->cells) {
        CellInfo *ci = cell.second.get();
        if (ctx->verbose)
            log_info("cell '%s' is of type '%s'\n", ci->name.c_str(ctx), ci->type.c_str(ctx));
        if (is_lut(ctx, ci)) {
            std::unique_ptr<CellInfo> packed =
                    create_generic_cell(ctx, ctx->id("GENERIC_SLICE"), ci->name.str(ctx) + "_LC");
            for (auto &attr : ci->attrs)
                packed->attrs[attr.first] = attr.second;
            packed_cells.insert(ci->name);
            if (ctx->verbose)
                log_info("packed cell %s into %s\n", ci->name.c_str(ctx), packed->name.c_str(ctx));
            // See if we can pack into a DFF
            // TODO: LUT cascade
            NetInfo *o = ci->ports.at(ctx->id("Q")).net;
            CellInfo *dff = net_only_drives(ctx, o, is_ff, ctx->id("D"), true);
            auto lut_bel = ci->attrs.find(ctx->id("BEL"));
            bool packed_dff = false;
            if (dff) {
                if (ctx->verbose)
                    log_info("found attached dff %s\n", dff->name.c_str(ctx));
                auto dff_bel = dff->attrs.find(ctx->id("BEL"));
                if (lut_bel != ci->attrs.end() && dff_bel != dff->attrs.end() && lut_bel->second != dff_bel->second) {
                    // Locations don't match, can't pack
                } else {
                    lut_to_lc(ctx, ci, packed.get(), false);
                    dff_to_lc(ctx, dff, packed.get(), false);
                    ctx->nets.erase(o->name);
                    if (dff_bel != dff->attrs.end())
                        packed->attrs[ctx->id("BEL")] = dff_bel->second;
                    packed_cells.insert(dff->name);
                    if (ctx->verbose)
                        log_info("packed cell %s into %s\n", dff->name.c_str(ctx), packed->name.c_str(ctx));
                    packed_dff = true;
                }
            }
            if (!packed_dff) {
                lut_to_lc(ctx, ci, packed.get(), true);
            }
            new_cells.push_back(std::move(packed));
        }
    }
    for (auto pcell : packed_cells) {
        ctx->cells.erase(pcell);
    }
    for (auto &ncell : new_cells) {
        ctx->cells[ncell->name] = std::move(ncell);
    }
}

// Pack FFs not packed as LUTFFs
static void pack_nonlut_ffs(Context *ctx)
{
    log_info("Packing non-LUT FFs..\n");

    pool<IdString> packed_cells;
    std::vector<std::unique_ptr<CellInfo>> new_cells;

    for (auto &cell : ctx->cells) {
        CellInfo *ci = cell.second.get();
        if (is_ff(ctx, ci)) {
            std::unique_ptr<CellInfo> packed =
                    create_generic_cell(ctx, ctx->id("GENERIC_SLICE"), ci->name.str(ctx) + "_DFFLC");
            for (auto &attr : ci->attrs)
                packed->attrs[attr.first] = attr.second;
            if (ctx->verbose)
                log_info("packed cell %s into %s\n", ci->name.c_str(ctx), packed->name.c_str(ctx));
            packed_cells.insert(ci->name);
            dff_to_lc(ctx, ci, packed.get(), true);
            new_cells.push_back(std::move(packed));
        }
    }
    for (auto pcell : packed_cells) {
        ctx->cells.erase(pcell);
    }
    for (auto &ncell : new_cells) {
        ctx->cells[ncell->name] = std::move(ncell);
    }
}

// Merge a net into a constant net
static void set_net_constant(const Context *ctx, NetInfo *orig, NetInfo *constnet, bool constval)
{
    orig->driver.cell = nullptr;
    for (auto user : orig->users) {
        if (user.cell != nullptr) {
            CellInfo *uc = user.cell;
            if (ctx->verbose)
                log_info("%s user %s\n", orig->name.c_str(ctx), uc->name.c_str(ctx));
            if ((is_lut(ctx, uc) || is_lc(ctx, uc)) && (user.port.str(ctx).at(0) == 'I') && !constval) {
                uc->ports[user.port].net = nullptr;
                uc->ports[user.port].user_idx = {};
            } else {
                uc->ports[user.port].net = constnet;
                uc->ports[user.port].user_idx = constnet->users.add(user);
            }
        }
    }
    orig->users.clear();
}

// Pack constants (simple implementation)
static void pack_constants(Context *ctx)
{
    log_info("Packing constants..\n");

    std::unique_ptr<CellInfo> gnd_cell = create_generic_cell(ctx, ctx->id("GENERIC_SLICE"), "$PACKER_GND");
    gnd_cell->params[ctx->id("INIT")] = Property(0, 1 << ctx->args.K);
    std::unique_ptr<NetInfo> gnd_net = std::make_unique<NetInfo>(ctx->id("$PACKER_GND_NET"));
    gnd_net->driver.cell = gnd_cell.get();
    gnd_net->driver.port = ctx->id("F");
    gnd_cell->ports.at(ctx->id("F")).net = gnd_net.get();

    std::unique_ptr<CellInfo> vcc_cell = create_generic_cell(ctx, ctx->id("GENERIC_SLICE"), "$PACKER_VCC");
    // Fill with 1s
    vcc_cell->params[ctx->id("INIT")] = Property(Property::S1).extract(0, (1 << ctx->args.K), Property::S1);
    std::unique_ptr<NetInfo> vcc_net = std::make_unique<NetInfo>(ctx->id("$PACKER_VCC_NET"));
    vcc_net->driver.cell = vcc_cell.get();
    vcc_net->driver.port = ctx->id("F");
    vcc_cell->ports.at(ctx->id("F")).net = vcc_net.get();

    std::vector<IdString> dead_nets;

    bool gnd_used = false, vcc_used = false;

    for (auto &net : ctx->nets) {
        NetInfo *ni = net.second.get();
        if (ni->driver.cell != nullptr && ni->driver.cell->type == ctx->id("GND")) {
            IdString drv_cell = ni->driver.cell->name;
            set_net_constant(ctx, ni, gnd_net.get(), false);
            gnd_used = true;
            dead_nets.push_back(net.first);
            ctx->cells.erase(drv_cell);
        } else if (ni->driver.cell != nullptr && ni->driver.cell->type == ctx->id("VCC")) {
            IdString drv_cell = ni->driver.cell->name;
            set_net_constant(ctx, ni, vcc_net.get(), true);
            vcc_used = true;
            dead_nets.push_back(net.first);
            ctx->cells.erase(drv_cell);
        }
    }

    if (gnd_used) {
        ctx->cells[gnd_cell->name] = std::move(gnd_cell);
        ctx->nets[gnd_net->name] = std::move(gnd_net);
    }

    if (vcc_used) {
        ctx->cells[vcc_cell->name] = std::move(vcc_cell);
        ctx->nets[vcc_net->name] = std::move(vcc_net);
    }

    for (auto dn : dead_nets) {
        ctx->nets.erase(dn);
    }
}

static bool is_nextpnr_iob(Context *ctx, CellInfo *cell)
{
    return cell->type == ctx->id("$nextpnr_ibuf") || cell->type == ctx->id("$nextpnr_obuf") ||
           cell->type == ctx->id("$nextpnr_iobuf");
}

static bool is_generic_iob(const Context *ctx, const CellInfo *cell) { return cell->type == ctx->id("GENERIC_IOB"); }

// Pack IO buffers
static void pack_io(Context *ctx)
{
    pool<IdString> packed_cells;
    pool<IdString> delete_nets;

    std::vector<std::unique_ptr<CellInfo>> new_cells;
    log_info("Packing IOs..\n");

    for (auto &cell : ctx->cells) {
        CellInfo *ci = cell.second.get();
        if (is_nextpnr_iob(ctx, ci)) {
            CellInfo *iob = nullptr;
            if (ci->type == ctx->id("$nextpnr_ibuf") || ci->type == ctx->id("$nextpnr_iobuf")) {
                iob = net_only_drives(ctx, ci->ports.at(ctx->id("O")).net, is_generic_iob, ctx->id("PAD"), true, ci);

            } else if (ci->type == ctx->id("$nextpnr_obuf")) {
                NetInfo *net = ci->ports.at(ctx->id("I")).net;
                iob = net_only_drives(ctx, net, is_generic_iob, ctx->id("PAD"), true, ci);
            }
            if (iob != nullptr) {
                // Trivial case, GENERIC_IOB used. Just destroy the net and the
                // iobuf
                log_info("%s feeds GENERIC_IOB %s, removing %s %s.\n", ci->name.c_str(ctx), iob->name.c_str(ctx),
                         ci->type.c_str(ctx), ci->name.c_str(ctx));
                NetInfo *net = iob->ports.at(ctx->id("PAD")).net;
                if (((ci->type == ctx->id("$nextpnr_ibuf") || ci->type == ctx->id("$nextpnr_iobuf")) &&
                     net->users.entries() > 1) ||
                    (ci->type == ctx->id("$nextpnr_obuf") && (net->users.entries() > 2 || net->driver.cell != nullptr)))
                    log_error("PAD of %s '%s' connected to more than a single top level IO.\n", iob->type.c_str(ctx),
                              iob->name.c_str(ctx));

                if (net != nullptr) {
                    delete_nets.insert(net->name);
                    iob->ports.at(ctx->id("PAD")).net = nullptr;
                }
                if (ci->type == ctx->id("$nextpnr_iobuf")) {
                    NetInfo *net2 = ci->ports.at(ctx->id("I")).net;
                    if (net2 != nullptr) {
                        delete_nets.insert(net2->name);
                    }
                }
            } else if (bool_or_default(ctx->settings, ctx->id("disable_iobs"))) {
                // No IO buffer insertion; just remove nextpnr_[io]buf
                for (auto &p : ci->ports)
                    ci->disconnectPort(p.first);
            } else {
                // Create a GENERIC_IOB buffer
                std::unique_ptr<CellInfo> ice_cell =
                        create_generic_cell(ctx, ctx->id("GENERIC_IOB"), ci->name.str(ctx) + "$iob");
                nxio_to_iob(ctx, ci, ice_cell.get(), packed_cells);
                new_cells.push_back(std::move(ice_cell));
                iob = new_cells.back().get();
            }
            packed_cells.insert(ci->name);
            if (iob != nullptr)
                for (auto &attr : ci->attrs)
                    iob->attrs[attr.first] = attr.second;
        }
    }
    for (auto pcell : packed_cells) {
        ctx->cells.erase(pcell);
    }
    for (auto dnet : delete_nets) {
        ctx->nets.erase(dnet);
    }
    for (auto &ncell : new_cells) {
        ctx->cells[ncell->name] = std::move(ncell);
    }
}

// Main pack function
bool Arch::pack()
{
    Context *ctx = getCtx();
    try {
        log_break();
        if (uarch) {
            uarch->pack();
        } else {
            pack_constants(ctx);
            pack_io(ctx);
            pack_lut_lutffs(ctx);
            pack_nonlut_ffs(ctx);
        }
        ctx->assignArchInfo();
        ctx->settings[ctx->id("pack")] = 1;
        log_info("Checksum: 0x%08x\n", ctx->checksum());
        return true;
    } catch (log_execution_error_exception) {
        return false;
    }
}

NEXTPNR_NAMESPACE_END