bitcoin/src/timedata.cpp

// Copyright (c) 2014-2020 The Bitcoin Core developers
// Distributed under the MIT software license, see the accompanying
// file COPYING or http://www.opensource.org/licenses/mit-license.php.

#if defined(HAVE_CONFIG_H)
#include <config/bitcoin-config.h>
#endif

#include <timedata.h>

#include <netaddress.h>
#include <node/ui_interface.h>
#include <sync.h>
#include <tinyformat.h>
#include <util/system.h>
#include <util/translation.h>
#include <warnings.h>

static Mutex g_timeoffset_mutex;
static int64_t nTimeOffset GUARDED_BY(g_timeoffset_mutex) = 0;

/**
 * "Never go to sea with two chronometers; take one or three."
 * Our three time sources are:
 *  - System clock
 *  - Median of other nodes clocks
 *  - The user (asking the user to fix the system clock if the first two disagree)
 */
int64_t GetTimeOffset()
{
    LOCK(g_timeoffset_mutex);
    return nTimeOffset;
}

int64_t GetAdjustedTime()
{
    return GetTime() + GetTimeOffset();
}

#define BITCOIN_TIMEDATA_MAX_SAMPLES 200

void AddTimeData(const CNetAddr& ip, int64_t nOffsetSample)
{
    LOCK(g_timeoffset_mutex);
    // Ignore duplicates
    static std::set<CNetAddr> setKnown;
    if (setKnown.size() == BITCOIN_TIMEDATA_MAX_SAMPLES)
        return;
    if (!setKnown.insert(ip).second)
        return;

    // Add data
    static CMedianFilter<int64_t> vTimeOffsets(BITCOIN_TIMEDATA_MAX_SAMPLES, 0);
    vTimeOffsets.input(nOffsetSample);
    LogPrint(BCLog::NET, "added time data, samples %d, offset %+d (%+d minutes)\n", vTimeOffsets.size(), nOffsetSample, nOffsetSample / 60);

    // There is a known issue here (see issue #4521):
    //
    // - The structure vTimeOffsets contains up to 200 elements, after which
    // any new element added to it will not increase its size, replacing the
    // oldest element.
    //
    // - The condition to update nTimeOffset includes checking whether the
    // number of elements in vTimeOffsets is odd, which will never happen after
    // there are 200 elements.
    //
    // But in this case the 'bug' is protective against some attacks, and may
    // actually explain why we've never seen attacks which manipulate the
    // clock offset.
    //
    // So we should hold off on fixing this and clean it up as part of
    // a timing cleanup that strengthens it in a number of other ways.
    //
    if (vTimeOffsets.size() >= 5 && vTimeOffsets.size() % 2 == 1) {
        int64_t nMedian = vTimeOffsets.median();
        std::vector<int64_t> vSorted = vTimeOffsets.sorted();
        // Only let other nodes change our time by so much
        int64_t max_adjustment = std::max<int64_t>(0, gArgs.GetArg("-maxtimeadjustment", DEFAULT_MAX_TIME_ADJUSTMENT));
        if (nMedian >= -max_adjustment && nMedian <= max_adjustment) {
            nTimeOffset = nMedian;
        } else {
            nTimeOffset = 0;

            static bool fDone;
            if (!fDone) {
                // If nobody has a time different than ours but within 5 minutes of ours, give a warning
                bool fMatch = false;
                for (const int64_t nOffset : vSorted) {
                    if (nOffset != 0 && nOffset > -5 * 60 && nOffset < 5 * 60) fMatch = true;
                }

                if (!fMatch) {
                    fDone = true;
                    bilingual_str strMessage = strprintf(_("Please check that your computer's date and time are correct! If your clock is wrong, %s will not work properly."), PACKAGE_NAME);
                    SetMiscWarning(strMessage);
                    uiInterface.ThreadSafeMessageBox(strMessage, "", CClientUIInterface::MSG_WARNING);
                }
            }
        }

        if (LogAcceptCategory(BCLog::NET)) {
            std::string log_message{"time data samples: "};
            for (const int64_t n : vSorted) {
                log_message += strprintf("%+d  ", n);
            }
            log_message += strprintf("|  median offset = %+d  (%+d minutes)", nTimeOffset, nTimeOffset / 60);
            LogPrint(BCLog::NET, "%s\n", log_message);
        }
    }
}