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esp-seqno.c
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/*
* OpenConnect (SSL + DTLS) VPN client
*
* Copyright © 2008-2015 Intel Corporation.
*
* Author: David Woodhouse <dwmw2@infradead.org>
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public License
* version 2.1, as published by the Free Software Foundation.
*
* This program is distributed in the hope that it will be useful, but
* WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*/
#include <config.h>
#include <stdint.h>
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#include <inttypes.h>
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#include <stdlib.h>
#include <errno.h>
#include "openconnect-internal.h"
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#define DTLS_EMPTY_BITMAP (0xFFFFFFFFFFFFFFFFULL)
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/* Eventually we're going to have to have more than one incoming ESP
context at a time, to allow for the overlap period during a rekey.
So pass the 'esp' even though for now it's redundant. */
int verify_packet_seqno(struct openconnect_info *vpninfo,
struct esp *esp, uint32_t seq)
{
/*
* For incoming, esp->seq is the next *expected* packet, being
* the sequence number *after* the latest we have received.
*
* Since it must always be true that packet esp->seq-1 has been
* received, so there's no need to explicitly record that.
*
* So the backlog bitmap covers the 64 packets prior to that,
* with the LSB representing packet (esp->seq - 2), and the MSB
* representing (esp->seq - 65). A received packet is represented
* by a zero bit, and a missing packet is represented by a one.
*
* Thus we can allow out-of-order reception of packets that are
* within a reasonable interval of the latest packet received.
*/
if (seq == esp->seq) {
/* The common case. This is the packet we expected next. */
esp->seq_backlog <<= 1;
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/* This might reach a value higher than the 32-bit ESP sequence
* numbers can actually reach. Which is fine. When that
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* happens, we'll do the right thing and just not accept any
* newer packets. Someone needs to start a new epoch. */
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esp->seq++;
vpn_progress(vpninfo, PRG_TRACE,
_("Accepting expected ESP packet with seq %u\n"),
seq);
return 0;
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} else if (seq > esp->seq) {
/* The packet we were expecting has gone missing; this one is newer.
* We always advance the window to accommodate it. */
uint32_t delta = seq - esp->seq;
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if (delta >= 64) {
/* We jumped a long way into the future. We have not seen
* any of the previous 32 packets so set the backlog bitmap
* to all ones. */
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esp->seq_backlog = DTLS_EMPTY_BITMAP;
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} else if (delta == 63) {
/* Avoid undefined behaviour that shifting by 64 would incur.
* The (clear) top bit represents the packet which is currently
* esp->seq - 1, which we know was already received. */
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esp->seq_backlog = DTLS_EMPTY_BITMAP >> 1;
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} else {
/* We have missed (delta) packets. Shift the backlog by that
* amount *plus* the one we would have shifted it anyway if
* we'd received the packet we were expecting. The zero bit
* representing the packet which is currently esp->seq - 1,
* which we know has been received, ends up at bit position
* (1<<delta). Then we set all the bits lower than that, which
* represent the missing packets. */
esp->seq_backlog <<= delta + 1;
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esp->seq_backlog |= (1ULL << delta) - 1;
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}
vpn_progress(vpninfo, PRG_TRACE,
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_("Accepting later-than-expected ESP packet with seq %u (expected %" PRIu64 ")\n"),
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seq, esp->seq);
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esp->seq = (uint64_t)seq + 1;
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return 0;
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} else {
/* This packet is older than the one we were expecting. By how much...? */
uint32_t delta = esp->seq - seq;
/* delta==0 is the overflow case where esp->seq is 0x100000000 and seq is 0 */
if (delta > 65 || delta == 0) {
/* Too old. We can't know if it's a replay. */
vpn_progress(vpninfo, PRG_DEBUG,
_("Discarding ancient ESP packet with seq %u (expected %" PRIu64 ")\n"),
seq, esp->seq);
return -EINVAL;
} else if (delta == 1) {
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/* Not in the bitmask since it is by definition already received. */
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replayed:
vpn_progress(vpninfo, PRG_DEBUG,
_("Discarding replayed ESP packet with seq %u\n"),
seq);
return -EINVAL;
} else {
/* Within the backlog window, so we remember whether we've seen it or not. */
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uint64_t mask = 1ULL << (delta - 2);
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if (!(esp->seq_backlog & mask))
goto replayed;
esp->seq_backlog &= ~mask;
vpn_progress(vpninfo, PRG_TRACE,
_("Accepting out-of-order ESP packet with seq %u (expected %" PRIu64 ")\n"),
seq, esp->seq);
return 0;
}
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}
}