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wireshark/epan/dissectors/packet-ssyncp.c

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/* packet-ssyncp.c
* Routines for dissecting mosh's State Synchronization Protocol
* Copyright 2020 Google LLC
*
* Wireshark - Network traffic analyzer
* By Gerald Combs <gerald@wireshark.org>
* Copyright 1998 Gerald Combs
*
* SPDX-License-Identifier: GPL-2.0-or-later
*/
/*
* State Synchronization Protocol is the protocol used by mosh:
* <https://mosh.org/mosh-paper-draft.pdf>
*
* The protocol name is abbreviated as SSyncP to avoid conflict with the
* "Scripting Service Protocol".
*
* The protocol is based on UDP, with a plaintext header followed by an
* encrypted payload. For now we just support decrypting a single connection at
* a time, using the MOSH_KEY dumped from the environment variables
* (`cat /proc/$pid/environ | tr '\0' '\n' | grep MOSH_KEY` on Linux).
* Note that to display the embedded protobuf properly, you'll have to add
* src/protobufs/ from mosh's source code to the ProtoBuf search path.
* For now we stop decoding after reaching the first level of protobufs; in
* them, a second layer of protobufs is sometimes embedded (e.g. for
* transmitting screen contents and such). Implementing that is left as an
* exercise for the reader.
*/
#include <config.h>
#include <epan/packet.h> /* Should be first Wireshark include (other than config.h) */
#include <epan/conversation.h>
#include <epan/wmem_scopes.h>
#include <epan/proto_data.h>
#include <epan/prefs.h>
#include <epan/expert.h>
#include <wsutil/report_message.h>
#include <wsutil/wsgcrypt.h>
void proto_reg_handoff_ssyncp(void);
void proto_register_ssyncp(void);
static int proto_ssyncp = -1;
static int hf_ssyncp_direction = -1;
static int hf_ssyncp_seq = -1;
static int hf_ssyncp_encrypted = -1;
static int hf_ssyncp_seq_delta = -1;
static int hf_ssyncp_timestamp = -1;
static int hf_ssyncp_timestamp_reply = -1;
static int hf_ssyncp_frag_seq = -1;
static int hf_ssyncp_frag_final = -1;
static int hf_ssyncp_frag_idx = -1;
static int hf_ssyncp_rtt_to_server = -1;
static int hf_ssyncp_rtt_to_client = -1;
/* Initialize the subtree pointers */
static gint ett_ssyncp = -1;
static gint ett_ssyncp_decrypted = -1;
static expert_field ei_ssyncp_fragmented = EI_INIT;
static expert_field ei_ssyncp_bad_key = EI_INIT;
static const char *pref_ssyncp_key;
static char ssyncp_raw_aes_key[16];
static gboolean have_ssyncp_key;
static dissector_handle_t dissector_protobuf;
typedef struct _ssyncp_conv_info_t {
/* last sequence numbers per direction */
guint64 last_seq[2];
/* for each direction, have we seen any traffic yet? */
gboolean seen_packet[2];
guint16 clock_offset[2];
gboolean clock_seen[2];
} ssyncp_conv_info_t;
typedef struct _ssyncp_packet_info_t {
gboolean first_packet;
gint64 seq_delta;
gboolean have_rtt_estimate;
gint16 rtt_estimate;
} ssyncp_packet_info_t;
#define SSYNCP_IV_PAD 4
#define SSYNCP_SEQ_LEN 8
#define SSYNCP_DATAGRAM_HEADER_LEN (SSYNCP_SEQ_LEN + 2 + 2) /* 64-bit IV and two 16-bit timestamps */
#define SSYNCP_TRANSPORT_HEADER_LEN (8 + 2)
#define SSYNCP_AUTHTAG_LEN 16 /* 128-bit auth tag */
/*
* We only match on 60001, which mosh uses for its first connection.
* If there are more connections in the range 60002-61000, the user will have to
* mark those as ssyncp traffic manually - we'd have too many false positives
* otherwise.
*/
#define SSYNCP_UDP_PORT 60001
static int
dissect_ssyncp(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree,
void *data _U_)
{
/* Check that we have at least a datagram plus an OCB auth tag. */
if (tvb_reported_length(tvb) < SSYNCP_DATAGRAM_HEADER_LEN + SSYNCP_TRANSPORT_HEADER_LEN + SSYNCP_AUTHTAG_LEN)
return 0;
guint64 direction_and_seq = tvb_get_guint64(tvb, 0, ENC_BIG_ENDIAN);
guint direction = direction_and_seq >> 63;
guint64 seq = direction_and_seq & ~(1ULL << 63);
/* Heuristic: The 63-bit sequence number starts from zero and increments
* from there. Even if you send 1000 packets per second over 10 years, you
* won't reach 2^35. So check that the sequence number is not outrageously
* high.
*/
if (seq > (1ULL << 35))
return 0;
/* On the first pass, track the previous sequence numbers per direction,
* compute deltas between sequence numbers, and save those deltas.
* On subsequent passes, use the computed deltas.
*/
ssyncp_packet_info_t *ssyncp_pinfo;
ssyncp_conv_info_t *ssyncp_info = NULL;
if (pinfo->fd->visited) {
ssyncp_pinfo = (ssyncp_packet_info_t *)p_get_proto_data(wmem_file_scope(), pinfo, proto_ssyncp, 0);
} else {
conversation_t *conversation = find_or_create_conversation(pinfo);
ssyncp_info = (ssyncp_conv_info_t *)conversation_get_proto_data(conversation, proto_ssyncp);
if (!ssyncp_info) {
ssyncp_info = wmem_new(wmem_file_scope(), ssyncp_conv_info_t);
conversation_add_proto_data(conversation, proto_ssyncp, ssyncp_info);
ssyncp_info->seen_packet[0] = FALSE;
ssyncp_info->seen_packet[1] = FALSE;
ssyncp_info->clock_seen[0] = FALSE;
ssyncp_info->clock_seen[1] = FALSE;
}
ssyncp_pinfo = wmem_new(wmem_file_scope(), ssyncp_packet_info_t);
ssyncp_pinfo->first_packet = !ssyncp_info->seen_packet[direction];
if (ssyncp_pinfo->first_packet) {
ssyncp_info->seen_packet[direction] = TRUE;
} else {
ssyncp_pinfo->seq_delta = seq - ssyncp_info->last_seq[direction];
}
ssyncp_pinfo->have_rtt_estimate = FALSE;
p_add_proto_data(wmem_file_scope(), pinfo, proto_ssyncp, 0, ssyncp_pinfo);
ssyncp_info->last_seq[direction] = seq;
}
/*** COLUMN DATA ***/
col_set_str(pinfo->cinfo, COL_PROTOCOL, "ssyncp");
col_clear(pinfo->cinfo, COL_INFO);
char *direction_str = direction ? "Server->Client" : "Client->Server";
col_set_str(pinfo->cinfo, COL_INFO, direction_str);
/*** PROTOCOL TREE ***/
/* create display subtree for the protocol */
proto_item *ti = proto_tree_add_item(tree, proto_ssyncp, tvb, 0, -1, ENC_NA);
proto_tree *ssyncp_tree = proto_item_add_subtree(ti, ett_ssyncp);
/* Add an item to the subtree, see section 1.5 of README.dissector for more
* information. */
proto_tree_add_item(ssyncp_tree, hf_ssyncp_direction, tvb,
0, 1, ENC_BIG_ENDIAN);
proto_tree_add_item(ssyncp_tree, hf_ssyncp_seq, tvb,
0, 8, ENC_BIG_ENDIAN);
#ifdef GCRY_OCB_BLOCK_LEN
proto_item *encrypted_item =
#endif
proto_tree_add_item(ssyncp_tree, hf_ssyncp_encrypted,
tvb, 8, -1, ENC_NA);
if (!ssyncp_pinfo->first_packet) {
proto_item *delta_item =
proto_tree_add_int64(ssyncp_tree, hf_ssyncp_seq_delta, tvb, 0, 0,
ssyncp_pinfo->seq_delta);
proto_item_set_generated(delta_item);
}
unsigned char *decrypted = NULL;
guint decrypted_len = 0;
/* avoid build failure on ancient libgcrypt without OCB support */
#ifdef GCRY_OCB_BLOCK_LEN
if (have_ssyncp_key) {
gcry_error_t gcry_err;
/* try to decrypt the rest of the packet */
gcry_cipher_hd_t gcry_hd;
gcry_err = gcry_cipher_open(&gcry_hd, GCRY_CIPHER_AES128, GCRY_CIPHER_MODE_OCB, 0);
if (gcry_err_code(gcry_err)) {
/* this shouldn't happen (even if the packet is garbage) */
report_failure("ssyncp: unable to initialize cipher???");
return tvb_captured_length(tvb);
}
gcry_err = gcry_cipher_setkey(gcry_hd, ssyncp_raw_aes_key, sizeof(ssyncp_raw_aes_key));
if (gcry_err_code(gcry_err)) {
/* this shouldn't happen (even if the packet is garbage) */
report_failure("ssyncp: unable to set key???");
gcry_cipher_close(gcry_hd);
return tvb_captured_length(tvb);
}
char nonce[SSYNCP_IV_PAD + SSYNCP_SEQ_LEN];
memset(nonce, 0, SSYNCP_IV_PAD);
tvb_memcpy(tvb, nonce + SSYNCP_IV_PAD, 0, SSYNCP_SEQ_LEN);
gcry_err = gcry_cipher_setiv(gcry_hd, nonce, sizeof(nonce));
if (gcry_err_code(gcry_err)) {
/* this shouldn't happen (even if the packet is garbage) */
report_failure("ssyncp: unable to set iv???");
gcry_cipher_close(gcry_hd);
return tvb_captured_length(tvb);
}
decrypted_len = tvb_captured_length(tvb) - SSYNCP_SEQ_LEN - SSYNCP_AUTHTAG_LEN;
decrypted = (unsigned char *)tvb_memdup(pinfo->pool, tvb,
SSYNCP_SEQ_LEN, decrypted_len);
gcry_cipher_final(gcry_hd);
gcry_err = gcry_cipher_decrypt(gcry_hd, decrypted, decrypted_len, NULL, 0);
if (gcry_err_code(gcry_err)) {
/* this shouldn't happen (even if the packet is garbage) */
report_failure("ssyncp: unable to decrypt???");
gcry_cipher_close(gcry_hd);
return tvb_captured_length(tvb);
}
gcry_err = gcry_cipher_checktag(gcry_hd,
tvb_get_ptr(tvb, SSYNCP_SEQ_LEN+decrypted_len, SSYNCP_AUTHTAG_LEN),
SSYNCP_AUTHTAG_LEN);
if (gcry_err_code(gcry_err) && gcry_err_code(gcry_err) != GPG_ERR_CHECKSUM) {
/* this shouldn't happen (even if the packet is garbage) */
report_failure("ssyncp: unable to check auth tag???");
gcry_cipher_close(gcry_hd);
return tvb_captured_length(tvb);
}
if (gcry_err_code(gcry_err)) {
/* if the tag is wrong, the key was wrong and the decrypted data is useless */
decrypted = NULL;
expert_add_info(pinfo, encrypted_item, &ei_ssyncp_bad_key);
}
gcry_cipher_close(gcry_hd);
}
#endif
if (decrypted) {
tvbuff_t *decrypted_tvb = tvb_new_child_real_data(tvb, decrypted, decrypted_len, decrypted_len);
add_new_data_source(pinfo, decrypted_tvb, "Decrypted data");
if (!pinfo->fd->visited) {
guint16 our_clock16 = ((guint64)pinfo->abs_ts.secs * 1000 + pinfo->abs_ts.nsecs / 1000000) & 0xffff;
guint16 sender_ts = tvb_get_guint16(decrypted_tvb, 0, ENC_BIG_ENDIAN);
guint16 reply_ts = tvb_get_guint16(decrypted_tvb, 2, ENC_BIG_ENDIAN);
ssyncp_info->clock_offset[direction] = sender_ts - our_clock16;
ssyncp_info->clock_seen[direction] = TRUE;
if (reply_ts != 0xffff && ssyncp_info->clock_seen[1-direction]) {
guint16 projected_send_time_our_clock = reply_ts - ssyncp_info->clock_offset[1-direction];
ssyncp_pinfo->rtt_estimate = our_clock16 - projected_send_time_our_clock;
ssyncp_pinfo->have_rtt_estimate = TRUE;
}
}
proto_tree *dec_tree = proto_tree_add_subtree(ssyncp_tree, decrypted_tvb,
0, -1, ett_ssyncp_decrypted, NULL, "Decrypted data");
proto_tree_add_item(dec_tree, hf_ssyncp_timestamp, decrypted_tvb,
0, 2, ENC_BIG_ENDIAN);
proto_tree_add_item(dec_tree, hf_ssyncp_timestamp_reply, decrypted_tvb,
2, 2, ENC_BIG_ENDIAN);
if (ssyncp_pinfo->have_rtt_estimate) {
int rtt_id = direction ? hf_ssyncp_rtt_to_server : hf_ssyncp_rtt_to_client;
proto_item *rtt_item = proto_tree_add_int(dec_tree, rtt_id, decrypted_tvb, 2, 2, ssyncp_pinfo->rtt_estimate);
proto_item_set_generated(rtt_item);
}
proto_tree_add_item(dec_tree, hf_ssyncp_frag_seq, decrypted_tvb,
4, 8, ENC_BIG_ENDIAN);
proto_tree_add_item(dec_tree, hf_ssyncp_frag_final, decrypted_tvb,
12, 2, ENC_BIG_ENDIAN);
proto_item *frag_idx_item = proto_tree_add_item(dec_tree,
hf_ssyncp_frag_idx, decrypted_tvb, 12, 2, ENC_BIG_ENDIAN);
/* TODO actually handle fragmentation; for now just bail out on fragmentation */
if (tvb_get_guint16(decrypted_tvb, 12, ENC_BIG_ENDIAN) != 0x8000) {
expert_add_info(pinfo, frag_idx_item, &ei_ssyncp_fragmented);
return tvb_captured_length(tvb);
}
tvbuff_t *inflated_tvb = tvb_child_uncompress(decrypted_tvb, decrypted_tvb, 14, decrypted_len - 14);
if (inflated_tvb == NULL)
return tvb_captured_length(tvb);
add_new_data_source(pinfo, inflated_tvb, "Inflated data");
if (dissector_protobuf) {
call_dissector_with_data(dissector_protobuf, inflated_tvb, pinfo,
dec_tree, "message,TransportBuffers.Instruction");
}
}
return tvb_captured_length(tvb);
}
/* Register the protocol with Wireshark.
*
* This format is required because a script is used to build the C function that
* calls all the protocol registration.
*/
void
proto_register_ssyncp(void)
{
static const true_false_string direction_name = {
"Server->Client",
"Client->Server"
};
/* Setup list of header fields See Section 1.5 of README.dissector for
* details. */
static hf_register_info hf[] = {
{ &hf_ssyncp_direction,
{ "Direction", "ssyncp.direction",
FT_BOOLEAN, 8, TFS(&direction_name), 0x80,
"Direction of packet", HFILL }
},
{ &hf_ssyncp_seq,
{ "Sequence number", "ssyncp.seq",
FT_UINT64, BASE_HEX, NULL, 0x7fffffffffffffff,
"Monotonically incrementing packet sequence number", HFILL }
},
{ &hf_ssyncp_encrypted,
{ "Encrypted data", "ssyncp.enc_data",
FT_BYTES, BASE_NONE, NULL, 0,
"Encrypted RTT estimation fields and Transport Layer payload, encrypted with AES-128-OCB",
HFILL }
},
{ &hf_ssyncp_seq_delta,
{ "Sequence number delta", "ssyncp.seq_delta",
FT_INT64, BASE_DEC, NULL, 0,
"Delta from last sequence number; 1 is normal, 0 is duplicated packet, <0 is reordering, >1 is reordering or packet loss", HFILL }
},
{ &hf_ssyncp_timestamp,
{ "Truncated timestamp", "ssyncp.timestamp",
FT_UINT16, BASE_HEX, NULL, 0,
"Low 16 bits of sender's time in milliseconds", HFILL }
},
{ &hf_ssyncp_timestamp_reply,
{ "Last timestamp received", "ssyncp.timestamp_reply",
FT_UINT16, BASE_HEX, NULL, 0,
"Low 16 bits of timestamp of last received packet plus time since it was received (for RTT estimation)", HFILL }
},
{ &hf_ssyncp_frag_seq,
{ "Fragment ID", "ssyncp.frag_seq",
FT_UINT64, BASE_HEX, NULL, 0,
"Transport-level sequence number, used for fragment reassembly", HFILL }
},
{ &hf_ssyncp_frag_final,
{ "Final fragment", "ssyncp.frag_final",
FT_BOOLEAN, 16, NULL, 0x8000,
"Is this the last fragment?", HFILL }
},
{ &hf_ssyncp_frag_idx,
{ "Fragment Index", "ssyncp.frag_idx",
FT_UINT16, BASE_HEX, NULL, 0x7fff,
"Index of this fragment in the list of fragments of the transport-level message", HFILL }
},
{ &hf_ssyncp_rtt_to_server,
{ "RTT estimate to server (in ms)", "ssyncp.rtt_est_to_server",
FT_INT16, BASE_DEC, NULL, 0,
"Estimated round trip time from point of capture to server", HFILL }
},
{ &hf_ssyncp_rtt_to_client,
{ "RTT estimate to client (in ms)", "ssyncp.rtt_est_to_client",
FT_INT16, BASE_DEC, NULL, 0,
"Estimated round trip time from point of capture to client", HFILL }
}
};
/* Setup protocol subtree array */
static gint *ett[] = {
&ett_ssyncp,
&ett_ssyncp_decrypted
};
/* Setup protocol expert items */
static ei_register_info ei[] = {
{ &ei_ssyncp_fragmented,
{ "ssyncp.fragmented", PI_REASSEMBLE, PI_WARN,
"SSYNCP-level fragmentation, dissector can't handle that", EXPFILL }
},
{ &ei_ssyncp_bad_key,
{ "ssyncp.badkey", PI_DECRYPTION, PI_WARN,
"Encrypted data could not be decrypted with the provided key", EXPFILL }
}
};
/* Register the protocol name and description */
proto_ssyncp = proto_register_protocol("State Synchronization Protocol", "SSyncP", "ssyncp");
/* Required function calls to register the header fields and subtrees */
proto_register_field_array(proto_ssyncp, hf, array_length(hf));
proto_register_subtree_array(ett, array_length(ett));
expert_module_t *expert_ssyncp = expert_register_protocol(proto_ssyncp);
expert_register_field_array(expert_ssyncp, ei, array_length(ei));
module_t *ssyncp_module = prefs_register_protocol(proto_ssyncp, proto_reg_handoff_ssyncp);
prefs_register_string_preference(ssyncp_module, "key",
"ssyncp MOSH_KEY",
"MOSH_KEY AES key (from mosh-{client,server} environment variable)",
&pref_ssyncp_key);
}
void
proto_reg_handoff_ssyncp(void)
{
static dissector_handle_t ssyncp_handle;
static gboolean initialized = FALSE;
if (!initialized) {
ssyncp_handle = create_dissector_handle(dissect_ssyncp, proto_ssyncp);
dissector_add_uint("udp.port", SSYNCP_UDP_PORT, ssyncp_handle);
dissector_protobuf = find_dissector("protobuf");
if (dissector_protobuf == NULL) {
report_failure("unable to find protobuf dissector");
}
initialized = TRUE;
}
have_ssyncp_key = FALSE;
if (strlen(pref_ssyncp_key) != 0) {
if (strlen(pref_ssyncp_key) != 22) {
report_failure("ssyncp: invalid key, must be 22 characters long");
return;
}
char base64_key[25];
memcpy(base64_key, pref_ssyncp_key, 22);
memcpy(base64_key+22, "==\0", 3);
gsize out_len;
if (g_base64_decode_inplace(base64_key, &out_len) == NULL || out_len != sizeof(ssyncp_raw_aes_key)) {
report_failure("ssyncp: invalid key, base64 decoding (with \"==\" appended) failed");
return;
}
memcpy(ssyncp_raw_aes_key, base64_key, sizeof(ssyncp_raw_aes_key));
have_ssyncp_key = TRUE;
}
}
/*
* Editor modelines - https://www.wireshark.org/tools/modelines.html
*
* Local variables:
* c-basic-offset: 4
* tab-width: 8
* indent-tabs-mode: nil
* End:
*
* vi: set shiftwidth=4 tabstop=8 expandtab:
* :indentSize=4:tabSize=8:noTabs=true:
*/
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