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git: show expert info for unrecognized special packet 0003 

In Git protocol, a pkt-line consists of a 4-hexdigit pkt-length,
followed by several bytes of pkt-data.  The pkt-length represents the
length of the entire pkt-line including the length field, so for an
ordinary pkt-line the length is always >= 4.  This allows the protocol
to use values less than 4 as special values --- for example, 0000 is a
so-called flush-pkt, representing the end of a command.

There's one particular pkt-length value that should never appear: 0003
is not >= 4 and is not a flush-pkt, delim-pkt, or response-end-pkt, so
it is not permitted in Git protocol.  Currently the dissector handles
this case by returning length 0 so it doesn't show up in wireshark as
Git protocol.  Better to treat it as Git protocol and add expert-info
describing what is wrong in case it shows up in a corrupt capture.

Part of #17093.  Based on a hint from Pascal Quantin at [1].

[1] https://gitlab.com/wireshark/wireshark/-/merge_requests/1946#note_515567051
This commit is contained in:
Jonathan Nieder 2021-02-23 16:44:40 -08:00 committed by Wireshark GitLab Utility
parent a3f1408649
commit 23e6b32855
1 changed files with 27 additions and 11 deletions
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38
epan/dissectors/packet-git.c
View File

@ -16,6 +16,7 @@
#include <stdio.h>
#include <epan/packet.h>
#include <epan/expert.h>
#include <epan/prefs.h>
#include "packet-tcp.h"
@ -25,6 +26,7 @@ void proto_reg_handoff_git(void);
static dissector_handle_t git_handle;
static int proto_git = -1;
static expert_field ei_git_bad_pkt_len = EI_INIT;
static gint ett_git = -1;
@ -84,17 +86,15 @@ get_git_pdu_len(packet_info *pinfo _U_, tvbuff_t *tvb, int offset, void *data _U
if (!get_packet_length(tvb, offset, &plen))
return 0; /* No idea what this is */
if (plen == 0 || plen == 1 || plen == 2) {
/* Terminator, Delimiter, or Response_end packets */
return 4;
}
return plen;
return plen < 4
? 4 // Special packet (e.g., flush-pkt)
: plen;
}
/* Parse pkt-lines one-by-one
*
* @param tvb The buffer to dissect.
* @param pinfo Packet info associated to this buffer.
* @param git_tree The git protocol subtree.
* @param offset The offset at which to start the dissection.
* @return bool After successful/unsuccessful parsing.
@ -104,17 +104,22 @@ get_git_pdu_len(packet_info *pinfo _U_, tvbuff_t *tvb, int offset, void *data _U
* remainder of the data.
*/
static gboolean
dissect_pkt_line(tvbuff_t *tvb, proto_tree *git_tree, int *offset)
dissect_pkt_line(tvbuff_t *tvb, packet_info *pinfo, proto_tree *git_tree,
int *offset)
{
guint16 plen;
// what type of pkt-line is it?
if (!get_packet_length(tvb, *offset, &plen))
return FALSE;
if (plen == 0 || plen == 1 || plen == 2) {
proto_tree_add_uint(git_tree, hf_git_packet_type, tvb, *offset,
4, plen);
if (plen < 4) { // a special packet (e.g., flush-pkt)
proto_item *ti =
proto_tree_add_uint(git_tree, hf_git_packet_type, tvb,
*offset, 4, plen);
*offset += 4;
if (!try_val_to_str(plen, packet_type_vals))
expert_add_info(pinfo, ti, &ei_git_bad_pkt_len);
return TRUE;
}
@ -170,7 +175,7 @@ dissect_git_pdu(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, void* data
ti = proto_tree_add_item(tree, proto_git, tvb, offset, -1, ENC_NA);
git_tree = proto_item_add_subtree(ti, ett_git);
if (!dissect_pkt_line(tvb, git_tree, &offset))
if (!dissect_pkt_line(tvb, pinfo, git_tree, &offset))
return 0;
return tvb_captured_length(tvb);
@ -212,12 +217,23 @@ proto_register_git(void)
&ett_git,
};
static ei_register_info ei[] = {
{ &ei_git_bad_pkt_len,
{ "git.bad_pkt_len", PI_PROTOCOL, PI_ERROR,
"unrecognized special pkt-len value", EXPFILL }
}
};
module_t *git_module;
expert_module_t *expert_git;
proto_git = proto_register_protocol(PNAME, PSNAME, PFNAME);
proto_register_field_array(proto_git, hf, array_length(hf));
proto_register_subtree_array(ett, array_length(ett));
expert_git = expert_register_protocol(proto_git);
expert_register_field_array(expert_git, ei, array_length(ei));
git_handle = register_dissector(PFNAME, dissect_git, proto_git);
git_module = prefs_register_protocol(proto_git, NULL);