/*
* hgsh.c - restricted login shell for mercurial
*
* Copyright 2006 Vadim Gelfer <vadim.gelfer@gmail.com>
*
* This software may be used and distributed according to the terms of the
* GNU General Public License, incorporated herein by reference.
*
* this program is login shell for dedicated mercurial user account. it
* only allows few actions:
*
* 1. run hg in server mode on specific repository. no other hg commands
* are allowed. we try to verify that repo to be accessed exists under
* given top-level directory.
*
* 2. (optional) forward ssh connection from firewall/gateway machine to
* "real" mercurial host, to let users outside intranet pull and push
* changes through firewall.
*
* 3. (optional) run normal shell, to allow to "su" to mercurial user, use
* "sudo" to run programs as that user, or run cron jobs as that user.
*
* only tested on linux yet. patches for non-linux systems welcome.
*/
#ifndef _GNU_SOURCE
#define _GNU_SOURCE /* for asprintf */
#endif
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sysexits.h>
#include <unistd.h>
/*
* user config.
*
* if you see a hostname below, just use first part of hostname. example,
* if you have host named foo.bar.com, use "foo".
*/
/*
* HG_GATEWAY: hostname of gateway/firewall machine that people outside your
* intranet ssh into if they need to ssh to other machines. if you do not
* have such machine, set to NULL.
*/
#ifndef HG_GATEWAY
#define HG_GATEWAY "gateway"
#endif
/*
* HG_HOST: hostname of mercurial server. if any machine is allowed, set to
* NULL.
*/
#ifndef HG_HOST
#define HG_HOST "mercurial"
#endif
/*
* HG_USER: username to log in from HG_GATEWAY to HG_HOST. if gateway and
* host username are same, set to NULL.
*/
#ifndef HG_USER
#define HG_USER "hg"
#endif
/*
* HG_ROOT: root of tree full of mercurial repos. if you do not want to
* validate location of repo when someone is try to access, set to NULL.
*/
#ifndef HG_ROOT
#define HG_ROOT "/home/hg/repos"
#endif
/*
* HG: path to the mercurial executable to run.
*/
#ifndef HG
#define HG "/home/hg/bin/hg"
#endif
/*
* HG_SHELL: shell to use for actions like "sudo" and "su" access to
* mercurial user, and cron jobs. if you want to make these things
* impossible, set to NULL.
*/
#ifndef HG_SHELL
#define HG_SHELL NULL
/* #define HG_SHELL "/bin/bash" */
#endif
/*
* HG_HELP: some way for users to get support if they have problem. if they
* should not get helpful message, set to NULL.
*/
#ifndef HG_HELP
#define HG_HELP "please contact support@example.com for help."
#endif
/*
* SSH: path to ssh executable to run, if forwarding from HG_GATEWAY to
* HG_HOST. if you want to use rsh instead (why?), you need to modify
* arguments it is called with. see forward_through_gateway.
*/
#ifndef SSH
#define SSH "/usr/bin/ssh"
#endif
/*
* tell whether to print command that is to be executed. useful for
* debugging. should not interfere with mercurial operation, since
* mercurial only cares about stdin and stdout, and this prints to stderr.
*/
static const int debug = 0;
static void print_cmdline(int argc, char **argv)
{
FILE *fp = stderr;
int i;
fputs("command: ", fp);
for (i = 0; i < argc; i++) {
char *spc = strpbrk(argv[i], " \t\r\n");
if (spc) {
fputc('\'', fp);
}
fputs(argv[i], fp);
if (spc) {
fputc('\'', fp);
}
if (i < argc - 1) {
fputc(' ', fp);
}
}
fputc('\n', fp);
fflush(fp);
}
static void usage(const char *reason, int exitcode)
{
char *hg_help = HG_HELP;
if (reason) {
fprintf(stderr, "*** Error: %s.\n", reason);
}
fprintf(stderr, "*** This program has been invoked incorrectly.\n");
if (hg_help) {
fprintf(stderr, "*** %s\n", hg_help);
}
exit(exitcode ? exitcode : EX_USAGE);
}
/*
* run on gateway host to make another ssh connection, to "real" mercurial
* server. it sends its command line unmodified to far end.
*
* never called if HG_GATEWAY is NULL.
*/
static void forward_through_gateway(int argc, char **argv)
{
char *ssh = SSH;
char *hg_host = HG_HOST;
char *hg_user = HG_USER;
char **nargv = alloca((10 + argc) * sizeof(char *));
int i = 0, j;
nargv[i++] = ssh;
nargv[i++] = "-q";
nargv[i++] = "-T";
nargv[i++] = "-x";
if (hg_user) {
nargv[i++] = "-l";
nargv[i++] = hg_user;
}
nargv[i++] = hg_host;
/*
* sshd called us with added "-c", because it thinks we are a shell.
* drop it if we find it.
*/
j = 1;
if (j < argc && strcmp(argv[j], "-c") == 0) {
j++;
}
for (; j < argc; i++, j++) {
nargv[i] = argv[j];
}
nargv[i] = NULL;
if (debug) {
print_cmdline(i, nargv);
}
execv(ssh, nargv);
perror(ssh);
exit(EX_UNAVAILABLE);
}
/*
* run shell. let administrator "su" to mercurial user's account to do
* administrative works.
*
* never called if HG_SHELL is NULL.
*/
static void run_shell(int argc, char **argv)
{
char *hg_shell = HG_SHELL;
char **nargv;
char *c;
int i;
nargv = alloca((argc + 3) * sizeof(char *));
c = strrchr(hg_shell, '/');
/* tell "real" shell it is login shell, if needed. */
if (argv[0][0] == '-' && c) {
nargv[0] = strdup(c);
if (nargv[0] == NULL) {
perror("malloc");
exit(EX_OSERR);
}
nargv[0][0] = '-';
} else {
nargv[0] = hg_shell;
}
for (i = 1; i < argc; i++) {
nargv[i] = argv[i];
}
nargv[i] = NULL;
if (debug) {
print_cmdline(i, nargv);
}
execv(hg_shell, nargv);
perror(hg_shell);
exit(EX_OSFILE);
}
enum cmdline {
hg_init,
hg_serve,
};
/*
* attempt to verify that a directory is really a hg repo, by testing
* for the existence of a subdirectory.
*/
static int validate_repo(const char *repo_root, const char *subdir)
{
char *abs_path;
struct stat st;
int ret;
if (asprintf(&abs_path, "%s.hg/%s", repo_root, subdir) == -1) {
ret = -1;
goto bail;
}
/* verify that we really are looking at valid repo. */
if (stat(abs_path, &st) == -1) {
ret = 0;
} else {
ret = 1;
}
bail:
return ret;
}
/*
* paranoid wrapper, runs hg executable in server mode.
*/
static void serve_data(int argc, char **argv)
{
char *hg_root = HG_ROOT;
char *repo, *repo_root;
enum cmdline cmd;
char *nargv[6];
size_t repolen;
int i;
/*
* check argv for looking okay. we should be invoked with argv
* resembling like this:
*
* hgsh
* -c
* hg -R some/path serve --stdio
*
* the "-c" is added by sshd, because it thinks we are login shell.
*/
if (argc != 3) {
goto badargs;
}
if (strcmp(argv[1], "-c") != 0) {
goto badargs;
}
if (sscanf(argv[2], "hg init %as", &repo) == 1) {
cmd = hg_init;
} else if (sscanf(argv[2], "hg -R %as serve --stdio", &repo) == 1) {
cmd = hg_serve;
} else {
goto badargs;
}
repolen = repo ? strlen(repo) : 0;
if (repolen == 0) {
goto badargs;
}
if (hg_root) {
if (asprintf(&repo_root, "%s/%s/", hg_root, repo) == -1) {
goto badargs;
}
/*
* attempt to stop break out from inside the
* repository tree. could do something more clever
* here, because e.g. we could traverse a symlink that
* looks safe, but really breaks us out of tree.
*/
if (strstr(repo_root, "/../") != NULL) {
goto badargs;
}
/* only hg init expects no repo. */
if (cmd != hg_init) {
int valid;
valid = validate_repo(repo_root, "data");
if (valid == -1) {
goto badargs;
}
if (valid == 0) {
valid = validate_repo(repo_root, "store");
if (valid == -1) {
goto badargs;
}
}
if (valid == 0) {
perror(repo);
exit(EX_DATAERR);
}
}
if (chdir(hg_root) == -1) {
perror(hg_root);
exit(EX_SOFTWARE);
}
}
i = 0;
switch (cmd) {
case hg_serve:
nargv[i++] = HG;
nargv[i++] = "-R";
nargv[i++] = repo;
nargv[i++] = "serve";
nargv[i++] = "--stdio";
break;
case hg_init:
nargv[i++] = HG;
nargv[i++] = "init";
nargv[i++] = repo;
break;
}
nargv[i] = NULL;
if (debug) {
print_cmdline(i, nargv);
}
execv(HG, nargv);
perror(HG);
exit(EX_UNAVAILABLE);
badargs:
/* print useless error message. */
usage("invalid arguments", EX_DATAERR);
}
int main(int argc, char **argv)
{
char host[1024];
char *c;
if (gethostname(host, sizeof(host)) == -1) {
perror("gethostname");
exit(EX_OSERR);
}
if ((c = strchr(host, '.')) != NULL) {
*c = '\0';
}
if (getenv("SSH_CLIENT")) {
char *hg_gateway = HG_GATEWAY;
char *hg_host = HG_HOST;
if (hg_gateway && strcmp(host, hg_gateway) == 0) {
forward_through_gateway(argc, argv);
}
if (hg_host && strcmp(host, hg_host) != 0) {
usage("invoked on unexpected host", EX_USAGE);
}
serve_data(argc, argv);
} else if (HG_SHELL) {
run_shell(argc, argv);
} else {
usage("invalid arguments", EX_DATAERR);
}
return 0;
}