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client: add support for dropping root privileges

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To minimize the impact of potential attacks targeting chronyc started
under root (e.g. performed by a local chronyd process running without
root privileges, a remote chronyd process, or a MITM attacker on the
network), add support for changing the effective UID/GID in chronyc
after start.

The user can be specified by the -u option, similarly to chronyd. The
default chronyc user can be changed by the --with-chronyc-user
configure option. The default value of the default chronyc user is
"root", i.e. chronyc doesn't try to change the identity by default.

The default chronyc user does not follow the default chronyd user
set by the configure --with-user option to avoid errors on systems where
chronyc is not allowed to change its UID/GID (e.g. by a SELinux policy).
This commit is contained in:
Miroslav Lichvar
2025-08-04 16:34:52 +02:00
parent 5e2cd47ad1
commit be7f5e8916
5 changed files with 34 additions and 4 deletions
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20
client.c
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@@ -3590,9 +3590,11 @@ print_help(const char *progname)
" -m\t\tAccept multiple commands\n"
" -h HOST\tSpecify server (%s)\n"
" -p PORT\tSpecify UDP port (%d)\n"
" -u USER\tSpecify user (%s)\n"
" -v, --version\tPrint version and exit\n"
" --help\tPrint usage and exit\n",
progname, DEFAULT_COMMAND_SOCKET",127.0.0.1,::1", DEFAULT_CANDM_PORT);
progname, DEFAULT_COMMAND_SOCKET",127.0.0.1,::1",
DEFAULT_CANDM_PORT, DEFAULT_CHRONYC_USER);
}
/* ================================================== */
@@ -3609,10 +3611,11 @@ int
main(int argc, char **argv)
{
char *line;
const char *hostnames = NULL, *user = DEFAULT_CHRONYC_USER;
const char *progname = argv[0];
const char *hostnames = NULL;
int opt, ret = 1, multi = 0, family = IPADDR_UNSPEC;
int port = DEFAULT_CANDM_PORT;
struct passwd *pw;
/* Parse long command-line options */
for (optind = 1; optind < argc; optind++) {
@@ -3628,7 +3631,7 @@ main(int argc, char **argv)
optind = 1;
/* Parse short command-line options */
while ((opt = getopt(argc, argv, "+46acdef:h:mnNp:v")) != -1) {
while ((opt = getopt(argc, argv, "+46acdef:h:mnNp:u:v")) != -1) {
switch (opt) {
case '4':
case '6':
@@ -3664,6 +3667,9 @@ main(int argc, char **argv)
case 'p':
port = atoi(optarg);
break;
case 'u':
user = optarg;
break;
case 'v':
print_version();
return 0;
@@ -3673,6 +3679,14 @@ main(int argc, char **argv)
}
}
/* Drop root privileges if configured to do so */
if (strcmp(user, "root") != 0 && geteuid() == 0) {
pw = getpwnam(user);
if (!pw)
LOG_FATAL("Could not get user/group ID of %s", user);
UTI_DropRoot(pw->pw_uid, pw->pw_gid);
}
if (isatty(0) && isatty(1) && isatty(2)) {
on_terminal = 1;
}