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4feeb2d986
libvirt/tools/virt-login-shell-helper.c
Daniel P. Berrangé 4feeb2d986 tools: split virt-login-shell into two binaries 
The virt-login-shell binary is a setuid program that takes
no arguments. When invoked it looks at the invoking uid,
resolves it to a username, and finds an LXC guest with the
same name. It then starts the guest and runs the shell in
side the namespaces of the container.

Given this set of tasks the virt-login-shell binary needs
to connect to libvirtd, make various other libvirt API calls.
This is a problem for setuid binaries as various libraries
that libvirt.so links to are not safe. For example, they have
constructor functions which execute an unknown amount of code
that can be influenced by env variables.

For this reason virt-login-shell doesn't use libvirt.so,
but instead links to a custom, cut down, set of source files
sufficient to be a local client only.

This introduces a problem for integrating glib2 into libvirt
though, as once integrated, there would be no way to build
virt-login-shell without an external dependancy on glib2 and
this is definitely not setuid safe.

To resolve this problem, we split the virt-login-shell binary
into two parts. The first part is setuid and does almost
nothing. It simply records the original uid+gid, and then
invokes the virt-login-shell-helper binary. Crucially when
it does this it completes scrubs all environment variables.
It is thus safe for virt-login-shell-helper to link to the
normal libvirt.so. Any things that constructor functions
do cannot be influenced by user control env vars or cli
args.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
2019-08-07 16:54:01 +01:00

440 lines
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/*
* virt-login-shell-helper.c: a shell to connect to a container
*
* Copyright (C) 2013-2014 Red Hat, Inc.
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 2.1 of the License, or (at your option) any later version.
*
* This library 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.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this library. If not, see
* <http://www.gnu.org/licenses/>.
*/
#include <config.h>
#include <fnmatch.h>
#include <getopt.h>
#include <signal.h>
#include <stdarg.h>
#include <unistd.h>
#include "internal.h"
#include "virerror.h"
#include "virconf.h"
#include "virutil.h"
#include "virfile.h"
#include "virprocess.h"
#include "configmake.h"
#include "virstring.h"
#include "viralloc.h"
#include "vircommand.h"
#include "virgettext.h"
#define VIR_FROM_THIS VIR_FROM_NONE
static const char *conf_file = SYSCONFDIR "/libvirt/virt-login-shell.conf";
static int virLoginShellAllowedUser(virConfPtr conf,
const char *name,
gid_t *groups,
size_t ngroups)
{
int ret = -1;
size_t i;
char *gname = NULL;
char **users = NULL, **entries;
if (virConfGetValueStringList(conf, "allowed_users", false, &users) < 0)
goto cleanup;
for (entries = users; entries && *entries; entries++) {
char *entry = *entries;
/*
If string begins with a % this indicates a linux group.
Check to see if the user is in the Linux Group.
*/
if (entry[0] == '%') {
entry++;
if (!*entry)
continue;
for (i = 0; i < ngroups; i++) {
if (!(gname = virGetGroupName(groups[i])))
continue;
if (fnmatch(entry, gname, 0) == 0) {
ret = 0;
goto cleanup;
}
VIR_FREE(gname);
}
} else {
if (fnmatch(entry, name, 0) == 0) {
ret = 0;
goto cleanup;
}
}
}
virReportSystemError(EPERM,
_("%s not matched against 'allowed_users' in %s"),
name, conf_file);
cleanup:
VIR_FREE(gname);
virStringListFree(users);
return ret;
}
static int virLoginShellGetShellArgv(virConfPtr conf,
char ***shargv,
size_t *shargvlen)
{
int rv;
if ((rv = virConfGetValueStringList(conf, "shell", true, shargv)) < 0)
return -1;
if (rv == 0) {
if (VIR_ALLOC_N(*shargv, 2) < 0)
return -1;
if (VIR_STRDUP((*shargv)[0], "/bin/sh") < 0) {
VIR_FREE(*shargv);
return -1;
}
*shargvlen = 1;
} else {
*shargvlen = virStringListLength((const char *const *)shargv);
}
return 0;
}
static char *progname;
/*
* Print usage
*/
static void
usage(void)
{
fprintf(stdout,
_("\n"
"Usage:\n"
" %s [option]\n\n"
"Options:\n"
" -h | --help Display program help\n"
" -V | --version Display program version\n"
" -c CMD Run CMD via shell\n"
"\n"
"libvirt login shell\n"),
progname);
return;
}
/* Display version information. */
static void
show_version(void)
{
printf("%s (%s) %s\n", progname, PACKAGE_NAME, PACKAGE_VERSION);
}
static void
hideErrorFunc(void *opaque ATTRIBUTE_UNUSED,
virErrorPtr err ATTRIBUTE_UNUSED)
{
}
int
main(int argc, char **argv)
{
virConfPtr conf = NULL;
const char *login_shell_path = conf_file;
pid_t cpid = -1;
int ret = EXIT_CANCELED;
int status;
unsigned long long uidval;
unsigned long long gidval;
uid_t uid;
gid_t gid;
char *name = NULL;
char **shargv = NULL;
size_t shargvlen = 0;
char *shcmd = NULL;
virSecurityModelPtr secmodel = NULL;
virSecurityLabelPtr seclabel = NULL;
virDomainPtr dom = NULL;
virConnectPtr conn = NULL;
char *homedir = NULL;
int arg;
int longindex = -1;
int ngroups;
gid_t *groups = NULL;
ssize_t nfdlist = 0;
int *fdlist = NULL;
int openmax;
size_t i;
const char *cmdstr = NULL;
char *tmp;
char *term = NULL;
virErrorPtr saved_err = NULL;
bool autoshell = false;
struct option opt[] = {
{"help", no_argument, NULL, 'h'},
{"version", optional_argument, NULL, 'V'},
{NULL, 0, NULL, 0}
};
if (virInitialize() < 0) {
fprintf(stderr, _("Failed to initialize libvirt error handling"));
return EXIT_CANCELED;
}
virSetErrorFunc(NULL, hideErrorFunc);
virSetErrorLogPriorityFunc(NULL);
progname = argv[0];
if (virGettextInitialize() < 0)
return ret;
if (geteuid() != 0) {
fprintf(stderr, _("%s: must be run as root\n"), argv[0]);
return ret;
}
if (getuid() != 0) {
fprintf(stderr, _("%s: must not be run setuid root\n"), argv[0]);
return ret;
}
while ((arg = getopt_long(argc, argv, "hVc:", opt, &longindex)) != -1) {
switch (arg) {
case 'h':
usage();
exit(EXIT_SUCCESS);
case 'V':
show_version();
exit(EXIT_SUCCESS);
case 'c':
cmdstr = optarg;
break;
case '?':
default:
usage();
exit(EXIT_CANCELED);
}
}
if (optind != (argc - 2)) {
virReportSystemError(EINVAL, _("%s expects UID and GID parameters"), progname);
goto cleanup;
}
if (virStrToLong_ull(argv[optind], NULL, 10, &uidval) < 0 ||
((uid_t)uidval) != uidval) {
virReportSystemError(EINVAL, _("%s cannot parse UID '%s'"),
progname, argv[optind]);
goto cleanup;
}
optind++;
if (virStrToLong_ull(argv[optind], NULL, 10, &gidval) < 0 ||
((gid_t)gidval) != gidval) {
virReportSystemError(EINVAL, _("%s cannot parse GID '%s'"),
progname, argv[optind]);
goto cleanup;
}
uid = (uid_t)uidval;
gid = (gid_t)gidval;
name = virGetUserName(uid);
if (!name)
goto cleanup;
homedir = virGetUserDirectoryByUID(uid);
if (!homedir)
goto cleanup;
if (!(conf = virConfReadFile(login_shell_path, 0)))
goto cleanup;
if ((ngroups = virGetGroupList(uid, gid, &groups)) < 0)
goto cleanup;
if (virLoginShellAllowedUser(conf, name, groups, ngroups) < 0)
goto cleanup;
if (virLoginShellGetShellArgv(conf, &shargv, &shargvlen) < 0)
goto cleanup;
if (virConfGetValueBool(conf, "auto_shell", &autoshell) < 0)
goto cleanup;
conn = virConnectOpen("lxc:///system");
if (!conn)
goto cleanup;
dom = virDomainLookupByName(conn, name);
if (!dom)
goto cleanup;
if (!virDomainIsActive(dom) && virDomainCreate(dom) < 0) {
virErrorPtr last_error;
last_error = virGetLastError();
if (last_error->code != VIR_ERR_OPERATION_INVALID) {
virReportSystemError(last_error->code,
_("Can't create %s container: %s"),
name, last_error->message);
goto cleanup;
}
}
openmax = sysconf(_SC_OPEN_MAX);
if (openmax < 0) {
virReportSystemError(errno, "%s",
_("sysconf(_SC_OPEN_MAX) failed"));
goto cleanup;
}
if ((nfdlist = virDomainLxcOpenNamespace(dom, &fdlist, 0)) < 0)
goto cleanup;
if (VIR_ALLOC(secmodel) < 0)
goto cleanup;
if (VIR_ALLOC(seclabel) < 0)
goto cleanup;
if (virNodeGetSecurityModel(conn, secmodel) < 0)
goto cleanup;
if (virDomainGetSecurityLabel(dom, seclabel) < 0)
goto cleanup;
if (virSetUIDGID(0, 0, NULL, 0) < 0)
goto cleanup;
if (virDomainLxcEnterSecurityLabel(secmodel, seclabel, NULL, 0) < 0)
goto cleanup;
if (virDomainLxcEnterCGroup(dom, 0) < 0)
goto cleanup;
if (nfdlist > 0 &&
virDomainLxcEnterNamespace(dom, nfdlist, fdlist, NULL, NULL, 0) < 0)
goto cleanup;
if (virSetUIDGID(uid, gid, groups, ngroups) < 0)
goto cleanup;
if (chdir(homedir) < 0) {
virReportSystemError(errno, _("Unable to chdir(%s)"), homedir);
goto cleanup;
}
if (autoshell) {
tmp = virGetUserShell(uid);
if (tmp) {
virStringListFree(shargv);
shargvlen = 1;
if (VIR_ALLOC_N(shargv[0], shargvlen + 1) < 0) {
VIR_FREE(tmp);
goto cleanup;
}
shargv[0] = tmp;
shargv[1] = NULL;
}
}
if (cmdstr) {
if (VIR_REALLOC_N(shargv, shargvlen + 3) < 0)
goto cleanup;
if (VIR_STRDUP(shargv[shargvlen++], "-c") < 0)
goto cleanup;
if (VIR_STRDUP(shargv[shargvlen++], cmdstr) < 0)
goto cleanup;
shargv[shargvlen] = NULL;
}
/* We need to modify the first elementin shargv
* so that it has the relative filename and has
* a leading '-' to indicate it is a login shell
*/
shcmd = shargv[0];
if (shcmd[0] != '/') {
virReportSystemError(errno,
_("Shell '%s' should have absolute path"),
shcmd);
goto cleanup;
}
tmp = strrchr(shcmd, '/');
if (VIR_STRDUP(shargv[0], tmp) < 0)
goto cleanup;
shargv[0][0] = '-';
/* We're duping the string because the clearenv()
* call will shortly release the pointer we get
* back from virGetEnvAllowSUID() right here */
if (VIR_STRDUP(term, virGetEnvAllowSUID("TERM")) < 0)
goto cleanup;
/* A fork is required to create new process in correct pid namespace. */
if ((cpid = virFork()) < 0)
goto cleanup;
if (cpid == 0) {
int tmpfd;
for (i = 3; i < openmax; i++) {
tmpfd = i;
VIR_MASS_CLOSE(tmpfd);
}
clearenv();
setenv("PATH", "/bin:/usr/bin", 1);
setenv("SHELL", shcmd, 1);
setenv("USER", name, 1);
setenv("LOGNAME", name, 1);
setenv("HOME", homedir, 1);
if (term)
setenv("TERM", term, 1);
if (execv(shcmd, (char *const*) shargv) < 0) {
virReportSystemError(errno, _("Unable to exec shell %s"),
shcmd);
virDispatchError(NULL);
return errno == ENOENT ? EXIT_ENOENT : EXIT_CANNOT_INVOKE;
}
}
/* At this point, the parent is now waiting for the child to exit,
* but as that may take a long time, we release resources now. */
cleanup:
saved_err = virSaveLastError();
if (nfdlist > 0)
for (i = 0; i < nfdlist; i++)
VIR_FORCE_CLOSE(fdlist[i]);
VIR_FREE(fdlist);
virConfFree(conf);
if (dom)
virDomainFree(dom);
if (conn)
virConnectClose(conn);
virStringListFree(shargv);
VIR_FREE(shcmd);
VIR_FREE(term);
VIR_FREE(name);
VIR_FREE(homedir);
VIR_FREE(seclabel);
VIR_FREE(secmodel);
VIR_FREE(groups);
if (virProcessWait(cpid, &status, true) == 0)
virProcessExitWithStatus(status);
if (saved_err) {
virSetError(saved_err);
fprintf(stderr, "%s: %s\n", argv[0], virGetLastErrorMessage());
}
return ret;
}
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