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touches to shell proxy

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reworked the privilege escalation works by changing the way target user info
is passed around, now we pass struct passwd* and struct group* instead of
strings wich allows much greater flexibility.
Also now us mimics the behavior of sudo and doas regarding the shell proxies:
- commands are started under a subshell, that shell is the user's
- if no command is specified just the shell is executed
noproxy
Alessandro Mauri 4 years ago
parent 37c594c1bd
commit a666081599
2 changed files with 125 additions and 102 deletions
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  1. 2
      TODO
  2. 203
      us.c

2
TODO
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@ -3,3 +3,5 @@
* LOGNAME -> to target user
* SHELL -> to the target user's SHELL
* HOME -> to the target user's HOME
- Reconsider executing programs in a subshell

203
us.c
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@ -32,8 +32,10 @@
#include <security/pam_misc.h>
static void usage (void);
static int perm_set (const char *, const char *);
static int perm_set (struct passwd *, struct group *);
static int authenticate (const char *);
static struct passwd* user_to_passwd (const char *);
static struct group* group_to_grp (const char *);
// FIXME: misc_conv is a separate library, should stick to plain PAM or make
// our own pam module
@ -43,9 +45,12 @@ int main (int argc, char *argv[])
{
// TODO: Add arguments
// FIXME: change the default program to execute SHELL
char *t_usr = NULL, *t_grp = NULL;
char *t_usr = "root", *t_grp = NULL;
struct passwd *t_pw;
struct group *t_gr;
int opt;
while ((opt = getopt(argc, argv, "A:u:g:C:")) != -1) {
int shellflag = 0;
while ((opt = getopt(argc, argv, "A:u:g:C:s")) != -1) {
switch (opt) {
case 'A':
printf("-A is not yet implemented\n");
@ -61,6 +66,9 @@ int main (int argc, char *argv[])
printf("-C is not yet implemented\n");
exit(EXIT_FAILURE);
break;
case 's':
shellflag = 1;
break;
case '?':
usage();
exit(EINVAL);
@ -68,64 +76,51 @@ int main (int argc, char *argv[])
}
}
/*
int c_argc = argc - optind + 2;
char **c_argv = malloc((c_argc + 1) * sizeof(char *));
if (!c_argv) {
fprintf(stderr, "malloc: %s\n", strerror(errno));
goto fail_end;
}
c_argv[0] = shell;
c_argv[1] = "-c";
for (int i = 2; optind < argc; i++, optind++) {
c_argv[i] = strdup(argv[optind]);
if (!c_argv[i]) {
/* Get user info */
const char *uname;
char *shell;
uid_t ruid = getuid();
struct passwd *my_pw = getpwuid(ruid);
if (!my_pw) {
fprintf(stderr, "getpwid: %s\n", strerror(errno));
exit(errno);
return errno;
}
uname = my_pw->pw_name;
t_pw = user_to_passwd(t_usr);
if (!t_pw) {
fprintf(stderr, "user_to_passwd: %s\n", strerror(errno));
return errno;
}
c_argc[c_argv] = NULL;
*/
t_gr = group_to_grp(t_grp);
/* Get target user's shell */
if (!shellflag)
shell = t_pw->pw_shell;
else
shell = getenv("SHELL");
if (!shell)
shell = "/bin/sh";
/* Set argc and argv */
// TODO: get the right shell for the user (it is in the passwd struct)
char *shell = "/bin/sh";
char *command, **c_argv;
char *command = NULL;
int size, popind = optind;
int c_argc = 3; // /bin/sh -c command
c_argv = malloc((c_argc + 1) * sizeof(char *));
if (!c_argv) {
fprintf(stderr, "malloc: %s\n", strerror(errno));
exit(errno);
}
if (argc - optind) {
for (size = 0; optind < argc; optind++)
size += strlen(argv[optind]) + 1;
command = malloc(sizeof(char) * size);
command = malloc(sizeof(char) * size + 1);
if (!command) {
fprintf(stderr, "malloc: %s\n", strerror(errno));
exit(errno);
}
memset(command, 0, size + 1);
for (optind = popind; optind < argc; optind++) {
strcat(command, argv[optind]);
strcat(command, " ");
}
c_argv[0] = shell;
// FIXME: this should be optional if size is zero: we want just a shell
c_argv[1] = "-c";
c_argv[2] = command;
c_argv[c_argc] = NULL;
}
/* Authenticate */
uid_t ruid = getuid();
struct passwd *pw = getpwuid(ruid);
if (!pw) {
fprintf(stderr, "getpwid: %s\n", strerror(errno));
return errno;
}
const char *uname = pw->pw_name;
if (authenticate(uname) != PAM_SUCCESS)
exit(EXIT_FAILURE);
@ -133,21 +128,24 @@ int main (int argc, char *argv[])
errno = 0;
/* Set permissions */
if (perm_set(t_usr, t_grp) == -1) { // 0 = root
if (perm_set(t_pw, t_gr) == -1) {
fprintf(stderr, "perm_set: %s\n", strerror(errno));
goto fail_end;
}
/* Execute the command */
if (execv(*c_argv, c_argv) == -1) // execvp searches in path
fprintf(stderr, "execv: %s\n", strerror(errno));
int err;
if (command)
err = execl(shell, shell, "-c", command, (char *)NULL);
else
err = execl(shell, shell, (char *)NULL);
if (err == -1)
fprintf(stderr, "execl: %s\n", strerror(errno));
/* Cleanup and return */
fail_end:
/* Free up the copied argv */
for (int i = 0; i < c_argc; i++)
free(c_argv[i]);
free(c_argv);
free(command);
return errno;
}
@ -161,60 +159,24 @@ static inline void usage (void)
// -c [file]: manually select config file
// something about environment
// something about non interactiveness
printf("usage: us [-u user] [-g group] command [args]\n");
printf("usage: us [-s] [-u user] [-g group] command [args]\n");
}
static int perm_set (const char *user, const char *group)
static int perm_set (struct passwd *pw, struct group *gr)
{
if (!user)
user = "root";
struct passwd *pw;
struct group *gr;
uid_t uid;
gid_t gid;
long uid_l, gid_l;
errno = 0;
if (user[0] != '#') {
pw = getpwnam(user);
} else {
uid_l = strtol(&user[1], NULL, 10);
if (uid_l < 0 || errno) {
errno = errno ? errno : EINVAL;
return -1;
}
pw = getpwuid((uid_t)uid_l);
}
if (!pw) {
if (!errno)
errno = EINVAL;
return -1;
}
uid_t uid;
gid_t gid;
uid = pw->pw_uid;
gid = pw->pw_gid;
if (group) {
if (group[0] != '#') {
gr = getgrnam(group);
} else {
gid_l = strtol(&group[1], NULL, 10);
if (gid_l < 0 || errno) {
errno = errno ? errno : EINVAL;
return -1;
}
gr = getgrgid((gid_t)gid_l);
}
if (!gr) {
if (!errno)
errno = EINVAL;
return -1;
}
if (gr)
gid = gr->gr_gid;
}
/* Set permissions, setting group perms first because in the case of
* dropping from higher permissions setting the uid first results in
@ -272,3 +234,62 @@ static int authenticate (const char *uname)
return pam_end(pamh, pam_err);
}
static struct passwd* user_to_passwd (const char *user)
{
if (!user) {
errno = EINVAL;
return NULL;
}
struct passwd* pw;
long uid_l;
errno = 0;
if (user[0] != '#') {
pw = getpwnam(user);
} else {
uid_l = strtol(&user[1], NULL, 10);
if (uid_l < 0 || errno) {
errno = errno ? errno : EINVAL;
return NULL;
}
pw = getpwuid((uid_t)uid_l);
}
if (!pw) {
if (!errno)
errno = EINVAL;
return NULL;
}
return pw;
}
static struct group* group_to_grp (const char *group)
{
if (!group) {
errno = EINVAL;
return NULL;
}
struct group* gr;
long gid_l;
errno = 0;
if (group[0] != '#') {
gr = getgrnam(group);
} else {
gid_l = strtol(&group[1], NULL, 10);
if (gid_l < 0 || errno) {
errno = errno ? errno : EINVAL;
return NULL;
}
gr = getgrgid((gid_t)gid_l);
}
if (!gr) {
if (!errno)
errno = EINVAL;
return NULL;
}
return gr;
}

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