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exec: Introduce osmo_system_nowait2() to allow specify a user 

For a process running as root, it may be desirable to drop privileges
down to a normal user  before executing an external command.  Let's
add a new API function for that.

Change-Id: If1431f930f72a8d6c1d102426874a11b7a2debd9
This commit is contained in:
Harald Welte 2020-04-17 18:23:52 +02:00
parent e7d0d70ce7
commit f3cc731d40
2 changed files with 49 additions and 2 deletions
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1
include/osmocom/core/exec.h
View File

@ -25,4 +25,5 @@ extern const char *osmo_environment_whitelist[];
int osmo_environment_filter(char **out, size_t out_len, char **in, const char **whitelist);
int osmo_environment_append(char **out, size_t out_len, char **in);
int osmo_close_all_fds_above(int last_fd_to_keep);
int osmo_system_nowait2(const char *command, const char **env_whitelist, char **addl_env, const char *user);
int osmo_system_nowait(const char *command, const char **env_whitelist, char **addl_env);

50
src/exec.c
View File

@ -23,6 +23,7 @@
#include "config.h"
#ifndef EMBEDDED
#define _GNU_SOURCE
#include <unistd.h>
#include <errno.h>
@ -31,6 +32,7 @@
#include <stdio.h>
#include <dirent.h>
#include <sys/types.h>
#include <pwd.h>
#include <osmocom/core/logging.h>
#include <osmocom/core/utils.h>
@ -192,23 +194,34 @@ int osmo_close_all_fds_above(int last_fd_to_keep)
/* Seems like POSIX has no header file for this, and even glibc + __USE_GNU doesn't help */
extern char **environ;
/*! call an external shell command without waiting for it.
/*! call an external shell command as 'user' without waiting for it.
*
* This mimics the behavior of system(3), with the following differences:
* - it doesn't wait for completion of the child process
* - it closes all non-stdio file descriptors by iterating /proc/self/fd
* - it constructs a reduced environment where only whitelisted keys survive
* - it (optionally) appends additional variables to the environment
* - it (optionally) changes the user ID to that of 'user' (requires execution as root)
*
* \param[in] command the shell command to be executed, see system(3)
* \param[in] env_whitelist A white-list of keys for environment variables
* \param[in] addl_env any additional environment variables to be appended
* \param[in] user name of the user to which we should switch before executing the command
* \returns PID of generated child process; negative on error
*/
int osmo_system_nowait(const char *command, const char **env_whitelist, char **addl_env)
int osmo_system_nowait2(const char *command, const char **env_whitelist, char **addl_env, const char *user)
{
struct passwd _pw, *pw;
int getpw_buflen = sysconf(_SC_GETPW_R_SIZE_MAX);
int rc;
if (user) {
char buf[getpw_buflen];
getpwnam_r(user, &_pw, buf, sizeof(buf), &pw);
if (!pw)
return -EINVAL;
}
rc = fork();
if (rc == 0) {
/* we are in the child */
@ -232,6 +245,20 @@ int osmo_system_nowait(const char *command, const char **env_whitelist, char **a
return rc;
}
/* drop privileges */
if (pw) {
if (setresgid(pw->pw_gid, pw->pw_gid, pw->pw_gid) < 0) {
perror("setresgid() during privilege drop");
exit(1);
}
if (setresuid(pw->pw_uid, pw->pw_uid, pw->pw_uid) < 0) {
perror("setresuid() during privilege drop");
exit(1);
}
}
/* if we want to behave like system(3), we must go via the shell */
execle("/bin/sh", "sh", "-c", command, (char *) NULL, new_env);
/* only reached in case of error */
@ -244,4 +271,23 @@ int osmo_system_nowait(const char *command, const char **env_whitelist, char **a
}
}
/*! call an external shell command without waiting for it.
*
* This mimics the behavior of system(3), with the following differences:
* - it doesn't wait for completion of the child process
* - it closes all non-stdio file descriptors by iterating /proc/self/fd
* - it constructs a reduced environment where only whitelisted keys survive
* - it (optionally) appends additional variables to the environment
*
* \param[in] command the shell command to be executed, see system(3)
* \param[in] env_whitelist A white-list of keys for environment variables
* \param[in] addl_env any additional environment variables to be appended
* \returns PID of generated child process; negative on error
*/
int osmo_system_nowait(const char *command, const char **env_whitelist, char **addl_env)
{
return osmo_system_nowait2(command, env_whitelist, addl_env, NULL);
}
#endif /* EMBEDDED */