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sec.c
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sec.c
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/*
* onpts is a too to securely access the input buffer of other pseudo terminals
* Copyright (C) 2017 Ralf Stemmer <ralf.stemmer@gmx.net>
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* This program 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 General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/ioctl.h>
#include <sys/types.h>
#include <dirent.h>
#include <string.h>
#include <errno.h>
#include <ctype.h>
#include <unistd.h>
#include <stdbool.h>
#include <fein/fein.h>
#include "sec.h"
static uid_t global_uid;
static gid_t global_gid;
#define RETVAL_ERROR -1 // Never change this value, it is related to the error-behavior of libfein
#define RETVAL_OK 0
#define RETVAL_UNSECURE RETVAL_ERROR
static int ForEachPIDCallback(const char *str, const char *delimiters, const char *parent_pid);
static int ForEachTaskCallback(const char *dirpath, struct dirent *entry);
static int ForEachPIDLineCallback(const char *filename, const char *line, size_t linelength, size_t linenumber);
static int CheckStatus(char *statusfilepath);
static int ForEachStatusLineCallback(const char *filename, const char *line, size_t linelength, size_t linenumber);
/*
* Structure of the recursive construct of callback functions.
*
* ┌───────────────────────────┐
* │ │
* │ CheckPrivileges │
* │ │
* └─────────────┬─────────────┘
* │
* │ For each PID given by
* │ "fuser $PTY"
* ▼
* ┌───────────────────────────┐ ┌───────────────────────────┐
* │ │ │ │
* ┌──▶│ ForEachPIDCallback ├──▶│ CheckStatus │
* │ │ │ │ │
* │ └─────────────┬─────────────┘ └──────────────┬────────────┘
* │ │ │
* │ │ For each TID in │ For each line in
* │ │ /proc/$PID/task │ /proc/$PID/status
* │ ▼ ▼
* │ ┌───────────────────────────┐ ┌───────────────────────────┐
* │ │ │ │ │
* │ │ ForEachTaskCallback │ │ ForEachStatusLineCallback │
* │ │ │ │ │
* │ └─────────────┬─────────────┘ └───────────────────────────┘
* │ │
* │ │ For each child PID in
* │ │ /proc/$PID/task/$TID/children
* │ ▼
* │ ┌───────────────────────────┐
* │ │ │
* └───┤ ForEachPIDLineCallback │
* │ │
* └───────────────────────────┘
*/
/*
* This function checks for all processes running on the given terminal that may be accessed
* by onpts, if they have different permissions than onpts.
*
* Args:
* pts_path: path to the pseudo terminal slave /dev/pts/X
* uid: The UID of the onpts caller
* gid: The GID of the onpts caller
*
* Returns:
* RETVAL_OK: On the PTS addressed by pty_path, there is NO process/task/child
* running with other permissions than the onpts has.
* otherwise: It is not secure to send input data to the other terminal!
* There may be a process running as root.
*/
int CheckPrivileges(const char* pts_path, uid_t uid, gid_t gid)
{
global_uid = uid;
global_gid = gid;
// Get PIDs that access PTY
FILE *pp;
char command[128];
strncpy(command, "fuser ", 7);
strncat(command, pts_path, 100);
strncat(command, " 2>/dev/null", 20);
#ifdef DEBUG
printf("\e[1;34m\tExecuting \e[0;36m%s\e[0m\n", command);
#endif
pp = popen(command, "r");
if(!pp)
{
fprintf(stderr, "\e[1;31mpopen(\"%s\", \"r\") failed with error: ", command);
fprintf(stderr, "%s\e[0m\n", strerror(errno));
return RETVAL_ERROR;
}
#ifdef DEBUG
printf("\e[1;34m\tReading PIDs: ");
#endif
char pidlist[1024];
fgets(pidlist, 1024, pp);
pclose(pp);
#ifdef DEBUG
printf("\e[0;36m%s\e[0m\n", pidlist);
#endif
int retval;
retval = ForEachTokenInString(pidlist, " ", ForEachPIDCallback);
return retval;
}
/*
* This function checks the permissions of a process with the given PID.
* If the processes permission are OK, its tasks and their child process
* permissions will be checked.
*
* Args:
* str: (not used) - DO NOT USE THIS ARGUMENT
* delimiters: (not used) - DO NOT USE THIS ARGUMENT
* parent_pid: the PID thats permission and child permissions shall be checked
*
* Returns:
* The security status of the process and its child processes
*/
int ForEachPIDCallback(const char *str, const char *delimiters, const char *parent_pid)
{
int retval;
// Check status
#ifdef DEBUG
printf("\e[1;34m\tCheck status of pid \e[0;36m%s\e[0m\n", parent_pid);
#endif
char *statuspath;
asprintf(&statuspath, "/proc/%s/status", parent_pid);
retval = CheckStatus(statuspath);
free(statuspath);
if(retval != RETVAL_OK)
return retval;
// For each task the child processes must be checked
char *taskdirpath;
asprintf(&taskdirpath, "/proc/%s/task", parent_pid);
retval = ForEachFileInDir(taskdirpath, ForEachTaskCallback);
free(taskdirpath);
return retval;
}
/*
* This function gets called for each task of a process.
* And for each task the child processes (if there are any) permissions
* will be checked later on in the ForEachPIDLineCallback function.
* The PIDs of the child processes were read from the file
* /proc/$PID/task/$TID/children
*
* Args:
* dirpath: path to the task directory: /proc/$PID/task
* entry: One entry inside the task directory except "." and ".."
*
* Returns:
* The security status of the tasks child processes
*/
int ForEachTaskCallback(const char *dirpath, struct dirent *entry)
{
// check children
char *childlistpath;
int retval;
asprintf(&childlistpath, "%s/%s/children", dirpath, entry->d_name);
retval = ForEachLineInFile(childlistpath, ForEachPIDLineCallback);
free(childlistpath);
return retval;
}
/*
* This is a wrapper function to ForEachPIDCallback.
* This callback gets called for each PID in the /proc/PARENTPID/task/PARENTTID/children
* file.
* The line containing a child PID gets trimmed (removing trailing space).
* Then ForEachPIDCallback gets called with str=NULL, delimiters=NULL and parent_pid as the
* child PID in the children file to recursively check those permissions
* and the permissions of those child processes
*
* Args:
* filename: (not used) - path to the children file of a task
* line: One line of the children file containing one PID of a child
* linelength: (not used) - length of the line in bytes
* linenumber: (not used) - number of the line starting by 1
*
* Returns:
* The security status of the child processes
*/
int ForEachPIDLineCallback(const char *filename, const char *line, size_t linelength, size_t linenumber)
{
// remove trailing spaces
char *pid;
pid = (char*)malloc(sizeof(char)*linelength);
if(pid == NULL)
{
fprintf(stderr, "\e[1;31mmalloc(%lu); failed with error: ", sizeof(char)*linelength);
fprintf(stderr, "\e[1;31m%s\e[0m\n", strerror(errno));
return -1;
}
strncpy(pid, line, linelength);
while(pid[linelength-1] == ' ')
pid[--linelength] = '\0';
// check the childs permissions
int retval;
retval = ForEachPIDCallback(NULL, NULL, (const char*)pid);
free(pid);
return retval;
}
/*
* This function reads the status file of a process: /proc/$PID/status.
* This file contains the UID and GID of the process. Those have to match
* the UID and GID of the user who called onpts (stored in global_{uid,gid}).
*
* Args:
* statusfilepath: Path to the status file of a process
*
* Returns:
* Result of the ForEachStatusLineCallback:
* RETVAL_OK: if the UID and GID match
* RETVAL_UNSECURE: if the UID or GID are different of the callers one
*/
int CheckStatus(char *statusfilepath)
{
#ifdef DEBUG
printf("\e[1;34m\t\tCheckin status in \e[0;36m%s\e[0m\n", statusfilepath);
#endif
int retval;
retval = ForEachLineInFile(statusfilepath, ForEachStatusLineCallback);
return retval;
}
/*
* This function gets applied to each line in /proc/$PID/status.
* If a line starts with "Uid:" or "Gid:", the listed ids will be compared
* to the IDs of the onpts callers user/group IDs.
* If one of the IDs don't match, the function returns RETVAL_USECURE and the
* whole security check gets stop.
* Furthermore an error messages gets printed to stderr telling the user that
* he has no permission to run onpts.
*
* Args:
* filename: (not used) - path to the statusfile
* line: One line of the status file
* linelength: (not used) - length of the line in bytes
* linenumber: (not used) - number of the line starting by 1
*
* Returns:
*/
int ForEachStatusLineCallback(const char *filename, const char *line, size_t linelength, size_t linenumber)
{
if(strncmp(line, "Uid:", 4) == 0 || strncmp(line, "Gid:", 4) == 0)
{
uid_t real, eff, saved, fs;
char tmp[128];
int n;
n = sscanf(line, "%127s\t%d\t%d\t%d\t%d", tmp, &real, &eff, &saved, &fs);
#ifdef DEBUG
printf("\e[1;34m\t\t\t\"\e[1;36m%s\e[1;34m\": ", tmp);
printf("\e[0;36m%d\e[1;34m, ", real);
printf("\e[0;36m%d\e[1;34m, ", eff);
printf("\e[0;36m%d\e[1;34m, ", saved);
printf("\e[0;36m%d\e[0m\n", fs);
#endif
if(n != 5)
{
fprintf(stderr, "\e[1;31mScanning %s failed!\e[0m\n", line);
fprintf(stderr, "\e[1;31mError: %s\e[0m\n", strerror(errno));
return RETVAL_ERROR;
}
uid_t id;
if(tmp[0] == 'U') id = global_uid;
if(tmp[0] == 'G') id = global_gid;
if(real != id || eff != id || saved != id || fs != id)
{
fprintf(stderr, "\e[1;31mPermission denied - One process on destination PTS has different privileges!\e[0m\n");
return RETVAL_UNSECURE;
}
}
return RETVAL_OK;
}
// vim: tabstop=4 expandtab shiftwidth=4 softtabstop=4