fssb.c

/**
 * fssb.c - Main source file for the FSSB project.
 *
 * Copyright (C) 2016 Adhityaa Chandrasekar
 *
 * 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/>.
 */

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/syscall.h>
#include <assert.h>
#include <fcntl.h>
#include <sys/ptrace.h>
#include <sys/user.h>
#include <sys/wait.h>
#include <sys/stat.h>

#include "proxyfile.h"
#include "arguments.h"
#include "utils.h"

#define RDONLY_MEM_WRITE_SIZE 256

long first_rxp_mem = -1;
long write_slots[6];

/* Hopefully we don't need a 90-digit number. */
char SANDBOX_DIR[100];
int PROXY_FILE_LEN;

proxyfile_list *list;

FILE *log_file, *debug_file;

int cleanup, print_list;

int finish_and_return(int child, int syscall, int *retval) {
    if(syscall == -1) {
        char buf[2];
        fgets(buf, sizeof(buf), stdin);
    }

    if(syscall_breakpoint(child) != 0)
        return 0;

    *retval = get_reg(child, eax);
    return 1;
}

int handle_syscalls(pid_t child) {
    if(syscall_breakpoint(child) != 0)
        return 0;

    int syscall = get_reg(child, orig_eax);

    if(syscall != SYS_execve && first_rxp_mem == -1) {
        first_rxp_mem = get_readonly_mem(child);
        int i;
        for(i = 0; i < 6; i++)
            write_slots[i] = first_rxp_mem + i*RDONLY_MEM_WRITE_SIZE;
    }

    switch (syscall) {
        case SYS_exit:
        case SYS_exit_group: {
            int exit_code = get_syscall_arg(child, 0);
            fprintf(stderr, "fssb: child exited with %d\n", exit_code);
            fprintf(stderr, "fssb: sandbox directory: %s\n", SANDBOX_DIR);
            break;
        }
        case SYS_open:
        case SYS_creat: {
            /* int open(const char *pathname, int flags); */

            long orig_word = get_syscall_arg(child, 0);
            char *pathname = get_string(child, orig_word);

            int flags = get_syscall_arg(child, 1);

            proxyfile *cur;

            if(flags & O_APPEND || flags & O_CREAT || flags & O_WRONLY) {
                fprintf(debug_file, "open as write%s\n", pathname);
                cur = search_proxyfile(list, pathname);
                if(!cur)
                    cur = new_proxyfile(list, pathname);

                write_string(child, write_slots[0], cur->proxy_path);
                set_syscall_arg(child, 0, write_slots[0]);
            }

            if(flags == O_RDONLY) {
                /* If this file has been written to, then we should hijack the arg
                   with the proxyfile because we want the process to see its own
                   changes.  If this file has never been opened with write
                   permissions, we can just give the same file (this is more
                   performant than copying the file). */
                cur = search_proxyfile(list, pathname);
                fprintf(debug_file, "open as read %s\n", pathname);

                if(cur) {
                    write_string(child, write_slots[0], cur->proxy_path);
                    set_syscall_arg(child, 0, write_slots[0]);
                }
            }

            int retval;
            if(finish_and_return(child, syscall, &retval) == 0)
                return 0;

            set_syscall_arg(child, 0, orig_word);
            break;
        }
        case SYS_unlink:
        case SYS_unlinkat: {
            /* int unlink(const char *pathname); */
            /* int unlinkat(int dirfd, const char *pathname, int flags); */

            int swap_arg = 0;
            if(syscall == SYS_unlinkat)
                swap_arg = 1;

            long orig_word = get_syscall_arg(child, swap_arg);
            char *pathname = get_string(child, orig_word);

            fprintf(debug_file, "unlink %s\n", pathname);

            proxyfile *cur = search_proxyfile(list, pathname);
            char *new_name;

            if(cur) /* it's a file we've previously written to */
                new_name = cur->proxy_path;
            else {
                new_name = proxy_path(SANDBOX_DIR, pathname);

                struct stat sb;
                if(!stat(pathname, &sb)) /* this file actually exists */
                    fclose(fopen(new_name, "w"));
                else /* this file doesn't exist; so let them try to remove it */
                    new_name = pathname;
            }

            if(new_name != pathname) {
                write_string(child, write_slots[swap_arg], new_name);
                set_syscall_arg(child, swap_arg, write_slots[swap_arg]);
            }

            int retval;
            if(finish_and_return(child, syscall, &retval) == 0)
                return 0;

            set_syscall_arg(child, swap_arg, orig_word);

            if(cur) /* let's take this off our records */
                delete_proxyfile(list, cur);
            else /* we malloc'd some memory, let's free it */
                free(new_name);
            break;
        }
        case SYS_rename: {
            /* int rename(const char *oldpath, const char *newpath); */

            long orig_old_word = get_syscall_arg(child, 0),
                 orig_new_word = get_syscall_arg(child, 1);
            char *oldpath = get_string(child, orig_old_word),
                 *newpath = get_string(child, orig_new_word);

            fprintf(debug_file, "rename %s -> %s\n", oldpath, newpath);

            char *new_old_name = proxy_path(SANDBOX_DIR, oldpath),
                 *new_new_name = proxy_path(SANDBOX_DIR, newpath);

            write_string(child, write_slots[0], new_old_name);
            write_string(child, write_slots[1], new_new_name);
            set_syscall_arg(child, 0, write_slots[0]);
            set_syscall_arg(child, 1, write_slots[1]);

            int retval;
            if(finish_and_return(child, syscall, &retval) == 0)
                return 0;

            set_syscall_arg(child, 0, orig_old_word);
            set_syscall_arg(child, 1, orig_new_word);

            proxyfile *oldpf = search_proxyfile(list, oldpath);
            if(oldpf) { /* nothing to do if this is an invalid rename */
                delete_proxyfile(list, oldpf);

                /* register the new file as a known file for future reads */
                proxyfile *new = new_proxyfile(list, newpath);
            }

            free(new_old_name);
            free(new_new_name);
            break;
        }
        case SYS_stat:
        case SYS_lstat:
        case SYS_access: {
            /* int stat(const char *pathname, struct stat *buf); */
            /* int lstat(const char *pathname, struct stat *buf); */
            /* int access(const char *pathname, int mode); */

            long orig_word = get_syscall_arg(child, 0);
            char *pathname = get_string(child, orig_word);

            proxyfile *cur = search_proxyfile(list, pathname);

            if(cur) { /* it's a file we've previously written to */
                write_string(child, write_slots[0], cur->proxy_path);
                set_syscall_arg(child, 0, write_slots[0]);
            }

            int retval;
            if(finish_and_return(child, syscall, &retval) == 0)
                return 0;

            set_syscall_arg(child, 0, orig_word);
            break;
        }
    }
    return 1;
}

void trace(pid_t child) {
    int status;
    waitpid(child, &status, 0);

    assert(WIFSTOPPED(status));
    ptrace(PTRACE_SETOPTIONS, child, 0, PTRACE_O_TRACESYSGOOD);

    while(handle_syscalls(child));
}

int process_child(int argc, char **argv) {
    int i;
    char *args[argc+1];
    for(i=0; i < argc; i++)
        args[i] = argv[i];
    args[argc] = NULL;  /* execvp needs a NULL terminated list */

    ptrace(PTRACE_TRACEME);
    kill(getpid(), SIGSTOP);
    return execvp(args[0], args);
}

void init(int child) {
    struct stat sb;

    int i;
    for(i = 1; ; i++) {
        sprintf(SANDBOX_DIR, "/tmp/fssb-%d/", i);
        if(stat(SANDBOX_DIR, &sb))
            break;
    }
    mkdir(SANDBOX_DIR, 0775);

    PROXY_FILE_LEN = strlen(SANDBOX_DIR) + 32;

    list = new_proxyfile_list();
    list->SANDBOX_DIR = SANDBOX_DIR;
    list->PROXY_FILE_LEN = PROXY_FILE_LEN;
}

int main(int argc, char **argv) {
    build_help();
    check_args_validity(argc, argv);
    if(help_requested(argc, argv)) {
        print_help();
        return 0;
    }

    /* everything else must run a program */
    int pos = get_child_args_start_pos(argc, argv);
    int child_argc = argc - pos;
    char **child_argv = argv + pos;

    set_parameters(pos - 1,
                   argv,
                   &cleanup,
                   &log_file,
                   &debug_file,
                   &print_list);

    pid_t child = fork();

    if(child > 0) {
        init(child);
        trace(child);
    }
    else if(child == 0) {
        int return_code = 0;
        if(process_child(child_argc, child_argv) == -1) {
            fprintf(stderr, "fssb: %s: command not found\n", child_argv[0]);
            return_code = 1;
        }

        return return_code;
    }
    else {
        fprintf(stderr, "fssb: error: cannot fork\n");
        return 1;
    }

    write_map(list, SANDBOX_DIR);
    if(print_list)
        print_map(list, log_file);

    if(cleanup) {
        remove_proxy_files(list);
        rmdir(SANDBOX_DIR);
    }

    return 0;
}