Using the Snow Leopard sandbox

First and foremost, a warning that's prominently displayed in all sandbox definition files:


WARNING: The sandbox rules [..] currently constitute Apple System Private Interface and are subject to change at any time and
without notice. The contents of this file are also auto-generated and not user editable; it may be overwritten at any time.


The sandbox being a private interface is probably the reason why the sandbox policy files are un-documented. However, in the standard Snow Leopard install there are a bunch of files that define sandboxing policies ( under /System/Library/Sandbox/Profiles/ and /usr/share/sandbox/) that can be used to derive new policy files.

When creating a policy file for HTTP, the most verbose section out of our sandbox definition turns out to be the section for allowing DNS queries.

http.sb

(version 1)
(debug deny)

;; DNS
(allow file-read-metadata
        (literal "/var")
        (literal "/etc"))

(allow ipc-posix-shm)

(allow mach-lookup
  (global-name "com.apple.SystemConfiguration.configd")
  (global-name "com.apple.system.notification_center"))


(allow network-outbound
       (remote unix-socket (path-literal "/private/var/run/mDNSResponder")))

;; HTTP traffic
(allow network-outbound (remote ip4 "*:80"))

(deny default)

Our test program runs a couple of functions - a curl based HTTP client and a file read operation.

test.c

#include <sandbox.h>
#include <stdio.h>
#include <curl/curl.h>

size_t write_data(void *buffer, size_t size, size_t nmemb, void *userp)
{
 int i=0;
 //exit after 80 chars
 for(;i<size * nmemb && i < 80;i++)
  fprintf(stderr, "%c", (unsigned char)((char *)buffer)[i]);
 fprintf(stderr, "[..]\n");
}
int httpRequest()
{
 CURL * easyhandle = curl_easy_init();
 curl_easy_setopt(easyhandle, CURLOPT_URL, "http://loudhush.ro/changelog.txt");
 curl_easy_setopt(easyhandle, CURLOPT_WRITEFUNCTION, write_data);
 curl_easy_perform(easyhandle);
}
int readFile()
{
 FILE * f = fopen("/etc/passwd", "r");
 if(f == NULL)
 {
  perror("Error opening file");
  return 1;
 }

 char line[1024];
 while(fgets(line, 1023, f) != NULL)
  fprintf(stdout, "Line: %s", line);

 fclose(f);
 return 0;
}
int main()
{
 char * errbuf;
 int retval = sandbox_init("/Users/diciu/Programming/sandbox/http.sb", SANDBOX_NAMED, &errbuf);
 if(retval)
 {
  fprintf(stderr, "Sandbox init error: %s\n", errbuf);
  return 1;
 }
 fprintf(stderr, "--- HTTP request ---\n");
 httpRequest();
 fprintf(stderr, "\n--- File read request ---\n");
 readFile();
}

As expected, once the sandbox is initialized, HTTP requests go through but the file access operation is denied:

cristi:sandbox diciu$ ./a.out 
--- HTTP request ---
LoudHush 1.3.21

- Fixed a couple of leaks in AccountController/Management
- Fix[..]

--- File read request ---
Error opening file: Operation not permitted

Attempts to go out of the sandbox are logged in syslog:

Jan  6 14:13:37 cristi sandboxd[59224]: a.out(59222) deny file-read-data /private/etc/passwd