Seatbelt

Seatbelt, also known as sandbox, is a kernel extension that restricts a set of features from being used for some processes.

Applying sandbox to your app

It is a good idea to apply a sandbox to restrict what your app can do, esp. for setuid apps, in case a vulnerability is found (well maybe not needed for iPhoneOS...). The documented way of applying a sandbox is

#include <sandbox.h>

char* errbuf;
int errcode = sandbox_init("profile", SANDBOX_NAMED, &errbuf);

...

sandbox_free_error(errbuf);

Once a sandbox is applied, you cannot jump out from the sandbox anymore.

There are 5 documented profiles

• kSBXProfileNoWriteExceptTemporary (= "write-tmp-only")
• kSBXProfileNoInternet (= "nointernet")
• kSBXProfileNoNetwork (= "nonet")
• kSBXProfileNoWrite (= "nowrite")
• kSBXProfilePureComputation (= "pure-computation")

and 9 undocumented profiles:

• "sandbox-compilerd"
• "mDNSResponder"
• "apsd"
• "AppleDiags"
• "PasteBoard"
• "MobileSafari"
• "MobileMail"
• "MobileMaps"
• "container" (Having the same restriction as AppStore apps.)

Definition of profiles

container

The 2.x .sb template

(version 1) (deny default) ; Sandbox violations get logged to syslog via kernel logging. (debug deny) (allow sysctl-read) ; Mount / umount commands (deny file-write-mount file-write-umount) ; System is read only (allow file-read*) (deny file-write*) ; NOTE: Later rules override earlier rules. ; Private areas (deny file-write* (regex "^/private/var/mobile/Applications/.*$")) (deny file-read* (regex "^/private/var/mobile/Applications/.*$")) ; SQLite uses /private/var/tmp ; TBR: <rdar://problem/5805879> SQLite doesn't honor the TMPDIR environment variable (allow file-write* (regex "^/private/var/tmp(/|$)")) (allow file-read* (regex "^/private/var/tmp(/|$)")) ; TBR: <rdar://problem/5806524> (allow process-exec (regex "^/private/var/tmp$")) ; TBR: <rdar://problem/5830139> (allow file-write* (regex "^/private/var/tmp/UpdatedSnapshots/$")) ; Permit reading and writing in the App container (allow file-read* (regex "^/private/var/mobile/Applications/XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX(/|$)")) (allow file-write* (regex "^/private/var/mobile/Applications/XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX/(tmp|Library|Documents)(/|$)")) (allow process-exec (regex #"^/private/var/mobile/Applications/XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX/.*\.app(/|$)")) ; Allow Address book access via filesystem ; This is an SQLite3 database - there is room to make the rules tighter (allow file-write* (regex "^/private/var/mobile/Library/AddressBook(/|$)")) (allow file-read* (regex "^/private/var/mobile/Library/AddressBook(/|$)")) ; Allow keyboard db access via filesystem ; This is a custom file format. There is room to make the rules tighter (allow file-write* (regex "^/private/var/mobile/Library(/Keyboard)?(/|$)")) (allow file-read* (regex "^/private/var/mobile/Library(/Keyboard)?(/|$)")) ; Pictures, but not other media ; Allow photo access via filesystem. There is room to make the rules tighter (deny file-write* (regex "^/private/var/mobile/Media(/|$)")) (deny file-read* (regex "^/private/var/mobile/Media/")) (allow file-write* (regex "^/private/var/mobile/Media/com.apple.itunes.lock_sync$")) (allow file-read* (regex "^/private/var/mobile/Media/com.apple.itunes.lock_sync$")) (allow file-write* (regex "^/private/var/mobile/Media/DCIM(/|$)")) (allow file-read* (regex "^/private/var/mobile/Media/DCIM(/|$)")) (allow file-read* (regex "^/private/var/mobile/Media/Photos(/|$)")) ; Mach lookups. There is room to make the rule tighter. (allow mach-lookup) ;; (global-name "PurpleSystemEventPort") ;; (global-name "com.apple.CARenderServer") ;; (global-name "com.apple.eventpump") ;; (global-name "com.apple.springboard.migserver") ;; (global-name "com.apple.system.notification_center")) (deny process-fork) ; For ASL logs - /var/run/asl_input (XXX: socket can now be named) ; (allow network-outbound) ; (to unix-socket "/private/var/run/asl_input")) (allow network*) ; To allow crash reporter / exceptions to kill the process (allow signal (target self))

MobileSafari

Same as MobileMail but /var/mobile/Media/Safari/ is also read/writable.

MobileMail

• UNIX sockets cannot be used.
• The following files and directories cannot be read:
  • /var/mobile/Applications/*
  • /var/mobile/Media/* (except DCIM/, com.apple.itdbprep.postprocess.lock and com.apple.itunes.lock_sync)
  • /var/mobile/Library/Caches/com.apple.mobile.installation.plist
  • /usr/sbin/fairplayd
• The filesystem by default is read-only, but the following directories can be written:
  • /var/mobile/Library/*
  • /var/mobile/Media/DCIM/*
  • /tmp/*

MobileMaps

Based on MobileMail but

• /var/logs cannot be read.
• /var/mobile/Library/Caches/com.apple.UIKit.pboard/* cannot be read.
• /var/mobile/Media/* cannot be written.
• /var/mobile/Library/* cannot be written, except:
  • /var/mobile/Library/Caches/Maps/*
  • /var/mobile/Library/Keyboard/*
  • /var/mobile/Library/Maps/*
  • /var/mobile/Library/SMS/*

PasteBoard

Everywhere is read-only, except:

• /var/mobile/Library/Caches/com.apple.UIKit.pboard/*
• /tmp/*

AppleDiags

Based on MobileMail but

• /var/mobile/Media/* can be read.
• /var/mobile/Library/Caches/com.apple.mobile.installation.plist can be read.
• /var/mobile/Library/* cannot be written, except:
  • /var/mobile/Library/AddressBook/*
  • /var/mobile/Library/Keyboard/*
  • /var/mobile/Library/Logs/CrashReporter/*
  • /var/mobile/Library/Preferences/*

apsd, mDNSResponder

The filesystem cannot be accessed at all.

sandbox-compilerd

Only /var/* can be accessed freely.

Creating your own sandbox definition

A sandbox definition should be a Scheme file with extension .sb. If a non-built-in profile is given, a .sb placed in one of:

• /usr/share/sandbox/*.sb
• /private/var/mobile/Applications/*.sb

shall be compiled and sandboxed by this definition. (You can also provide a complete path.) However, due to the lack of sandboxd the .sb files cannot be compiled, so assume this feature isn't there yet.

Oasis in the sandbox around /var/mobile

While sandbox is a nice security feature, it is sometimes too restrictive for a MobileSubstrate extension. Since the sandbox is implemented as a kernel feature it's impossible to workaround it. For example, an MS extension targeting AppStore app can only write in /var/mobile, but most of the /var/mobile itself is unreadable due to sandboxing. But there are a few places that the sandbox cannot apply, so AppStore apps may store data in these places. They are:

• Anything in the container itself (of course).
• ~/* except the Media and Library subfolders, which only the following directories are readable:
• ~/Media/DCIM/*
• ~/Media/Photos/*
• ~/Library/AddressBook/*
• ~/Library/Keyboard/*
• ~/Library/Preferences/*

An AppStore app itself can also read/write to these folders, even without jailbreaking (in principle), but they must be accessed via a symlink. Doing so allows easy cross-app data sharing, but that may violate the AppStore rules.

Kernel debug messages regarding the sandbox

You can turn on kernel debug messages from the Seatbelt.kext with this command:

sysctl -w security.mac.seatbelt.debug=<integer>

where the integer is a bit-field, with purposes:

• 1 = Memory management (reference counting) messages. Very verbose, do not turn it on.
• 2 = Function hook messages.
• 4, 8, 16, 32, 64, 128 = ???

Set to 0 to turn off everything.