AOSP Privileged vs System app

http://stackoverflow.com/questions/19868138/aosp-privileged-vs-system-app

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So in 4.3 there was a concept of System applications. Apks that were placed in System/app were given system privellages. As of 4.4, there is a new concept of Privellaged app. Privellaged apps are stored in system/priv-app and seem to be treated differently. If you look in the AOSP Source code, under PackageManagerService, you will see new methods such as

static boolean locationIsPrivileged(File path) {    try {        final String privilegedAppDir = new File(Environment.getRootDirectory(), "priv-app")                .getCanonicalPath();        return path.getCanonicalPath().startsWith(privilegedAppDir);    } catch (IOException e) {        Slog.e(TAG, "Unable to access code path " + path);    }    return false;}

So here is an example of a situation where these differ.

public final void addActivity(PackageParser.Activity a, String type) {...if (!systemApp && intent.getPriority() > 0 && "activity".equals(type)) {                intent.setPriority(0);                Log.w(TAG, "Package " + a.info.applicationInfo.packageName + " has activity "                        + a.className + " with priority > 0, forcing to 0");            }...

This affects the priority of any activities that are not defined as system applications. This seems to imply you can not add an activity to the package manager who's priority is higher than 0, unless you are a system app. This does NOT preclude privileged apps as far as I can tell (theres a lot of logic here, i may be wrong.)

My question is what exactly does this imply? If my app is privellaged, but not system, what difference will that make? In PackageManagerService you can find various things that differ between system and privileged apps, they are not exactly the same. There should be some kind of ideology behind privileged apps, otherwise they would have just said:

if locationIsPrivilleged: app.flags |= FLAG_SYSTEM

and been done with it. This is a new concept, and I think it would be important to know the difference between these kinds of apps for anyone who is doing AOSP development as of 4.4.

android android-source

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edited Nov 11 '13 at 20:38

asked Nov 8 '13 at 20:41

Andrew T.
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2 Answers

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So after some digging, it's clear that apps in priv-app get system privileges, the same way that old apps used to get system privileges by being in system-app. The only official Google documentation I could find on this came in the form of a commit message:

Commit hash: ccbf84f44c9e6a5ed3c08673614826bb237afc54

Some system apps are more system than others

"signatureOrSystem" permissions are no longer available to all apps residing en the /system partition. Instead, there is a new /system/priv-app directory, and only apps whose APKs are in that directory are allowed to use signatureOrSystem permissions without sharing the platform cert.This will reduce the surface area for possible exploits of system- bundled applications to try to gain access to permission-guarded operations.

The ApplicationInfo.FLAG_SYSTEM flag continues to mean what it is says in the documentation: it indicates that the application apk was bundled on the /system partition.A new hidden flag FLAG_PRIVILEGED has been introduced that reflects the actual right to access these permissions.

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edited Nov 20 '13 at 18:54

answered Nov 20 '13 at 18:49

Andrew T.
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stackoverflow.com/a/19813031/1306452 –  Andrew T. Jan 28 at 20:22

 

So if from 4.4, only /system/priv-app applications can get SignatureOrSystem permissions, what's the implication for privileges of apps that are kept in /system/app/ ? Thanks. –  JakeFeb 22 at 12:12 

 

More specifically, what's the purpose of /system/app/ folder in 4.4 ? Thanks. –  Jake Feb 22 at 12:18 

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@Jake Apps put in system/app are typically things that you might want to have less permissions. for instance, you probably don't want your email client or random vendor bloatware to be able to change your system security settings. –  Andrew T. Feb 22 at 18:19

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Apps in system/app have no special permissions. They don't differ from 3rd party apps unless they are signed with the system key (hence SigOrSystem check). As for the methods that helped, theres a variety. I started grepping for priv-app, and then followed that to PackageManagerService which now refers to Privilleged packages. –  Andrew T. Feb 23 at 5:10