M7350/base/docs/html/guide/topics/security/security.jd

page.title=Security and Permissions
@jd:body

<div id="qv-wrapper">
<div id="qv">

<h2>In this document</h2>
<ol>
<li><a href="#arch">Security Architecture</a></li>
<li><a href="#signing">Application Signing</a></li>
<li><a href="#userid">User IDs and File Access</a></li>
<li><a href="#permissions">Using Permissions</a></li>
<li><a href="#declaring">Declaring and Enforcing Permissions</a>
	<ol>
	<li><a href="#manifest">...in AndroidManifest.xml</a></li>
	<li><a href="#broadcasts">...when Sending Broadcasts</a></li>
	<li><a href="#enforcement">Other Permission Enforcement</a></li>
	</ol></li>
<li><a href="#uri">URI Permissions</a></li>
</ol>
</div>
</div>

<p>Android is a privilege-separated operating system, in which each
application runs with a distinct system identity (Linux user ID and group
ID).  Parts of the system are also separated into distinct identities.
Linux thereby isolates applications from each other and from the system.</p>

<p>Additional finer-grained security features are provided through a
"permission" mechanism that enforces restrictions on the specific operations
that a particular process can perform, and per-URI permissions for granting
ad-hoc access to specific pieces of data.</p>

<a name="arch"></a>
<h2>Security Architecture</h2>

<p>A central design point of the Android security architecture is that no
application, by default, has permission to perform any operations that would
adversely impact other applications, the operating system, or the user.  This
includes reading or writing the user's private data (such as contacts or
e-mails), reading or writing another application's files, performing
network access, keeping the device awake, etc.</p>

<p>Because the kernel sandboxes applications from each other, applications
must explicitly share resources and data. They do this by declaring the
<em>permissions</em> they need for additional capabilities not provided by
the basic sandbox. Applications statically declare the permissions they
require, and the Android system prompts the user for consent at the time the
application is installed. Android has no mechanism for granting permissions
dynamically (at run-time) because it complicates the user experience to the
detriment of security.</p>

<p>The kernel is solely responsible for sandboxing applications from each
other. In particular the Dalvik VM is not a security boundary, and any app
can run native code (see <a href="/sdk/ndk/index.html">the Android NDK</a>).
All types of applications &mdash; Java, native, and hybrid &mdash; are
sandboxed in the same way and have the same degree of security from each
other.</p>

<a name="signing"></a>
<h2>Application Signing</h2>

<p>All Android applications (.apk files) must be signed with a certificate
whose private key is held by their developer.  This certificate identifies
the author of the application.  The certificate does <em>not</em> need to be
signed by a certificate authority: it is perfectly allowable, and typical,
for Android applications to use self-signed certificates. The purpose of
certificates in Android is to distinguish application authors. This allows
the system to grant or deny applications access to <a
href="/guide/topics/manifest/permission-element.html#plevel">signature-level
permissions</a> and to grant or deny an application's <a
href="/guide/topics/manifest/manifest-element.html#uid">request to be given
the same Linux identity</a> as another application.</p>

<a name="userid"></a>
<h2>User IDs and File Access</h2>

<p>At install time, Android gives each package a distinct Linux user ID. The
identity remains constant for the duration of the package's life on that
device. On a different device, the same package may have a different UID;
what matters is that each package has a distinct UID on a given device.</p>

<p>Because security enforcement happens at the
process level, the code of any two packages can not normally
run in the same process, since they need to run as different Linux users.
You can use the {@link android.R.attr#sharedUserId} attribute in the
<code>AndroidManifest.xml</code>'s
{@link android.R.styleable#AndroidManifest manifest} tag of each package to
have them assigned the same user ID.  By doing this, for purposes of security
the two packages are then treated as being the same application, with the same
user ID and file permissions.  Note that in order to retain security, only two applications
signed with the same signature (and requesting the same sharedUserId) will
be given the same user ID.</p>

<p>Any data stored by an application will be assigned that application's user
ID, and not normally accessible to other packages.  When creating a new file
with {@link android.content.Context#getSharedPreferences},
{@link android.content.Context#openFileOutput}, or
{@link android.content.Context#openOrCreateDatabase},
you can use the
{@link android.content.Context#MODE_WORLD_READABLE} and/or
{@link android.content.Context#MODE_WORLD_WRITEABLE} flags to allow any other
package to read/write the file.  When setting these flags, the file is still
owned by your application, but its global read and/or write permissions have
been set appropriately so any other application can see it.</p>


<a name="permissions"></a>
<h2>Using Permissions</h2>

<p>A basic Android application has no permissions associated with it,
meaning it can not do anything that would adversely impact the user experience
or any data on the device.  To make use of protected features of the device,
you must include in your <code>AndroidManifest.xml</code> one or more
<code>{@link android.R.styleable#AndroidManifestUsesPermission &lt;uses-permission&gt;}</code>
tags declaring the permissions that your application needs.</p>

<p>For example, an application that needs to monitor incoming SMS messages would
specify:</p>

<pre>&lt;manifest xmlns:android=&quot;http://schemas.android.com/apk/res/android&quot;
    package=&quot;com.android.app.myapp&quot; &gt;
    &lt;uses-permission android:name=&quot;android.permission.RECEIVE_SMS&quot; /&gt;
    ...
&lt;/manifest&gt;</pre>

<p>At application install time, permissions requested by the application are
granted to it by the package installer, based on checks against the
signatures of the applications declaring those permissions and/or interaction
with the user. <em>No</em> checks with the user
are done while an application is running: it either was granted a particular
permission when installed, and can use that feature as desired, or the
permission was not granted and any attempt to use the feature will fail
without prompting the user.</p>

<p>Often times a permission failure will result in a {@link
java.lang.SecurityException} being thrown back to the application. However,
this is not guaranteed to occur everywhere. For example, the {@link
android.content.Context#sendBroadcast} method checks permissions as data is
being delivered to each receiver, after the method call has returned, so you
will not receive an exception if there are permission failures. In almost all
cases, however, a permission failure will be printed to the system log.</p>

<p>The permissions provided by the Android system can be found at {@link
android.Manifest.permission}. Any application may also define and enforce its
own permissions, so this is not a comprehensive list of all possible
permissions.</p>

<p>A particular permission may be enforced at a number of places during your
program's operation:</p>

<ul>
<li>At the time of a call into the system, to prevent an application from
executing certain functions.</li>
<li>When starting an activity, to prevent applications from launching
activities of other applications.</li>
<li>Both sending and receiving broadcasts, to control who can receive
your broadcast or who can send a broadcast to you.</li>
<li>When accessing and operating on a content provider.</li>
<li>Binding to or starting a service.</li>
</ul>


<a name="declaring"></a>
<h2>Declaring and Enforcing Permissions</h2>

<p>To enforce your own permissions, you must first declare them in your
<code>AndroidManifest.xml</code> using one or more
<code>{@link android.R.styleable#AndroidManifestPermission &lt;permission&gt;}</code>
tags.</p>

<p>For example, an application that wants to control who can start one
of its activities could declare a permission for this operation as follows:</p>

<pre>&lt;manifest xmlns:android=&quot;http://schemas.android.com/apk/res/android&quot;
    package=&quot;com.me.app.myapp&quot; &gt;
    &lt;permission android:name=&quot;com.me.app.myapp.permission.DEADLY_ACTIVITY&quot;
        android:label=&quot;&#64;string/permlab_deadlyActivity&quot;
        android:description=&quot;&#64;string/permdesc_deadlyActivity&quot;
        android:permissionGroup=&quot;android.permission-group.COST_MONEY&quot;
        android:protectionLevel=&quot;dangerous&quot; /&gt;
    ...
&lt;/manifest&gt;</pre>

<p>The {@link android.R.styleable#AndroidManifestPermission_protectionLevel
&lt;protectionLevel&gt;} attribute is required, telling the system how the
user is to be informed of applications requiring the permission, or who is
allowed to hold that permission, as described in the linked documentation.</p>

<p>The {@link android.R.styleable#AndroidManifestPermission_permissionGroup
&lt;permissionGroup&gt;} attribute is optional, and only used to help the system display
permissions to the user.  You will usually want to set this to either a standard
system group (listed in {@link android.Manifest.permission_group
android.Manifest.permission_group}) or in more rare cases to one defined by
yourself.  It is preferred to use an existing group, as this simplifies the
permission UI shown to the user.</p>

<p>Note that both a label and description should be supplied for the
permission. These are string resources that can be displayed to the user when
they are viewing a list of permissions
(<code>{@link android.R.styleable#AndroidManifestPermission_label android:label}</code>)
or details on a single permission (
<code>{@link android.R.styleable#AndroidManifestPermission_description android:description}</code>).
The label should be short, a few words
describing the key piece of functionality the permission is protecting. The
description should be a couple sentences describing what the permission allows
a holder to do. Our convention for the description is two sentences, the first
describing the permission, the second warning the user of what bad things
can happen if an application is granted the permission.</p>

<p>Here is an example of a label and description for the CALL_PHONE
permission:</p>

<pre>
    &lt;string name=&quot;permlab_callPhone&quot;&gt;directly call phone numbers&lt;/string&gt;
    &lt;string name=&quot;permdesc_callPhone&quot;&gt;Allows the application to call
        phone numbers without your intervention. Malicious applications may
        cause unexpected calls on your phone bill. Note that this does not
        allow the application to call emergency numbers.&lt;/string&gt;
</pre>

<p>You can look at the permissions currently defined in the system with the
shell command <code>adb shell pm list permissions</code>.  In particular,
the '-s' option displays the permissions in a form roughly similar to how the
user will see them:</p>

<pre>
$ adb shell pm list permissions -s      
All Permissions:

Network communication: view Wi-Fi state, create Bluetooth connections, full
Internet access, view network state

Your location: access extra location provider commands, fine (GPS) location,
mock location sources for testing, coarse (network-based) location

Services that cost you money: send SMS messages, directly call phone numbers

...</pre>

<a name="manifest"></a>
<h3>Enforcing Permissions in AndroidManifest.xml</h3>

<p>High-level permissions restricting access to entire components of the
system or application can be applied through your
<code>AndroidManifest.xml</code>. All that this requires is including an {@link
android.R.attr#permission android:permission} attribute on the desired
component, naming the permission that will be used to control access to
it.</p>

<p><strong>{@link android.app.Activity}</strong> permissions
(applied to the
{@link android.R.styleable#AndroidManifestActivity &lt;activity&gt;} tag)
restrict who can start the associated
activity.  The permission is checked during
{@link android.content.Context#startActivity Context.startActivity()} and
{@link android.app.Activity#startActivityForResult Activity.startActivityForResult()};
if the caller does not have
the required permission then {@link java.lang.SecurityException} is thrown
from the call.</p>

<p><strong>{@link android.app.Service}</strong> permissions
(applied to the
{@link android.R.styleable#AndroidManifestService &lt;service&gt;} tag)
restrict who can start or bind to the
associated service.  The permission is checked during
{@link android.content.Context#startService Context.startService()},
{@link android.content.Context#stopService Context.stopService()} and
{@link android.content.Context#bindService Context.bindService()};
if the caller does not have
the required permission then {@link java.lang.SecurityException} is thrown
from the call.</p>

<p><strong>{@link android.content.BroadcastReceiver}</strong> permissions
(applied to the
{@link android.R.styleable#AndroidManifestReceiver &lt;receiver&gt;} tag)
restrict who can send broadcasts to the associated receiver.
The permission is checked <em>after</em>
{@link android.content.Context#sendBroadcast Context.sendBroadcast()} returns,
as the system tries
to deliver the submitted broadcast to the given receiver.  As a result, a
permission failure will not result in an exception being thrown back to the
caller; it will just not deliver the intent.  In the same way, a permission
can be supplied to
{@link android.content.Context#registerReceiver(android.content.BroadcastReceiver, android.content.IntentFilter, String, android.os.Handler)
Context.registerReceiver()}
to control who can broadcast to a programmatically registered receiver.
Going the other way, a permission can be supplied when calling
{@link android.content.Context#sendBroadcast(Intent, String) Context.sendBroadcast()}
to restrict which BroadcastReceiver objects are allowed to receive the broadcast (see
below).</p>

<p><strong>{@link android.content.ContentProvider}</strong> permissions
(applied to the
{@link android.R.styleable#AndroidManifestProvider &lt;provider&gt;} tag)
restrict who can access the data in
a {@link android.content.ContentProvider}.  (Content providers have an important
additional security facility available to them called
<a href="#uri">URI permissions</a> which is described later.)
Unlike the other components,
there are two separate permission attributes you can set:
{@link android.R.attr#readPermission android:readPermission} restricts who
can read from the provider, and
{@link android.R.attr#writePermission android:writePermission} restricts
who can write to it.  Note that if a provider is protected with both a read
and write permission, holding only the write permission does not mean
you can read from a provider.  The permissions are checked when you first
retrieve a provider (if you don't have either permission, a SecurityException
will be thrown), and as you perform operations on the provider.  Using
{@link android.content.ContentResolver#query ContentResolver.query()} requires
holding the read permission; using
{@link android.content.ContentResolver#insert ContentResolver.insert()},
{@link android.content.ContentResolver#update ContentResolver.update()},
{@link android.content.ContentResolver#delete ContentResolver.delete()}
requires the write permission.
In all of these cases, not holding the required permission results in a
{@link java.lang.SecurityException} being thrown from the call.</p>


<a name="broadcasts"></a>
<h3>Enforcing Permissions when Sending Broadcasts</h3>

<p>In addition to the permission enforcing who can send Intents to a
registered {@link android.content.BroadcastReceiver} (as described above), you
can also specify a required permission when sending a broadcast. By calling {@link
android.content.Context#sendBroadcast(android.content.Intent,String)
Context.sendBroadcast()} with a
permission string, you require that a receiver's application must hold that
permission in order to receive your broadcast.</p>

<p>Note that both a receiver and a broadcaster can require a permission. When
this happens, both permission checks must pass for the Intent to be delivered
to the associated target.</p>


<a name="enforcement"></a>
<h3>Other Permission Enforcement</h3>

<p>Arbitrarily fine-grained permissions can be enforced at any call into a
service. This is accomplished with the {@link
android.content.Context#checkCallingPermission Context.checkCallingPermission()}
method. Call with a desired
permission string and it will return an integer indicating whether that
permission has been granted to the current calling process. Note that this can
only be used when you are executing a call coming in from another process,
usually through an IDL interface published from a service or in some other way
given to another process.</p>

<p>There are a number of other useful ways to check permissions. If you have
the pid of another process, you can use the Context method {@link
android.content.Context#checkPermission(String, int, int) Context.checkPermission(String, int, int)}
to check a permission against that pid. If you have the package name of another
application, you can use the direct PackageManager method {@link
android.content.pm.PackageManager#checkPermission(String, String)
PackageManager.checkPermission(String, String)}
to find out whether that particular package has been granted a specific permission.</p>


<a name="uri"></a>
<h2>URI Permissions</h2>

<p>The standard permission system described so far is often not sufficient
when used with content providers.  A content provider may want to
protect itself with read and write permissions, while its direct clients
also need to hand specific URIs to other applications for them to operate on.
A typical example is attachments in a mail application.  Access to the mail
should be protected by permissions, since this is sensitive user data.  However,
if a URI to an image attachment is given to an image viewer, that image viewer
will not have permission to open the attachment since it has no reason to hold
a permission to access all e-mail.</p>

<p>The solution to this problem is per-URI permissions: when starting an
activity or returning a result to an activity, the caller can set
{@link android.content.Intent#FLAG_GRANT_READ_URI_PERMISSION
Intent.FLAG_GRANT_READ_URI_PERMISSION} and/or
{@link android.content.Intent#FLAG_GRANT_WRITE_URI_PERMISSION
Intent.FLAG_GRANT_WRITE_URI_PERMISSION}.  This grants the receiving activity
permission access the specific data URI in the Intent, regardless of whether
it has any permission to access data in the content provider corresponding
to the Intent.</p>

<p>This mechanism allows a common capability-style model where user interaction
(opening an attachment, selecting a contact from a list, etc) drives ad-hoc
granting of fine-grained permission.  This can be a key facility for reducing
the permissions needed by applications to only those directly related to their
behavior.</p>

<p>The granting of fine-grained URI permissions does, however, require some
cooperation with the content provider holding those URIs.  It is strongly
recommended that content providers implement this facility, and declare that
they support it through the
{@link android.R.styleable#AndroidManifestProvider_grantUriPermissions
android:grantUriPermissions} attribute or
{@link android.R.styleable#AndroidManifestGrantUriPermission
&lt;grant-uri-permissions&gt;} tag.</p>

<p>More information can be found in the
{@link android.content.Context#grantUriPermission Context.grantUriPermission()},
{@link android.content.Context#revokeUriPermission Context.revokeUriPermission()}, and
{@link android.content.Context#checkUriPermission Context.checkUriPermission()}
methods.</p>