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AphrontWriteGuard.php
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Wed, Dec 18, 06:31

AphrontWriteGuard.php
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<?php
/**
* Guard writes against CSRF. The Aphront structure takes care of most of this
* for you, you just need to call:
*
* AphrontWriteGuard::willWrite();
*
* ...before executing a write against any new kind of storage engine. MySQL
* databases and the default file storage engines are already covered, but if
* you introduce new types of datastores make sure their writes are guarded. If
* you don't guard writes and make a mistake doing CSRF checks in a controller,
* a CSRF vulnerability can escape undetected.
*
* If you need to execute writes on a page which doesn't have CSRF tokens (for
* example, because you need to do logging), you can temporarily disable the
* write guard by calling:
*
* AphrontWriteGuard::beginUnguardedWrites();
* do_logging_write();
* AphrontWriteGuard::endUnguardedWrites();
*
* This is dangerous, because it disables the backup layer of CSRF protection
* this class provides. You should need this only very, very rarely.
*
* @task protect Protecting Writes
* @task disable Disabling Protection
* @task manage Managing Write Guards
* @task internal Internals
*
* @group aphront
*/
final class AphrontWriteGuard {
private static $instance;
private static $allowUnguardedWrites = false;
private $callback;
private $allowDepth = 0;
/* -( Managing Write Guards )---------------------------------------------- */
/**
* Construct a new write guard for a request. Only one write guard may be
* active at a time. You must explicitly call @{method:dispose} when you are
* done with a write guard:
*
* $guard = new AphrontWriteGuard($callback);
* // ...
* $guard->dispose();
*
* Normally, you do not need to manage guards yourself -- the Aphront stack
* handles it for you.
*
* This class accepts a callback, which will be invoked when a write is
* attempted. The callback should validate the presence of a CSRF token in
* the request, or abort the request (e.g., by throwing an exception) if a
* valid token isn't present.
*
* @param callable CSRF callback.
* @return this
* @task manage
*/
public function __construct($callback) {
if (self::$instance) {
throw new Exception(
"An AphrontWriteGuard already exists. Dispose of the previous guard ".
"before creating a new one.");
}
if (self::$allowUnguardedWrites) {
throw new Exception(
"An AphrontWriteGuard is being created in a context which permits ".
"unguarded writes unconditionally. This is not allowed and indicates ".
"a serious error.");
}
$this->callback = $callback;
self::$instance = $this;
}
/**
* Dispose of the active write guard. You must call this method when you are
* done with a write guard. You do not normally need to call this yourself.
*
* @return void
* @task manage
*/
public function dispose() {
if ($this->allowDepth > 0) {
throw new Exception(
"Imbalanced AphrontWriteGuard: more beginUnguardedWrites() calls than ".
"endUnguardedWrites() calls.");
}
self::$instance = null;
}
/**
* Determine if there is an active write guard.
*
* @return bool
* @task manage
*/
public static function isGuardActive() {
return (bool)self::$instance;
}
/* -( Protecting Writes )-------------------------------------------------- */
/**
* Declare intention to perform a write, validating that writes are allowed.
* You should call this method before executing a write whenever you implement
* a new storage engine where information can be permanently kept.
*
* Writes are permitted if:
*
* - The request has valid CSRF tokens.
* - Unguarded writes have been temporarily enabled by a call to
* @{method:beginUnguardedWrites}.
* - All write guarding has been disabled with
* @{method:allowDangerousUnguardedWrites}.
*
* If none of these conditions are true, this method will throw and prevent
* the write.
*
* @return void
* @task protect
*/
public static function willWrite() {
if (!self::$instance) {
if (!self::$allowUnguardedWrites) {
throw new Exception(
"Unguarded write! There must be an active AphrontWriteGuard to ".
"perform writes.");
} else {
// Unguarded writes are being allowed unconditionally.
return;
}
}
$instance = self::$instance;
if ($instance->allowDepth == 0) {
call_user_func($instance->callback);
}
}
/* -( Disabling Write Protection )----------------------------------------- */
/**
* Enter a scope which permits unguarded writes. This works like
* @{method:beginUnguardedWrites} but returns an object which will end
* the unguarded write scope when its __destruct() method is called. This
* is useful to more easily handle exceptions correctly in unguarded write
* blocks:
*
* // Restores the guard even if do_logging() throws.
* function unguarded_scope() {
* $unguarded = AphrontWriteGuard::beginScopedUnguardedWrites();
* do_logging();
* }
*
* @return AphrontScopedUnguardedWriteCapability Object which ends unguarded
* writes when it leaves scope.
* @task disable
*/
public static function beginScopedUnguardedWrites() {
self::beginUnguardedWrites();
return new AphrontScopedUnguardedWriteCapability();
}
/**
* Begin a block which permits unguarded writes. You should use this very
* sparingly, and only for things like logging where CSRF is not a concern.
*
* You must pair every call to @{method:beginUnguardedWrites} with a call to
* @{method:endUnguardedWrites}:
*
* AphrontWriteGuard::beginUnguardedWrites();
* do_logging();
* AphrontWriteGuard::endUnguardedWrites();
*
* @return void
* @task disable
*/
public static function beginUnguardedWrites() {
if (!self::$instance) {
return;
}
self::$instance->allowDepth++;
}
/**
* Declare that you have finished performing unguarded writes. You must
* call this exactly once for each call to @{method:beginUnguardedWrites}.
*
* @return void
* @task disable
*/
public static function endUnguardedWrites() {
if (!self::$instance) {
return;
}
if (self::$instance->allowDepth <= 0) {
throw new Exception(
"Imbalanced AphrontWriteGuard: more endUnguardedWrites() calls than ".
"beginUnguardedWrites() calls.");
}
self::$instance->allowDepth--;
}
/**
* Allow execution of unguarded writes. This is ONLY appropriate for use in
* script contexts or other contexts where you are guaranteed to never be
* vulnerable to CSRF concerns. Calling this method is EXTREMELY DANGEROUS
* if you do not understand the consequences.
*
* If you need to perform unguarded writes on an otherwise guarded workflow
* which is vulnerable to CSRF, use @{method:beginUnguardedWrites}.
*
* @return void
* @task disable
*/
public static function allowDangerousUnguardedWrites($allow) {
if (self::$instance) {
throw new Exception(
"You can not unconditionally disable AphrontWriteGuard by calling ".
"allowDangerousUnguardedWrites() while a write guard is active. Use ".
"beginUnguardedWrites() to temporarily allow unguarded writes.");
}
self::$allowUnguardedWrites = true;
}
/* -( Internals )---------------------------------------------------------- */
/**
* When the object is destroyed, make sure @{method:dispose} was called.
*
* @task internal
*/
public function __destruct() {
if (isset(self::$instance)) {
throw new Exception(
"AphrontWriteGuard was not properly disposed of! Call dispose() on ".
"every AphrontWriteGuard object you instantiate.");
}
}
}

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