Harden NSConnection security in handling third-party connections

Currently, MacVim uses Distributed Objects / NSConnection as the IPC
mechanism. The child Vim process connects to the parent MacVim process
using NSConnection and registers itself. A security issue with this is
that NSConnection is global, and any process can connect to the app, and
MacVim isn't too hardened against this issue. Note that one reason why
we do need the ability for the MacVim app to accept random connections
is to support the `:gui` command from a random Vim process, and to
supported listing server names.

One issue is that while the app protocol (MMAppProtocol) is only a few
functions, we were exposing the entire app, which exposes functions like
`executeInLoginShell`, which could be invoked by the caller, which is
quite unsafe as it could be invoked by any third-party app. Fix this
issue by using `NSProtocolChecker` to make sure we only expose the APIs
that we want to expose.

Each Vim controller now also gets a randomized ID instead of an
incremental one. Currently the API for sending messages from Vim to
MacVim is public, meaning any app can send message to MacVim. Using a
randomized ID makes it more difficult for an attacker to guess the ID
(which used to always start at 1) and injects random commands to MacVim
pretending to be the Vim process.

Also, make sure if MacVim failed to register the NSConnection on launch,
just display an error and terminate. This usually happens if multiple
MacVim instances are opened, but also if an attacking app is trying to
register a connection first using the same name. This way the user would
know something is wrong instead of MacVim being opened by not able to do
anything as it didn't register the connection.

In the near future, the IPC mechanism will be switched to XPC, which is
the preferred way by Apple as Distributed Objects has been deprecated
for a long time. It will have proper security to only accept processes
within the same app to message each other. It will be done in #1157.

src/MacVim/MMAppController.m

@@ -304,7 +304,9 @@ - (id)init
     // unlikely to fix it, we graciously give them the default connection.)
     connection = [[NSConnection alloc] initWithReceivePort:[NSPort port]
                                                   sendPort:nil];
-    [connection setRootObject:self];
+    NSProtocolChecker *rootObject = [NSProtocolChecker protocolCheckerWithTarget:self
+                                                                        protocol:@protocol(MMAppProtocol)];
+    [connection setRootObject:rootObject];
     [connection setRequestTimeout:MMRequestTimeout];
     [connection setReplyTimeout:MMReplyTimeout];
 
@@ -315,6 +317,20 @@ - (id)init
     if (![connection registerName:name]) {
         ASLogCrit(@"Failed to register connection with name '%@'", name);
         [connection release];  connection = nil;
+
+        NSAlert *alert = [[NSAlert alloc] init];
+        [alert addButtonWithTitle:NSLocalizedString(@"OK",
+            @"Dialog button")];
+        [alert setMessageText:NSLocalizedString(@"MacVim cannot be opened",
+            @"MacVim cannot be opened, title")];
+        [alert setInformativeText:[NSString stringWithFormat:NSLocalizedString(
+            @"MacVim could not set up its connection. It's likely you already have MacVim opened elsewhere.",
+            @"MacVim already opened, text")]];
+        [alert setAlertStyle:NSAlertStyleCritical];
+        [alert runModal];
+        [alert release];
+
+        [[NSApplication sharedApplication] terminate:nil];
     }
 
     // Register help search handler to support search Vim docs via the Help menu
@@ -859,7 +875,7 @@ - (NSMenuItem *)appMenuItemTemplate
 
 - (void)removeVimController:(id)controller
 {
-    ASLogDebug(@"Remove Vim controller pid=%d id=%d (processingFlag=%d)",
+    ASLogDebug(@"Remove Vim controller pid=%d id=%lu (processingFlag=%d)",
                [controller pid], [controller vimControllerId], processingFlag);
 
     NSUInteger idx = [vimControllers indexOfObject:controller];
@@ -1540,7 +1556,7 @@ - (MMVimController *)keyVimController
     return nil;
 }
 
-- (unsigned)connectBackend:(byref in id <MMBackendProtocol>)proxy pid:(int)pid
+- (unsigned long)connectBackend:(byref in id <MMBackendProtocol>)proxy pid:(int)pid
 {
     ASLogDebug(@"pid=%d", pid);
 
@@ -1570,21 +1586,21 @@ - (unsigned)connectBackend:(byref in id <MMBackendProtocol>)proxy pid:(int)pid
 }
 
 - (oneway void)processInput:(in bycopy NSArray *)queue
-              forIdentifier:(unsigned)identifier
+              forIdentifier:(unsigned long)identifier
 {
     // NOTE: Input is not handled immediately since this is a distributed
     // object call and as such can arrive at unpredictable times.  Instead,
     // queue the input and process it when the run loop is updated.
 
     if (!(queue && identifier)) {
-        ASLogWarn(@"Bad input for identifier=%d", identifier);
+        ASLogWarn(@"Bad input for identifier=%lu", identifier);
         return;
     }
 
-    ASLogDebug(@"QUEUE for identifier=%d: <<< %@>>>", identifier,
+    ASLogDebug(@"QUEUE for identifier=%lu: <<< %@>>>", identifier,
                debugStringForMessageQueue(queue));
 
-    NSNumber *key = [NSNumber numberWithUnsignedInt:identifier];
+    NSNumber *key = [NSNumber numberWithUnsignedLong:identifier];
     NSArray *q = [inputQueues objectForKey:key];
     if (q) {
         q = [q arrayByAddingObjectsFromArray:queue];
@@ -2715,7 +2731,7 @@ - (void)processInputQueues:(id)sender
     NSEnumerator *e = [queues keyEnumerator];
     NSNumber *key;
     while ((key = [e nextObject])) {
-        unsigned ukey = [key unsignedIntValue];
+        unsigned long ukey = [key unsignedLongValue];
         int i = 0, count = [vimControllers count];
         for (i = 0; i < count; ++i) {
             MMVimController *vc = [vimControllers objectAtIndex:i];
@@ -2737,7 +2753,7 @@ - (void)processInputQueues:(id)sender
         }
 
         if (i == count) {
-            ASLogWarn(@"No Vim controller for identifier=%d", ukey);
+            ASLogWarn(@"No Vim controller for identifier=%lu", ukey);
         }
     }
 
@@ -2758,7 +2774,7 @@ - (void)processInputQueues:(id)sender
 
 - (void)addVimController:(MMVimController *)vc
 {
-    ASLogDebug(@"Add Vim controller pid=%d id=%d",
+    ASLogDebug(@"Add Vim controller pid=%d id=%lu",
             [vc pid], [vc vimControllerId]);
 
     int pid = [vc pid];

src/MacVim/MMBackend.h

@@ -21,7 +21,7 @@
     NSConnection        *connection;
     NSConnection        *vimServerConnection;
     id                  appProxy;
-    unsigned            identifier;
+    unsigned long       identifier;
     NSDictionary        *colorDict;
     NSDictionary        *sysColorDict;
     NSDictionary        *actionDict;

src/MacVim/MMVimController.h

@@ -34,7 +34,7 @@
 #endif
     >
 {
-    unsigned            identifier;
+    unsigned long       identifier;
     BOOL                isInitialized;
     MMWindowController  *windowController;
     id                  backendProxy;
@@ -58,7 +58,7 @@
 
 - (id)initWithBackend:(id)backend pid:(int)processIdentifier;
 - (void)uninitialize;
-- (unsigned)vimControllerId;
+- (unsigned long)vimControllerId;
 - (id)backendProxy;
 - (int)pid;
 - (void)setServerName:(NSString *)name;

src/MacVim/MMVimController.m

@@ -56,8 +56,6 @@
 // Timeout used for setDialogReturn:.
 static NSTimeInterval MMSetDialogReturnTimeout = 1.0;
 
-static unsigned identifierCounter = 1;
-
 static BOOL isUnsafeMessage(int msgid);
 
 
@@ -168,8 +166,15 @@ - (id)initWithBackend:(id)backend pid:(int)processIdentifier
     if (!(self = [super init]))
         return nil;
 
-    // TODO: Come up with a better way of creating an identifier.
-    identifier = identifierCounter++;
+    // Use a random identifier. Currently, MMBackend connects using a public
+    // NSConnection, which has security implications. Using random identifiers
+    // make it much harder for third-party attacker to spoof.
+    int secSuccess = SecRandomCopyBytes(kSecRandomDefault, sizeof(identifier), &identifier);
+    if (secSuccess != errSecSuccess) {
+        // Don't know what concrete reasons secure random would fail, but just
+        // as a failsafe, use a less secure option.
+        identifier = ((unsigned long)arc4random()) << 32 | (unsigned long)arc4random();
+    }
 
     windowController =
         [[MMWindowController alloc] initWithVimController:self];
@@ -257,7 +262,7 @@ - (void)uninitialize
     isInitialized = NO;
 }
 
-- (unsigned)vimControllerId
+- (unsigned long)vimControllerId
 {
     return identifier;
 }
@@ -436,7 +441,7 @@ - (void)sendMessage:(int)msgid data:(NSData *)data
         [backendProxy processInput:msgid data:data];
     }
     @catch (NSException *ex) {
-        ASLogDebug(@"processInput:data: failed: pid=%d id=%d msg=%s reason=%@",
+        ASLogDebug(@"processInput:data: failed: pid=%d id=%lu msg=%s reason=%@",
                 pid, identifier, MMVimMsgIDStrings[msgid], ex);
     }
 }
@@ -468,7 +473,7 @@ - (BOOL)sendMessageNow:(int)msgid data:(NSData *)data
     }
     @catch (NSException *ex) {
         sendOk = NO;
-        ASLogDebug(@"processInput:data: failed: pid=%d id=%d msg=%s reason=%@",
+        ASLogDebug(@"processInput:data: failed: pid=%d id=%lu msg=%s reason=%@",
                 pid, identifier, MMVimMsgIDStrings[msgid], ex);
     }
     @finally {
@@ -500,7 +505,7 @@ - (NSString *)evaluateVimExpression:(NSString *)expr
         ASLogDebug(@"eval(%@)=%@", expr, eval);
     }
     @catch (NSException *ex) {
-        ASLogDebug(@"evaluateExpression: failed: pid=%d id=%d reason=%@",
+        ASLogDebug(@"evaluateExpression: failed: pid=%d id=%lu reason=%@",
                 pid, identifier, ex);
     }
 
@@ -517,7 +522,7 @@ - (id)evaluateVimExpressionCocoa:(NSString *)expr
                                          errorString:errstr];
         ASLogDebug(@"eval(%@)=%@", expr, eval);
     } @catch (NSException *ex) {
-        ASLogDebug(@"evaluateExpressionCocoa: failed: pid=%d id=%d reason=%@",
+        ASLogDebug(@"evaluateExpressionCocoa: failed: pid=%d id=%lu reason=%@",
                 pid, identifier, ex);
         *errstr = [ex reason];
     }
@@ -556,7 +561,7 @@ - (void)processInputQueue:(NSArray *)queue
         [windowController processInputQueueDidFinish];
     }
     @catch (NSException *ex) {
-        ASLogDebug(@"Exception: pid=%d id=%d reason=%@", pid, identifier, ex);
+        ASLogDebug(@"Exception: pid=%d id=%lu reason=%@", pid, identifier, ex);
     }
 }
 
@@ -1275,7 +1280,7 @@ - (void)savePanelDidEnd:(NSSavePanel *)panel code:(int)code
                                                 noteNewRecentFilePath:path];
     }
     @catch (NSException *ex) {
-        ASLogDebug(@"Exception: pid=%d id=%d reason=%@", pid, identifier, ex);
+        ASLogDebug(@"Exception: pid=%d id=%lu reason=%@", pid, identifier, ex);
     }
     @finally {
         [conn setRequestTimeout:oldTimeout];
@@ -1308,7 +1313,7 @@ - (void)alertDidEnd:(MMAlert *)alert code:(int)code context:(void *)context
         [backendProxy setDialogReturn:ret];
     }
     @catch (NSException *ex) {
-        ASLogDebug(@"setDialogReturn: failed: pid=%d id=%d reason=%@",
+        ASLogDebug(@"setDialogReturn: failed: pid=%d id=%lu reason=%@",
                 pid, identifier, ex);
     }
 }
@@ -2089,7 +2094,7 @@ - (void)connectionDidDie:(NSNotification *)notification
 
 - (void)scheduleClose
 {
-    ASLogDebug(@"pid=%d id=%d", pid, identifier);
+    ASLogDebug(@"pid=%d id=%lu", pid, identifier);
 
     // NOTE!  This message can arrive at pretty much anytime, e.g. while
     // the run loop is the 'event tracking' mode.  This means that Cocoa may

src/MacVim/MacVim.h

@@ -197,9 +197,9 @@ typedef NSString* NSAttributedStringKey;
 // connectBackend:pid: and processInput:forIdentifier:).
 //
 @protocol MMAppProtocol
-- (unsigned)connectBackend:(byref in id <MMBackendProtocol>)proxy pid:(int)pid;
+- (unsigned long)connectBackend:(byref in id <MMBackendProtocol>)proxy pid:(int)pid;
 - (oneway void)processInput:(in bycopy NSArray *)queue
-              forIdentifier:(unsigned)identifier;
+              forIdentifier:(unsigned long)identifier;
 - (NSArray *)serverList;
 @end
 

src/MacVim/MacVim.xcodeproj/project.pbxproj

@@ -74,6 +74,7 @@
 		909894382A56EB1E007B84A3 /* WhatsNew.xib in Resources */ = {isa = PBXBuildFile; fileRef = 909894362A56EB1E007B84A3 /* WhatsNew.xib */; };
 		9098943C2A56ECF6007B84A3 /* MMWhatsNewController.m in Sources */ = {isa = PBXBuildFile; fileRef = 9098943B2A56ECF6007B84A3 /* MMWhatsNewController.m */; };
 		90A33BEA28D563DF003A2E2F /* MMSparkle2Delegate.m in Sources */ = {isa = PBXBuildFile; fileRef = 90A33BE928D563DF003A2E2F /* MMSparkle2Delegate.m */; };
+		90AF83AB2A8C37F70046DA2E /* Security.framework in Frameworks */ = {isa = PBXBuildFile; fileRef = 90AF83A92A8C37F70046DA2E /* Security.framework */; };
 		90B9877D2A579F9500FC95D6 /* WebKit.framework in Frameworks */ = {isa = PBXBuildFile; fileRef = 90B9877B2A579F9500FC95D6 /* WebKit.framework */; settings = {ATTRIBUTES = (Weak, ); }; };
 /* End PBXBuildFile section */
 
@@ -423,6 +424,7 @@
 		90AF83B62AA15C660046DA2E /* nv_cmds.h */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.h; name = nv_cmds.h; path = ../nv_cmds.h; sourceTree = "<group>"; };
 		90AF83B72AA15C660046DA2E /* vim9cmds.c */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.c; name = vim9cmds.c; path = ../vim9cmds.c; sourceTree = "<group>"; };
 		90AF83B82AA15C660046DA2E /* termdefs.h */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.h; name = termdefs.h; path = ../termdefs.h; sourceTree = "<group>"; };
+		90AF83A92A8C37F70046DA2E /* Security.framework */ = {isa = PBXFileReference; lastKnownFileType = wrapper.framework; name = Security.framework; path = System/Library/Frameworks/Security.framework; sourceTree = SDKROOT; };
 		90B9877B2A579F9500FC95D6 /* WebKit.framework */ = {isa = PBXFileReference; lastKnownFileType = wrapper.framework; name = WebKit.framework; path = System/Library/Frameworks/WebKit.framework; sourceTree = SDKROOT; };
 		90F84F1E2521F2270000268B /* ko */ = {isa = PBXFileReference; lastKnownFileType = text.plist.strings; name = ko; path = ko.lproj/MainMenu.strings; sourceTree = "<group>"; };
 		90F84F232521F6480000268B /* ca */ = {isa = PBXFileReference; lastKnownFileType = text.plist.strings; name = ca; path = ca.lproj/MainMenu.strings; sourceTree = "<group>"; };
@@ -462,6 +464,7 @@
 			isa = PBXFrameworksBuildPhase;
 			buildActionMask = 2147483647;
 			files = (
+				90AF83AB2A8C37F70046DA2E /* Security.framework in Frameworks */,
 				90B9877D2A579F9500FC95D6 /* WebKit.framework in Frameworks */,
 				1DFE25A50C527BC4003000F7 /* PSMTabBarControl.framework in Frameworks */,
 				8D11072F0486CEB800E47090 /* Cocoa.framework in Frameworks */,
@@ -650,6 +653,7 @@
 		29B97323FDCFA39411CA2CEA /* Frameworks */ = {
 			isa = PBXGroup;
 			children = (
+				90AF83A92A8C37F70046DA2E /* Security.framework */,
 				90B9877B2A579F9500FC95D6 /* WebKit.framework */,
 				52A364721C4A5789005757EC /* Sparkle.framework */,
 				1D8B5A52104AF9FF002E59D5 /* Carbon.framework */,