santad: add critical system binaries

Podfile.lock

@@ -7,11 +7,11 @@ PODS:
   - MOLCertificate (1.9)
   - MOLCodesignChecker (1.10):
     - MOLCertificate (~> 1.8)
-  - MOLFCMClient (1.7):
+  - MOLFCMClient (1.8):
     - MOLAuthenticatingURLSession (~> 2.4)
   - MOLXPCConnection (1.2):
     - MOLCodesignChecker (~> 1.9)
-  - OCMock (3.4.1)
+  - OCMock (3.4.2)
 
 DEPENDENCIES:
   - FMDB
@@ -37,9 +37,9 @@ SPEC CHECKSUMS:
   MOLAuthenticatingURLSession: c238aa1c9a7b1077eb39a6f40204bfe76a7d204e
   MOLCertificate: e9e88a396c57032cab847f51a46e20c730cd752a
   MOLCodesignChecker: b0d5db9d2f9bd94e0fd093891a5d40e5ad77cbc0
-  MOLFCMClient: ee45348909351f232e2759c580329072ae7e02d4
+  MOLFCMClient: 2bfbacd45cc11e1ca3c077e97b80401c4e4a54f1
   MOLXPCConnection: c27af5cb1c43b18319698b0e568a8ddc2fc1e306
-  OCMock: 2cd0716969bab32a2283ff3a46fd26a8c8b4c5e3
+  OCMock: ebe9ee1dca7fbed0ff9193ac0b3e2d8862ea56f6
 
 PODFILE CHECKSUM: ddca043a7ace9ec600c108621c56d13a50d17236
 

Source/santad/DataLayer/SNTRuleTable.h

@@ -17,6 +17,7 @@
 #import "SNTCommonEnums.h"
 #import "SNTDatabaseTable.h"
 
+@class SNTCachedDecision;
 @class SNTRule;
 @class SNTNotificationMessage;
 
@@ -88,4 +89,12 @@
 ///
 - (void)removeOutdatedTransitiveRules;
 
+
+///
+///  A map of a file hashes to cached decisions. This is used to pre-validate and whitelist
+///  certain critical system binaries that are integral to Santa's functionality.
+///
+@property(readonly, nonatomic)
+    NSDictionary<NSString *, SNTCachedDecision *> *criticalSystemBinaries;
+
 @end

Source/santad/DataLayer/SNTRuleTable.m

@@ -17,7 +17,9 @@
 #import <MOLCertificate/MOLCertificate.h>
 #import <MOLCodesignChecker/MOLCodesignChecker.h>
 
+#import "SNTCachedDecision.h"
 #import "SNTConfigurator.h"
+#import "SNTFileInfo.h"
 #import "SNTLogging.h"
 #import "SNTRule.h"
 
@@ -31,6 +33,7 @@ @interface SNTRuleTable ()
 @property NSString *santadCertSHA;
 @property NSString *launchdCertSHA;
 @property NSDate *lastTransitiveRuleCulling;
+@property NSDictionary *criticalSystemBinaries;
 @end
 
 @implementation SNTRuleTable
@@ -64,7 +67,8 @@ - (uint32_t)initializeDatabase:(FMDatabase *)db fromVersion:(uint32_t)version {
   // Save hashes of the signing certs for launchd and santad.
   // Used to ensure rules for them are not removed.
   self.santadCertSHA = [[[[MOLCodesignChecker alloc] initWithSelf] leafCertificate] SHA256];
-  self.launchdCertSHA = [[[[MOLCodesignChecker alloc] initWithPID:1] leafCertificate] SHA256];
+  MOLCodesignChecker *launchdCSInfo = [[MOLCodesignChecker alloc] initWithPID:1];
+  self.launchdCertSHA = launchdCSInfo.leafCertificate.SHA256;
 
   // Ensure the certificates used to sign the running launchd/santad are whitelisted.
   // If they weren't previously and the database is not new, log an error.
@@ -87,6 +91,31 @@ - (uint32_t)initializeDatabase:(FMDatabase *)db fromVersion:(uint32_t)version {
         self.launchdCertSHA, @(SNTRuleStateWhitelist), @(SNTRuleTypeCertificate)];
   }
 
+  // Setup critical system binaries
+  // TODO(tburgin): Add the Santa components to this feature and remove the santadCertSHA rule.
+  NSMutableDictionary *bins = [NSMutableDictionary dictionary];
+  for (NSString *path in @[ @"/usr/libexec/trustd" ]) {
+    SNTFileInfo *binInfo = [[SNTFileInfo alloc] initWithPath:path];
+    MOLCodesignChecker *csInfo = [binInfo codesignCheckerWithError:NULL];
+
+    // Make sure the critical system binary is signed by the same chain as launchd.
+    if ([csInfo signingInformationMatches:launchdCSInfo]) {
+      SNTCachedDecision *cd = [[SNTCachedDecision alloc] init];
+
+      cd.decision = SNTEventStateAllowBinary;
+      cd.decisionExtra = @"critical system binary";
+      cd.sha256 = binInfo.SHA256;
+
+      // Not needed, but nice for logging.
+      cd.certSHA256 = csInfo.leafCertificate.SHA256;
+      cd.certCommonName = csInfo.leafCertificate.commonName;
+
+      bins[binInfo.SHA256] = cd;
+    }
+  }
+
+  self.criticalSystemBinaries = bins;
+
   return newVersion;
 }
 

Source/santad/SNTExecutionController.m

@@ -116,21 +116,28 @@ - (void)validateBinaryWithMessage:(santa_message_t)message {
   }
 
   // Get codesigning info about the file but only if it's a Mach-O.
+  // If the binary is a critical system binary, don't check its signiture. The binary was validated
+  // by santad at startup.
+  SNTCachedDecision *cd = self.ruleTable.criticalSystemBinaries[binInfo.SHA256];
   MOLCodesignChecker *csInfo;
-  if (binInfo.isMachO) {
+  if (!cd && binInfo.isMachO) {
     NSError *csError;
     csInfo = [[MOLCodesignChecker alloc] initWithBinaryPath:binInfo.path
                                              fileDescriptor:binInfo.fileHandle.fileDescriptor
                                                       error:&csError];
+
     // Ignore codesigning if there are any errors with the signature.
     if (csError) csInfo = nil;
   }
 
-  // Actually make the decision (and refresh rule access timestamp).
-  SNTCachedDecision *cd = [self.policyProcessor decisionForFileInfo:binInfo
+  // If needed, actually make the decision (and refresh rule access timestamp).
+  if (!cd) {
+    cd = [self.policyProcessor decisionForFileInfo:binInfo
                                                          fileSHA256:nil
                                                   certificateSHA256:csInfo.leafCertificate.SHA256];
-  cd.certCommonName = csInfo.leafCertificate.commonName;
+    cd.certCommonName = csInfo.leafCertificate.commonName;
+  }
+
   cd.vnodeId = message.vnode_id;
 
   // Formulate an initial action from the decision.