In integrity violations, use individual messages saying which check exactly failed.

src/blockstore/implementations/versioncounting/IntegrityViolationError.h

@@ -10,15 +10,20 @@ namespace blockstore {
 
         class IntegrityViolationError final : public std::exception {
         public:
-            IntegrityViolationError(const std::string &reason)
-                    : _reason("Integrity violation: " + reason) {
-            }
 
             const char *what() const throw() override {
                 return _reason.c_str();
             }
 
         private:
+            // Constructor is private to make sure that only VersionCountingBlockStore can throw this exception.
+            // This is because VersionCountingBlockStore wants to know about integrity violations and
+            // block all further file system access if it happens.
+            IntegrityViolationError(const std::string &reason)
+                    : _reason("Integrity violation: " + reason) {
+            }
+            friend class VersionCountingBlockStore;
+
             std::string _reason;
         };
 

src/blockstore/implementations/versioncounting/VersionCountingBlock.h

@@ -114,7 +114,7 @@ inline void VersionCountingBlock::_checkVersion() {
   uint32_t lastClientId = _readClientId();
   uint64_t version = _readVersion();
   if(!_blockStore->knownBlockVersions()->checkAndUpdateVersion(lastClientId, key(), version)) {
-    _blockStore->integrityViolationDetected();
+    _blockStore->integrityViolationDetected("The block version number is too low. Did an attacker try to roll back the block or to re-introduce a deleted block?");
   }
 }
 

src/blockstore/implementations/versioncounting/VersionCountingBlockStore.cpp

@@ -7,6 +7,7 @@ using cpputils::make_unique_ref;
 using cpputils::Data;
 using boost::none;
 using boost::optional;
+using std::string;
 namespace bf = boost::filesystem;
 
 namespace blockstore {
@@ -35,7 +36,7 @@ namespace blockstore {
             auto block = _baseBlockStore->load(key);
             if (block == boost::none) {
                 if (_missingBlockIsIntegrityViolation && _knownBlockVersions.blockShouldExist(key)) {
-                    integrityViolationDetected();
+                    integrityViolationDetected("A block that should exist wasn't found. Did an attacker delete it?");
                 }
                 return boost::none;
             }
@@ -52,9 +53,9 @@ namespace blockstore {
             }
         }
 
-        void VersionCountingBlockStore::integrityViolationDetected() {
+        void VersionCountingBlockStore::integrityViolationDetected(const string &reason) const {
             _integrityViolationDetected = true;
-            throw IntegrityViolationError("A block that should exist wasn't found. Did an attacker delete it?");
+            throw IntegrityViolationError(reason);
         }
 
         void VersionCountingBlockStore::remove(unique_ref<Block> block) {
@@ -93,7 +94,7 @@ namespace blockstore {
                 }
             });
             if (!existingBlocks.empty()) {
-                throw IntegrityViolationError("A block that should have existed wasn't found.");
+                integrityViolationDetected("A block that should have existed wasn't found.");
             }
         }
 

src/blockstore/implementations/versioncounting/VersionCountingBlockStore.h

@@ -24,7 +24,7 @@ public:
   uint64_t blockSizeFromPhysicalBlockSize(uint64_t blockSize) const override;
   void forEachBlock(std::function<void (const Key &)> callback) const override;
 
-  void integrityViolationDetected();
+  void integrityViolationDetected(const std::string &reason) const;
   KnownBlockVersions *knownBlockVersions();
 
 #ifndef CRYFS_NO_COMPATIBILITY
@@ -35,7 +35,7 @@ private:
   cpputils::unique_ref<BlockStore> _baseBlockStore;
   KnownBlockVersions _knownBlockVersions;
   const bool _missingBlockIsIntegrityViolation;
-  bool _integrityViolationDetected;
+  mutable bool _integrityViolationDetected;
 
   void _checkNoPastIntegrityViolations();