[Core] Improve hash collision avoidance in prefix caching (#12621)

Signed-off-by: Russell Bryant <rbryant@redhat.com>

tests/core/block/test_prefix_caching_block.py

@@ -65,8 +65,8 @@ class TestPrefixCachingBlock:
 
         previous_block = MagicMock(spec=PrefixCachingBlock)
         prev_block_hash = random.randint(0, 1000)
-        previous_block.content_hash = (prev_block_hash
-                                       if prev_block_has_hash else None)
+        previous_block.content_hash = (prev_block_hash if prev_block_has_hash
+                                       else hash('None'))
 
         num_to_fill = block_size if is_curr_block_full else random.randint(
             0, block_size - 1)

vllm/core/block/prefix_caching_block.py

@@ -65,6 +65,15 @@ class PrefixCachingBlockAllocator(BlockAllocator):
             from 0 to num_blocks - 1.
     """
 
+    # Note that we use 'None' as a string here instead of None because
+    # as of Python 3.12, hash(None) returns a constant predictable value.
+    # This could possibly make it easier to find and exploit hash
+    # collisions. 'None' as a string will be hashed differently per process,
+    # but consistently within the same process. This is the same as the
+    # behavior of None prior to Python 3.12.
+    _none_hash: int = hash('None')
+
+    # Implements Block.Factory.
     def __init__(
         self,
         num_blocks: int,
@@ -122,7 +131,6 @@ class PrefixCachingBlockAllocator(BlockAllocator):
 
         self.metric_data = CacheMetricData()
 
-    # Implements Block.Factory.
     def _create_block(
         self,
         prev_block: Optional[Block],
@@ -737,6 +745,14 @@ class PrefixCachingBlock(Block):
             such as adapters that influence the block, apart from the token_ids.
     """
 
+    # Note that we use 'None' as a string here instead of None because
+    # as of Python 3.12, hash(None) returns a constant predictable value.
+    # This could possibly make it easier to find and exploit hash
+    # collisions. 'None' as a string will be hashed differently per process,
+    # but consistently within the same process. This is the same as the
+    # behavior of None prior to Python 3.12.
+    _none_hash: int = hash('None')
+
     def __init__(
         self,
         prev_block: Optional[Block],
@@ -891,13 +907,13 @@ class PrefixCachingBlock(Block):
 
         is_first_block = self._prev_block is None
         prev_block_hash = (
-            None if is_first_block else
+            self._none_hash if is_first_block else
             self._prev_block.content_hash  # type: ignore
         )
 
         # Previous block exists but does not yet have a hash.
         # Return no hash in this case.
-        if prev_block_hash is None and not is_first_block:
+        if prev_block_hash == self._none_hash and not is_first_block:
             return None
 
         self._cached_content_hash = PrefixCachingBlock.hash_block_tokens(
@@ -907,8 +923,9 @@ class PrefixCachingBlock(Block):
             extra_hash=self._extra_hash)
         return self._cached_content_hash
 
-    @staticmethod
-    def hash_block_tokens(is_first_block: bool,
+    @classmethod
+    def hash_block_tokens(cls,
+                          is_first_block: bool,
                           prev_block_hash: Optional[int],
                           cur_block_token_ids: List[int],
                           extra_hash: Optional[int] = None) -> int:
@@ -929,7 +946,8 @@ class PrefixCachingBlock(Block):
         Returns:
         - int: The computed hash value for the block.
         """
-        assert (prev_block_hash is None) == is_first_block
+        if is_first_block and prev_block_hash is None:
+            prev_block_hash = cls._none_hash
         return hash((is_first_block, prev_block_hash, *cur_block_token_ids,
                      extra_hash))
 
@@ -949,6 +967,14 @@ class ComputedBlocksTracker:
     cached block hashes in the allocator.
     """
 
+    # Note that we use 'None' as a string here instead of None because
+    # as of Python 3.12, hash(None) returns a constant predictable value.
+    # This could possibly make it easier to find and exploit hash
+    # collisions. 'None' as a string will be hashed differently per process,
+    # but consistently within the same process. This is the same as the
+    # behavior of None prior to Python 3.12.
+    _none_hash: int = hash('None')
+
     def __init__(
         self,
         allocator: DeviceAwareBlockAllocator,
@@ -994,7 +1020,7 @@ class ComputedBlocksTracker:
         # We need to know the hash of the previous block to compute the hash of
         # the current block so that blocks could be uniquely identified across
         # sequences of prefixes.
-        prev_block_hash = (None if cur_num_blocks_recorded == 0 else
+        prev_block_hash = (self._none_hash if cur_num_blocks_recorded == 0 else
                            block_hashes_recorded[-1])
         # Only update the computed block hashes for the new blocks
         for i in range(cur_num_blocks_recorded, num_computed_blocks):
@@ -1009,7 +1035,7 @@ class ComputedBlocksTracker:
             # This has to be kept in sync with the allocator's hash
             # calculation.
             block_hash = PrefixCachingBlock.hash_block_tokens(
-                is_first_block=prev_block_hash is None,
+                is_first_block=prev_block_hash == self._none_hash,
                 prev_block_hash=prev_block_hash,
                 cur_block_token_ids=block_token_ids,
                 extra_hash=extra_hash,

vllm/v1/core/kv_cache_utils.py

@@ -263,6 +263,15 @@ def hash_block_tokens(
         The hash value of the block and the token ids in the block.
         The entire tuple is used as the hash key of the block.
     """
+    if not parent_block_hash:
+        # Note that we use 'None' as a string here instead of None because
+        # as of Python 3.12, hash(None) returns a constant predictable value.
+        # This could possibly make it easier to find and exploit hash
+        # collisions. 'None' as a string will be hashed differently per process,
+        # but consistently within the same process. This is the same as the
+        # behavior of None prior to Python 3.12.
+        parent_block_hash = hash('None')
+
     curr_block_token_ids_tuple = tuple(curr_block_token_ids)
     return BlockHashType(
         hash((parent_block_hash, curr_block_token_ids_tuple, extra_keys)),