fsverity: use unsigned long for level_start

fs/verity/ isn't consistent with whether Merkle tree block indices are
'unsigned long' or 'u64'.  There's no real point to using u64 for them,
though, since (a) a Merkle tree with over ULONG_MAX blocks would only be
needed for a file larger than MAX_LFS_FILESIZE, and (b) for reads, the
status of all Merkle tree blocks has to be tracked in memory.

Therefore, let's make things a bit more efficient on 32-bit systems by
using 'unsigned long[]' for merkle_tree_params::level_start, instead of
'u64[]'.  Also, to be extra safe, explicitly check that there aren't
more than ULONG_MAX Merkle tree blocks.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Andrey Albershteyn <aalbersh@redhat.com>
Tested-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Link: https://lore.kernel.org/r/20221223203638.41293-2-ebiggers@kernel.org

fs/verity/fsverity_private.h

@@ -52,7 +52,7 @@ struct merkle_tree_params {
 	 * Starting block index for each tree level, ordered from leaf level (0)
 	 * to root level ('num_levels - 1')
 	 */
-	u64 level_start[FS_VERITY_MAX_LEVELS];
+	unsigned long level_start[FS_VERITY_MAX_LEVELS];
 };
 
 /*

fs/verity/open.c

@@ -34,6 +34,7 @@ int fsverity_init_merkle_tree_params(struct merkle_tree_params *params,
 	struct fsverity_hash_alg *hash_alg;
 	int err;
 	u64 blocks;
+	u64 blocks_in_level[FS_VERITY_MAX_LEVELS];
 	u64 offset;
 	int level;
 
@@ -94,17 +95,26 @@ int fsverity_init_merkle_tree_params(struct merkle_tree_params *params,
 		}
 		blocks = (blocks + params->hashes_per_block - 1) >>
 			 params->log_arity;
-		/* temporarily using level_start[] to store blocks in level */
-		params->level_start[params->num_levels++] = blocks;
+		blocks_in_level[params->num_levels++] = blocks;
 	}
-	params->level0_blocks = params->level_start[0];
+	params->level0_blocks = blocks_in_level[0];
 
 	/* Compute the starting block of each level */
 	offset = 0;
 	for (level = (int)params->num_levels - 1; level >= 0; level--) {
-		blocks = params->level_start[level];
 		params->level_start[level] = offset;
-		offset += blocks;
+		offset += blocks_in_level[level];
+	}
+
+	/*
+	 * Since the data, and thus also the Merkle tree, cannot have more than
+	 * ULONG_MAX pages, hash block indices can always fit in an
+	 * 'unsigned long'.  To be safe, explicitly check for it too.
+	 */
+	if (offset > ULONG_MAX) {
+		fsverity_err(inode, "Too many blocks in Merkle tree");
+		err = -EFBIG;
+		goto out_err;
 	}
 
 	params->tree_size = offset << log_blocksize;