[PATCH v3 2/3] commit-graph: fix buffer read-overflow

[Josh Steadmon <steadmon@google.com>]

fuzz-commit-graph identified a case where Git will read past the end of
a buffer containing a commit graph if the graph's header has an
incorrect chunk count. A simple bounds check in parse_commit_graph()
prevents this.

Signed-off-by: Josh Steadmon <steadmon@google.com>
---
 commit-graph.c          | 14 ++++++++++++--
 t/t5318-commit-graph.sh | 15 +++++++++++++--
 2 files changed, 25 insertions(+), 4 deletions(-)

diff --git a/commit-graph.c b/commit-graph.c
index 07dd410f3c..836d65a1d3 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -165,10 +165,20 @@ struct commit_graph *parse_commit_graph(void *graph_map, int fd,
 	last_chunk_offset = 8;
 	chunk_lookup = data + 8;
 	for (i = 0; i < graph->num_chunks; i++) {
-		uint32_t chunk_id = get_be32(chunk_lookup + 0);
-		uint64_t chunk_offset = get_be64(chunk_lookup + 4);
+		uint32_t chunk_id;
+		uint64_t chunk_offset;
 		int chunk_repeated = 0;
 
+		if (data + graph_size - chunk_lookup <
+		    GRAPH_CHUNKLOOKUP_WIDTH) {
+			error(_("chunk lookup table entry missing; graph file may be incomplete"));
+			free(graph);
+			return NULL;
+		}
+
+		chunk_id = get_be32(chunk_lookup + 0);
+		chunk_offset = get_be64(chunk_lookup + 4);
+
 		chunk_lookup += GRAPH_CHUNKLOOKUP_WIDTH;
 
 		if (chunk_offset > graph_size - GIT_MAX_RAWSZ) {
diff --git a/t/t5318-commit-graph.sh b/t/t5318-commit-graph.sh
index 5fe21db99f..5b6b44b78e 100755
--- a/t/t5318-commit-graph.sh
+++ b/t/t5318-commit-graph.sh
@@ -366,24 +366,30 @@ GRAPH_OCTOPUS_DATA_OFFSET=$(($GRAPH_COMMIT_DATA_OFFSET + \
 GRAPH_BYTE_OCTOPUS=$(($GRAPH_OCTOPUS_DATA_OFFSET + 4))
 GRAPH_BYTE_FOOTER=$(($GRAPH_OCTOPUS_DATA_OFFSET + 4 * $NUM_OCTOPUS_EDGES))
 
-# usage: corrupt_graph_and_verify <position> <data> <string>
+# usage: corrupt_graph_and_verify <position> <data> <string> [<zero_pos>]
 # Manipulates the commit-graph file at the position
-# by inserting the data, then runs 'git commit-graph verify'
+# by inserting the data, optionally zeroing the file
+# starting at <zero_pos>, then runs 'git commit-graph verify'
 # and places the output in the file 'err'. Test 'err' for
 # the given string.
 corrupt_graph_and_verify() {
 	pos=$1
 	data="${2:-\0}"
 	grepstr=$3
+	orig_size=$(stat --format=%s $objdir/info/commit-graph)
+	zero_pos=${4:-${orig_size}}
 	cd "$TRASH_DIRECTORY/full" &&
 	test_when_finished mv commit-graph-backup $objdir/info/commit-graph &&
 	cp $objdir/info/commit-graph commit-graph-backup &&
 	printf "$data" | dd of="$objdir/info/commit-graph" bs=1 seek="$pos" conv=notrunc &&
+	truncate --size=$zero_pos $objdir/info/commit-graph &&
+	truncate --size=$orig_size $objdir/info/commit-graph &&
 	test_must_fail git commit-graph verify 2>test_err &&
 	grep -v "^+" test_err >err
 	test_i18ngrep "$grepstr" err
 }
 
+
 test_expect_success 'detect bad signature' '
 	corrupt_graph_and_verify 0 "\0" \
 		"graph signature"
@@ -484,6 +490,11 @@ test_expect_success 'detect invalid checksum hash' '
 		"incorrect checksum"
 '
 
+test_expect_success 'detect incorrect chunk count' '
+	corrupt_graph_and_verify $GRAPH_BYTE_CHUNK_COUNT "\xff" \
+		"chunk lookup table entry missing" $GRAPH_CHUNK_LOOKUP_OFFSET
+'
+
 test_expect_success 'git fsck (checks commit-graph)' '
 	cd "$TRASH_DIRECTORY/full" &&
 	git fsck &&
-- 
2.20.0.rc2.12.g4c11c11dec