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




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@xxxxxxxxxx>
Helped-by: Derrick Stolee <stolee@xxxxxxxxx>
---
 commit-graph.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/commit-graph.c b/commit-graph.c
index 0755359b1a..fee171a5f3 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -175,10 +175,19 @@ 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 (chunk_lookup + GRAPH_CHUNKLOOKUP_WIDTH > data + graph_size) {
+			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) {
-- 
2.20.0.rc2.403.gdbc3b29805-goog