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

[Josh Steadmon]

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 <stead...@google.com>
Helped-by: Derrick Stolee <sto...@gmail.com>
---
 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