[PATCH v2 2/3] 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>
---
 commit-graph.c          | 14 ++++++++++++--
 t/t5318-commit-graph.sh | 28 ++++++++++++++++++++++++++++
 2 files changed, 40 insertions(+), 2 deletions(-)

diff --git a/commit-graph.c b/commit-graph.c
index 07dd410f3c..224a5f161e 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 (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) {
diff --git a/t/t5318-commit-graph.sh b/t/t5318-commit-graph.sh
index 5fe21db99f..2503cb0345 100755
--- a/t/t5318-commit-graph.sh
+++ b/t/t5318-commit-graph.sh
@@ -384,6 +384,29 @@ corrupt_graph_and_verify() {
        test_i18ngrep "$grepstr" err
 }
 
+
+# usage: corrupt_and_zero_graph_then_verify <corrupt_position> <data> 
<zero_position> <string>
+# Manipulates the commit-graph file at <corrupt_position> by inserting the 
data,
+# then zeros the file starting at <zero_position>. Finally, runs
+# 'git commit-graph verify' and places the output in the file 'err'. Tests 
'err'
+# for the given string.
+corrupt_and_zero_graph_then_verify() {
+       corrupt_pos=$1
+       data="${2:-\0}"
+       zero_pos=$3
+       grepstr=$4
+       orig_size=$(stat --format=%s $objdir/info/commit-graph)
+       cd "$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="$objdir/info/commit-graph" bs=1 
seek="$corrupt_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 +507,11 @@ test_expect_success 'detect invalid checksum hash' '
                "incorrect checksum"
 '
 
+test_expect_success 'detect truncated graph' '
+       corrupt_and_zero_graph_then_verify $GRAPH_BYTE_CHUNK_COUNT "\xff" \
+               $GRAPH_CHUNK_LOOKUP_OFFSET "chunk lookup table entry missing"
+'
+
 test_expect_success 'git fsck (checks commit-graph)' '
        cd "$TRASH_DIRECTORY/full" &&
        git fsck &&
-- 
2.20.0.rc2.10.g7519fc76df