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@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/commit-graph.c b/commit-graph.c
index 15afad245a9022a96533a4187ff4cb9af1cd1fec..359e782deed65ec5478c59b1f439adb90aabf124 100644 (file)
--- 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 5fe21db99f43aa79dc36ab36bf6d40ca2f47264d..694f26079f028f11f88d5e55d5d67e346e56108a 100755 (executable)
--- a/t/t5318-commit-graph.sh
+++ b/t/t5318-commit-graph.sh
@@ -366,9 +366,10 @@ 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() {
@@ -376,11 +377,15 @@ corrupt_graph_and_verify() {
        data="${2:-\0}"
        grepstr=$3
        cd "$TRASH_DIRECTORY/full" &&
+       orig_size=$(wc -c < $objdir/info/commit-graph) &&
+       zero_pos=${4:-${orig_size}} &&
        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 &&
+       dd of="$objdir/info/commit-graph" bs=1 seek="$zero_pos" count=0 &&
+       dd if=/dev/zero of="$objdir/info/commit-graph" bs=1 seek="$zero_pos" count=$(($orig_size - $zero_pos)) &&
        test_must_fail git commit-graph verify 2>test_err &&
-       grep -v "^+" test_err >err
+       grep -v "^+" test_err >err &&
        test_i18ngrep "$grepstr" err
 }
 
@@ -484,6 +489,11 @@ test_expect_success 'detect invalid checksum hash' '
                "incorrect checksum"
 '
 
+test_expect_success 'detect incorrect chunk count' '
+       corrupt_graph_and_verify $GRAPH_BYTE_CHUNK_COUNT "\377" \
+               "chunk lookup table entry missing" $GRAPH_CHUNK_LOOKUP_OFFSET
+'
+
 test_expect_success 'git fsck (checks commit-graph)' '
        cd "$TRASH_DIRECTORY/full" &&
        git fsck &&