midx: check size of pack names chunk

We parse the pack-name chunk as a series of NUL-terminated strings. But
since we don't look at the chunk size, there's nothing to guarantee that
we don't parse off the end of the chunk (or even off the end of the
mapped file).

We can record the length, and then as we parse make sure that we never
walk past it.

The new test exercises the case, though note that it does not actually
segfault before this patch. It hits a NUL byte somewhere in one of the
other chunks, and comes up with a garbage pack name. You could construct
one that reads out-of-bounds (e.g., a PNAM chunk at the end of file),
but this case is simple and sufficient to check that we detect the
problem.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

3 files changed, 21 insertions, 2 deletions

diff --git a/midx.c b/midx.c
index 62e4c03e79..ec585cae1b 100644
--- a/midx.c
+++ b/midx.c
@@ -157,7 +157,7 @@ struct multi_pack_index *load_multi_pack_index(const char *object_dir, int local
 				   MIDX_HEADER_SIZE, m->num_chunks))
 		goto cleanup_fail;
 
-	if (pair_chunk_unsafe(cf, MIDX_CHUNKID_PACKNAMES, &m->chunk_pack_names))
+	if (pair_chunk(cf, MIDX_CHUNKID_PACKNAMES, &m->chunk_pack_names, &m->chunk_pack_names_len))
 		die(_("multi-pack-index required pack-name chunk missing or corrupted"));
 	if (read_chunk(cf, MIDX_CHUNKID_OIDFANOUT, midx_read_oid_fanout, m))
 		die(_("multi-pack-index required OID fanout chunk missing or corrupted"));
@@ -176,9 +176,16 @@ struct multi_pack_index *load_multi_pack_index(const char *object_dir, int local
 
 	cur_pack_name = (const char *)m->chunk_pack_names;
 	for (i = 0; i < m->num_packs; i++) {
+		const char *end;
+		size_t avail = m->chunk_pack_names_len -
+				(cur_pack_name - (const char *)m->chunk_pack_names);
+
 		m->pack_names[i] = cur_pack_name;
 
-		cur_pack_name += strlen(cur_pack_name) + 1;
+		end = memchr(cur_pack_name, '\0', avail);
+		if (!end)
+			die(_("multi-pack-index pack-name chunk is too short"));
+		cur_pack_name = end + 1;
 
 		if (i && strcmp(m->pack_names[i], m->pack_names[i - 1]) <= 0)
 			die(_("multi-pack-index pack names out of order: '%s' before '%s'"),
diff --git a/midx.h b/midx.h
index 5578cd7b83..5b2a7da043 100644
--- a/midx.h
+++ b/midx.h
@@ -32,6 +32,7 @@ struct multi_pack_index {
 	int local;
 
 	const unsigned char *chunk_pack_names;
+	size_t chunk_pack_names_len;
 	const uint32_t *chunk_oid_fanout;
 	const unsigned char *chunk_oid_lookup;
 	const unsigned char *chunk_object_offsets;
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index 2722e495b2..0a0ccec8a4 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -1083,4 +1083,15 @@ test_expect_success 'reader notices too-small oid lookup chunk' '
 	test_cmp expect err
 '
 
+test_expect_success 'reader notices too-small pack names chunk' '
+	# There is no NUL to terminate the name here, so the
+	# chunk is too short.
+	corrupt_chunk PNAM clear 70656666 &&
+	test_must_fail git log 2>err &&
+	cat >expect <<-\EOF &&
+	fatal: multi-pack-index pack-name chunk is too short
+	EOF
+	test_cmp expect err
+'
+
 test_done