[PATCH 05/20] midx: check size of oid lookup chunk

[Jeff King <peff@peff.net>]

When reading an on-disk multi-pack-index, we take the number of objects
in the midx from the final value of the fanout table. But we just
blindly assume that the chunk containing the actual oid entries is the
correct size. This can lead to us reading out-of-bounds memory if the
lookup chunk is too small (or if the fanout is corrupted; when they
don't agree we cannot tell which one is wrong).

Note that we bump the assignment of m->num_objects into the fanout
parser callback, so that it's set when we parse the lookup table
(otherwise we'd have to manually record the lookup table size and check
it later).

Signed-off-by: Jeff King <peff@peff.net>
---
As an aside, the first hunk of the diff annoyingly slides the "return 0"
into the wrong spot. I thought this was our heuristics gone wrong, but I
suspect it's actually the shortest diff because of the one-line change
in midx_read_oid_fanout(), which would otherwise require extra context
to split.

 midx.c                      | 18 +++++++++++++++---
 t/t5319-multi-pack-index.sh | 10 ++++++++++
 2 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/midx.c b/midx.c
index 21d7dd15ef..62e4c03e79 100644
--- a/midx.c
+++ b/midx.c
@@ -71,6 +71,20 @@ static int midx_read_oid_fanout(const unsigned char *chunk_start,
 		error(_("multi-pack-index OID fanout is of the wrong size"));
 		return 1;
 	}
+	m->num_objects = ntohl(m->chunk_oid_fanout[255]);
+	return 0;
+}
+
+static int midx_read_oid_lookup(const unsigned char *chunk_start,
+				size_t chunk_size, void *data)
+{
+	struct multi_pack_index *m = data;
+	m->chunk_oid_lookup = chunk_start;
+
+	if (chunk_size != st_mult(m->hash_len, m->num_objects)) {
+		error(_("multi-pack-index OID lookup chunk is the wrong size"));
+		return 1;
+	}
 	return 0;
 }
 
@@ -147,7 +161,7 @@ struct multi_pack_index *load_multi_pack_index(const char *object_dir, int local
 		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"));
-	if (pair_chunk_unsafe(cf, MIDX_CHUNKID_OIDLOOKUP, &m->chunk_oid_lookup))
+	if (read_chunk(cf, MIDX_CHUNKID_OIDLOOKUP, midx_read_oid_lookup, m))
 		die(_("multi-pack-index required OID lookup chunk missing or corrupted"));
 	if (pair_chunk_unsafe(cf, MIDX_CHUNKID_OBJECTOFFSETS, &m->chunk_object_offsets))
 		die(_("multi-pack-index required object offsets chunk missing or corrupted"));
@@ -157,8 +171,6 @@ struct multi_pack_index *load_multi_pack_index(const char *object_dir, int local
 	if (git_env_bool("GIT_TEST_MIDX_READ_RIDX", 1))
 		pair_chunk_unsafe(cf, MIDX_CHUNKID_REVINDEX, &m->chunk_revindex);
 
-	m->num_objects = ntohl(m->chunk_oid_fanout[255]);
-
 	CALLOC_ARRAY(m->pack_names, m->num_packs);
 	CALLOC_ARRAY(m->packs, m->num_packs);
 
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index b8fe85aeba..2722e495b2 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -1073,4 +1073,14 @@ test_expect_success 'reader notices too-small oid fanout chunk' '
 	test_cmp expect err
 '
 
+test_expect_success 'reader notices too-small oid lookup chunk' '
+	corrupt_chunk OIDL clear 00000000 &&
+	test_must_fail git log 2>err &&
+	cat >expect <<-\EOF &&
+	error: multi-pack-index OID lookup chunk is the wrong size
+	fatal: multi-pack-index required OID lookup chunk missing or corrupted
+	EOF
+	test_cmp expect err
+'
+
 test_done
-- 
2.42.0.884.g35e1fe1a6a