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midx: check size of pack names chunk

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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>
This commit is contained in:
Jeff King
2023-10-09 17:05:14 -04:00
committed by Junio C Hamano
parent 4169d89645
commit 72a9a08283
3 changed files with 21 additions and 2 deletions
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11
midx.c
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@ -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'"),