[11/20] midx: check size of revindex chunk

Commit Message

Jeff King Oct. 9, 2023, 9:05 p.m. UTC

When we load a revindex from disk, we check the size of the file
compared to the number of objects we expect it to have. But when we use
a RIDX chunk stored directly in the midx, we just access the memory
directly. This can lead to out-of-bounds memory access for a corrupted
or malicious multi-pack-index file.

We can catch this by recording the RIDX chunk size, and then checking it
against the expected size when we "load" the revindex. Note that this
check is much simpler than the one that load_revindex_from_disk() does,
because we just have the data array with no header (so we do not need
to account for the header size, and nor do we need to bother validating
the header values).

The test confirms both that we catch this case, and that we continue the
process (the revindex is required to use the midx bitmaps, but we
fallback to a non-bitmap traversal).

Signed-off-by: Jeff King <[e-mail]>
---
 midx.c                      |  3 ++-
 midx.h                      |  1 +
 pack-revindex.c             | 13 ++++++++++++-
 t/t5319-multi-pack-index.sh | 17 +++++++++++++++++
 4 files changed, 32 insertions(+), 2 deletions(-)

Comments

Taylor Blau Oct. 11, 2023, 6:41 p.m. UTC | #1

On Mon, Oct 09, 2023 at 05:05:33PM -0400, Jeff King wrote:
> +static int can_use_midx_ridx_chunk(struct multi_pack_index *m)
> +{
> +	if (!m->chunk_revindex)
> +		return 0;
> +	if (m->chunk_revindex_len != st_mult(sizeof(uint32_t), m->num_objects)) {
> +		error(_("multi-pack-index reverse-index chunk is the wrong size"));
> +		return 0;
> +	}
> +	return 1;
> +}

This all looks good to me. I was going to suggest that it might be worth
just NULL-ing out the chunk_revindex field here altogether. I think that
we have some prior art for doing that in the commit-graph code (though
I'd have to look to be sure...), but that's all within the same
compilation unit.

Having the pack-revindex machinery NULL out a field of the MIDX
structure feels icky to me, so I think that having a "can we use this
function?" instead makes much more sense.

Thanks,
Taylor

Patch

diff --git a/midx.c b/midx.c
index 3e768d0df0..2f3863c936 100644
--- a/midx.c
+++ b/midx.c
@@ -184,7 +184,8 @@  struct multi_pack_index *load_multi_pack_index(const char *object_dir, int local
 		   &m->chunk_large_offsets_len);
 
 	if (git_env_bool("GIT_TEST_MIDX_READ_RIDX", 1))
-		pair_chunk_unsafe(cf, MIDX_CHUNKID_REVINDEX, &m->chunk_revindex);
+		pair_chunk(cf, MIDX_CHUNKID_REVINDEX, &m->chunk_revindex,
+			   &m->chunk_revindex_len);
 
 	CALLOC_ARRAY(m->pack_names, m->num_packs);
 	CALLOC_ARRAY(m->packs, m->num_packs);
diff --git a/midx.h b/midx.h
index e8e8884d16..a5d98919c8 100644
--- a/midx.h
+++ b/midx.h
@@ -39,6 +39,7 @@  struct multi_pack_index {
 	const unsigned char *chunk_large_offsets;
 	size_t chunk_large_offsets_len;
 	const unsigned char *chunk_revindex;
+	size_t chunk_revindex_len;
 
 	const char **pack_names;
 	struct packed_git **packs;
diff --git a/pack-revindex.c b/pack-revindex.c
index 7fffcad912..6d8fd3645a 100644
--- a/pack-revindex.c
+++ b/pack-revindex.c
@@ -343,6 +343,17 @@  int verify_pack_revindex(struct packed_git *p)
 	return res;
 }
 
+static int can_use_midx_ridx_chunk(struct multi_pack_index *m)
+{
+	if (!m->chunk_revindex)
+		return 0;
+	if (m->chunk_revindex_len != st_mult(sizeof(uint32_t), m->num_objects)) {
+		error(_("multi-pack-index reverse-index chunk is the wrong size"));
+		return 0;
+	}
+	return 1;
+}
+
 int load_midx_revindex(struct multi_pack_index *m)
 {
 	struct strbuf revindex_name = STRBUF_INIT;
@@ -351,7 +362,7 @@  int load_midx_revindex(struct multi_pack_index *m)
 	if (m->revindex_data)
 		return 0;
 
-	if (m->chunk_revindex) {
+	if (can_use_midx_ridx_chunk(m)) {
 		/*
 		 * If the MIDX `m` has a `RIDX` chunk, then use its contents for
 		 * the reverse index instead of trying to load a separate `.rev`
diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh
index 16050f39d9..2a11dd1af6 100755
--- a/t/t5319-multi-pack-index.sh
+++ b/t/t5319-multi-pack-index.sh
@@ -1138,4 +1138,21 @@  test_expect_success 'reader bounds-checks large offset table' '
 	)
 '
 
+test_expect_success 'reader notices too-small revindex chunk' '
+	# We only get a revindex with bitmaps (and likewise only
+	# load it when they are asked for).
+	test_config repack.writeBitmaps true &&
+	corrupt_chunk RIDX clear 00000000 &&
+	git -c core.multipackIndex=false rev-list \
+		--all --use-bitmap-index >expect.out &&
+	git -c core.multipackIndex=true rev-list \
+		--all --use-bitmap-index >out 2>err &&
+	test_cmp expect.out out &&
+	cat >expect.err <<-\EOF &&
+	error: multi-pack-index reverse-index chunk is the wrong size
+	warning: multi-pack bitmap is missing required reverse index
+	EOF
+	test_cmp expect.err err
+'
+
 test_done