Subject: Re: [PATCH v6 2/3] commit-graph: fix buffer read-overflow

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Wed, 20 Feb 2019 15:55:58 +0100

On Tue, Jan 15 2019, Josh Steadmon wrote:

> 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.

This has a 2.21 regression where the test fails on NetBSD:
https://gitlab.com/git-vcs/git-ci/-/jobs/164224275

> Signed-off-by: Josh Steadmon <[email-masked]>
> ---
>  commit-graph.c          | 14 ++++++++++++--
>  t/t5318-commit-graph.sh | 16 +++++++++++++---
>  2 files changed, 25 insertions(+), 5 deletions(-)
>
> diff --git a/commit-graph.c b/commit-graph.c
> index 07dd410f3c..836d65a1d3 100644
> --- 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 5fe21db99f..694f26079f 100755
> --- 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)) &&

In the limited time I had to dig it starts failing at test 46, when
count=0 is given. dd on NetBSD exits with 127 when given count=0 it
seems.

>  	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
> +'
> +

Hacking around the above (e.g. "dd ... || :") makes all the failing
tests pass except this new one, which I didn't dig into.