Re: [PATCH v3 2/3] commit-graph: fix buffer read-overflow

From: Junio C Hamano <gitster@pobox.com>
To: Josh Steadmon <steadmon@google.com>
Cc: git@vger.kernel.org, stolee@gmail.com, avarab@gmail.com, peff@peff.net
Subject: Re: [PATCH v3 2/3] commit-graph: fix buffer read-overflow
Date: Sun, 09 Dec 2018 13:01:29 +0900	[thread overview]
Message-ID: <xmqqsgz74acm.fsf@gitster-ct.c.googlers.com> (raw)
In-Reply-To: <675d58ecea2f315bd05d2a21f6a473e9de1105a0.1544221121.git.steadmon@google.com> (Josh Steadmon's message of "Fri, 7 Dec 2018 14:27:23 -0800")

Josh Steadmon <steadmon@google.com> writes:

> diff --git a/t/t5318-commit-graph.sh b/t/t5318-commit-graph.sh
> index 5fe21db99f..5b6b44b78e 100755
> --- a/t/t5318-commit-graph.sh
> +++ b/t/t5318-commit-graph.sh
> @@ -366,24 +366,30 @@ 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() {
>  	pos=$1
>  	data="${2:-\0}"
>  	grepstr=$3
> +	orig_size=$(stat --format=%s $objdir/info/commit-graph)

"stat(1)" is not so portable, so you'll get complaints from minority
platform users later.  So is "truncate(1)".

> +	zero_pos=${4:-${orig_size}}
>  	cd "$TRASH_DIRECTORY/full" &&
>  	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 &&
> +	truncate --size=$zero_pos $objdir/info/commit-graph &&
> +	truncate --size=$orig_size $objdir/info/commit-graph &&
>  	test_must_fail git commit-graph verify 2>test_err &&
>  	grep -v "^+" test_err >err
>  	test_i18ngrep "$grepstr" err
>  }
>  
> +
>  test_expect_success 'detect bad signature' '
>  	corrupt_graph_and_verify 0 "\0" \
>  		"graph signature"
> @@ -484,6 +490,11 @@ test_expect_success 'detect invalid checksum hash' '
>  		"incorrect checksum"
>  '
>  
> +test_expect_success 'detect incorrect chunk count' '
> +	corrupt_graph_and_verify $GRAPH_BYTE_CHUNK_COUNT "\xff" \

Implementations of printf(1) may not grok "\xff" as a valid
representation of "\377".  The shell built-in of dash(1) for example
would not work with this.

> +		"chunk lookup table entry missing" $GRAPH_CHUNK_LOOKUP_OFFSET
> +'
> +
>  test_expect_success 'git fsck (checks commit-graph)' '
>  	cd "$TRASH_DIRECTORY/full" &&
>  	git fsck &&