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

From: Josh Steadmon <steadmon@google.com>
To: "SZEDER Gábor" <szeder.dev@gmail.com>
Cc: Junio C Hamano <gitster@pobox.com>,
	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: Mon, 10 Dec 2018 13:58:07 -0800	[thread overview]
Message-ID: <20181210215807.GD37614@google.com> (raw)
In-Reply-To: <20181210042843.GQ30222@szeder.dev>

On 2018.12.10 05:28, SZEDER Gábor wrote:
> On Sun, Dec 09, 2018 at 01:01:29PM +0900, Junio C Hamano wrote:
> > 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 $objdir/info/commit-graph)
> > 
> > "stat(1)" is not so portable, so you'll get complaints from minority
> > platform users later.  So is "truncate(1)".
> 
> I complain: this patch breaks on macOS (on Travis CI), but in a
> curious way.  First, 'stat' in the above line errors out with:
> 
>   +++stat --format=%s .git/objects/info/commit-graph
>   stat: illegal option -- -
>   usage: stat [-FlLnqrsx] [-f format] [-t timefmt] [file ...]
> 
> Alas, this doesn't immediately fail the test, because it's not part of
> the &&-chain.
> 
> > > +	zero_pos=${4:-${orig_size}}
> 
> No && here, either.
> 
> > >  	cd "$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 $objdir/info/commit-graph &&
> 
>   ++truncate --size= .git/objects/info/commit-graph
>   t5318-commit-graph.sh: line 385: truncate: command not found
> 
> Note that even if 'truncate' were available, it would most likely
> complain about the empty '--size=' argument resulting from the 'stat'
> error above.
> 
> Alas, this doesn't fail the test, either, because ...
> 
> > > +	truncate --size=$orig_size $objdir/info/commit-graph &&
> > >  	test_must_fail git commit-graph verify 2>test_err &&
> > >  	grep -v "^+" test_err >err
> 
> ... here the &&-chain was broken already before this patch.  However,
> since this above command was not executed due to the missing
> 'truncate', it didn't have a chance to create the 'err' file, ...
> 
> > >  	test_i18ngrep "$grepstr" err
> 
> ... so 'test_i18ngrep' can't find the file, which triggers its linting
> error, finally aborting the whole test script.
> 
> > >  }
> > >  
> > > +
> 
> Stray newline.
> 
> > >  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 &&

Thanks for the catch. All these will be fixed in V4.