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

From: SZEDER Gábor <[email-masked]>
Date: Mon, 10 Dec 2018 05:28:43 +0100

On Sun, Dec 09, 2018 at 01:01:29PM +0900, Junio C Hamano wrote:
> Josh Steadmon <[email-masked]> 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 &&