Re: [PATCH v4] gpg-interface.c: detect and reject multiple signatures on commits

["Michał Górny" <mgorny@gentoo.org>]

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On Sun, 2018-10-21 at 08:57 +0900, Junio C Hamano wrote:
> Michał Górny <mgorny@gentoo.org> writes:
> 
> > GnuPG supports creating signatures consisting of multiple signature
> > packets.  If such a signature is verified, it outputs all the status
> > messages for each signature separately.  However, git currently does not
> > account for such scenario and gets terribly confused over getting
> > multiple *SIG statuses.
> > 
> > For example, if a malicious party alters a signed commit and appends
> > a new untrusted signature, git is going to ignore the original bad
> > signature and report untrusted commit instead.  However, %GK and %GS
> > format strings may still expand to the data corresponding
> > to the original signature, potentially tricking the scripts into
> > trusting the malicious commit.
> > 
> > Given that the use of multiple signatures is quite rare, git does not
> > support creating them without jumping through a few hoops, and finally
> > supporting them properly would require extensive API improvement, it
> > seems reasonable to just reject them at the moment.
> > 
> > Signed-off-by: Michał Górny <mgorny@gentoo.org>
> > ---
> >  gpg-interface.c          | 90 +++++++++++++++++++++++++++-------------
> >  t/t7510-signed-commit.sh | 26 ++++++++++++
> >  2 files changed, 87 insertions(+), 29 deletions(-)
> > 
> > Changes in v4:
> > * switched to using skip_prefix(),
> > * renamed the variable to seen_exclusive_status,
> > * made the loop terminate early on first duplicate status seen.
> 
> Thanks for sticking to the topic and polishing it further.  Looks
> very good.  
> 
> Will replace.
> 
> > +	int seen_exclusive_status = 0;
> > +
> > +	/* Iterate over all lines */
> > +	for (line = buf; *line; line = strchrnul(line+1, '\n')) {
> > +		while (*line == '\n')
> > +			line++;
> > +		/* Skip lines that don't start with GNUPG status */
> > +		if (!skip_prefix(line, "[GNUPG:] ", &line))
> > +			continue;
> > +
> > +		/* Iterate over all search strings */
> > +		for (i = 0; i < ARRAY_SIZE(sigcheck_gpg_status); i++) {
> > +			if (skip_prefix(line, sigcheck_gpg_status[i].check, &line)) {
> > +				if (sigcheck_gpg_status[i].flags & GPG_STATUS_EXCLUSIVE) {
> > +					if (++seen_exclusive_status > 1)
> > +						goto found_duplicate_status;
> 
> Very minor point but by not using pre-increment, i.e.
> 
> 		if (seen_exclusive_status++)
> 			goto found_duplicate_status;
> 
> you can use the expression as a "have we already seen?" boolean,
> whic may probably be more idiomatic.
> 
> The patch is good in the way written as-is, and this is so minor
> that it is not worth rerolling to only update this part.
> 

Sure, thanks.  For the record, I've been taught to use pre-increment
whenever possible to avoid copying the variable but I suppose it doesn't
really matter here.  Just a habit.

I'll start working on my next ideas once this is merged and I rebase.

-- 
Best regards,
Michał Górny

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