On Mon, 2018-10-15 at 12:31 +0900, Junio C Hamano wrote:
> Michał Górny <mgorny@xxxxxxxxxx> 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@xxxxxxxxxx>
> > ---
> > gpg-interface.c | 94 +++++++++++++++++++++++++++-------------
> > t/t7510-signed-commit.sh | 26 +++++++++++
> > 2 files changed, 91 insertions(+), 29 deletions(-)
> >
> > Changes in v3: reworked the whole loop to iterate over lines rather
> > than scanning the whole buffer, as requested. Now it also catches
> > duplicate instances of the same status.
> >
> > diff --git a/gpg-interface.c b/gpg-interface.c
> > index db17d65f8..480aab4ee 100644
> > --- a/gpg-interface.c
> > +++ b/gpg-interface.c
> > @@ -75,48 +75,84 @@ void signature_check_clear(struct signature_check *sigc)
> > FREE_AND_NULL(sigc->key);
> > }
> >
> > +/* An exclusive status -- only one of them can appear in output */
> > +#define GPG_STATUS_EXCLUSIVE (1<<0)
> > +
> > static struct {
> > char result;
> > const char *check;
> > + unsigned int flags;
> > } sigcheck_gpg_status[] = {
> > - { 'G', "\n[GNUPG:] GOODSIG " },
> > - { 'B', "\n[GNUPG:] BADSIG " },
> > - { 'U', "\n[GNUPG:] TRUST_NEVER" },
> > - { 'U', "\n[GNUPG:] TRUST_UNDEFINED" },
> > - { 'E', "\n[GNUPG:] ERRSIG "},
> > - { 'X', "\n[GNUPG:] EXPSIG "},
> > - { 'Y', "\n[GNUPG:] EXPKEYSIG "},
> > - { 'R', "\n[GNUPG:] REVKEYSIG "},
> > + { 'G', "GOODSIG ", GPG_STATUS_EXCLUSIVE },
> > + { 'B', "BADSIG ", GPG_STATUS_EXCLUSIVE },
> > + { 'U', "TRUST_NEVER", 0 },
> > + { 'U', "TRUST_UNDEFINED", 0 },
> > + { 'E', "ERRSIG ", GPG_STATUS_EXCLUSIVE },
> > + { 'X', "EXPSIG ", GPG_STATUS_EXCLUSIVE },
> > + { 'Y', "EXPKEYSIG ", GPG_STATUS_EXCLUSIVE },
> > + { 'R', "REVKEYSIG ", GPG_STATUS_EXCLUSIVE },
> > };
> >
> > static void parse_gpg_output(struct signature_check *sigc)
> > {
> > const char *buf = sigc->gpg_status;
> > + const char *line, *next;
> > int i;
> > -
> > - /* Iterate over all search strings */
> > - for (i = 0; i < ARRAY_SIZE(sigcheck_gpg_status); i++) {
> > - const char *found, *next;
> > -
> > - if (!skip_prefix(buf, sigcheck_gpg_status[i].check + 1, &found)) {
> > - found = strstr(buf, sigcheck_gpg_status[i].check);
> > - if (!found)
> > - continue;
> > - found += strlen(sigcheck_gpg_status[i].check);
> > - }
> > - sigc->result = sigcheck_gpg_status[i].result;
> > - /* The trust messages are not followed by key/signer information */
> > - if (sigc->result != 'U') {
> > - next = strchrnul(found, ' ');
> > - sigc->key = xmemdupz(found, next - found);
> > - /* The ERRSIG message is not followed by signer information */
> > - if (*next && sigc-> result != 'E') {
> > - found = next + 1;
> > - next = strchrnul(found, '\n');
> > - sigc->signer = xmemdupz(found, next - found);
> > + int had_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 (strncmp(line, "[GNUPG:] ", 9))
> > + continue;
> > + line += 9;
>
> You do not want to count to 9 yourself. Instead
>
> if (!skip_prefix(line, "[GNUPG:] ", &line))
> continue;
>
>
> > + /* Iterate over all search strings */
> > + for (i = 0; i < ARRAY_SIZE(sigcheck_gpg_status); i++) {
> > + if (!strncmp(line, sigcheck_gpg_status[i].check,
> > + strlen(sigcheck_gpg_status[i].check))) {
> > + line += strlen(sigcheck_gpg_status[i].check);
>
> Likewise.
Both done.
>
> > + if (sigcheck_gpg_status[i].flags & GPG_STATUS_EXCLUSIVE)
> > + had_exclusive_status++;
>
> "has" is fine, but I think existing code elsewhere we use "seen" for
> things like this.
>
> > + sigc->result = sigcheck_gpg_status[i].result;
> > + /* The trust messages are not followed by key/signer information */
> > + if (sigc->result != 'U') {
> > + next = strchrnul(line, ' ');
> > + free(sigc->key);
> > + sigc->key = xmemdupz(line, next - line);
> > + /* The ERRSIG message is not followed by signer information */
> > + if (*next && sigc->result != 'E') {
> > + line = next + 1;
> > + next = strchrnul(line, '\n');
> > + free(sigc->signer);
> > + sigc->signer = xmemdupz(line, next - line);
> > + }
> > + }
> > + break;
> > }
> > }
> > }
>
> So unless U/E, we expect to see a key, and unless E, we also expect
> there is a signer; we keep the last value we see in the sequence in
> sigc. Because all of these that are not U are marked exclusive, if
> we check if sigc->key already has value at the point you free the
> sigc->key field above, we can see if there is a duplicate record
> that are of "exclusive" type? I am not suggesting to lose the
> addition of "flags = GPG_STATUS_EXCLUSIVE|0" field, but trying to
> see if I am getting the logic right.
>
> For gpg_status that is !GPG_STATUS_EXCLUSIVE (i.e. "U"), we do not
> do any replacement of already seen .key/.signer, and all the cases
> that we do the replacement are GPG_STATUS_EXCLUSIVE, which we know
> will become an error in the code below when we do see twice. So it
> is fine not to check if .key/.signer we see twice are the same or
> different. It is an error even if we see the same .key/.signer
> twice---having two records is already wrong no matter whose key/sign
> it is.
>
> OK, so the whole thing makes sense to me.
>
> Having said that, if we wanted to short-circuit, I think
>
> for (each line) {
> for (each sigcheck_gpg_status[]) {
> if (not the one on line)
> continue;
> if (sigc->result != 'U') {
> if (sigc->key)
> goto found_dup;
> sigc->key = make a copy;
> if (*next && sigc->result != 'E') {
> if (sigc->signer)
> goto found_dup;
> sigc->signer = make a copy;
> }
> }
> break;
> }
> }
> return;
>
> found_dup:
> sigc->result = 'E';
> FREE_AND_NULL(sigc->signer);
> FREE_AND_NULL(sigc->key);
> return;
>
> would also be fine.
Do I understand correctly that you mean to take advantage that 'seen
exclusive status' cases match 'seen key' cases? I think this would be
a little less readable.
That said, I was planning on next patch that replaced the "!= 'U'" test
with explicit flags for whether a particular status includes key
and UID. If you'd agree with this direction, I think having this one
separate as well would make sense.
> > +
> > + /*
> > + * GOODSIG, BADSIG etc. can occur only once for each signature.
> > + * Therefore, if we had more than one then we're dealing with multiple
> > + * signatures. We don't support them currently, and they're rather
> > + * hard to create, so something is likely fishy and we should reject
> > + * them altogether.
> > + */
> > + if (had_exclusive_status > 1) {
> > + sigc->result = 'E';
> > + /* Clear partial data to avoid confusion */
> > + if (sigc->signer)
> > + FREE_AND_NULL(sigc->signer);
> > + if (sigc->key)
> > + FREE_AND_NULL(sigc->key);
>
> I think it is OK to use FREE_AND_NULL() unconditionally (just like
> we can use free(x) on x==NULL).
Done as well.
>
> > + }
> > }
>
>
--
Best regards,
Michał Górny
Attachment:
signature.asc
Description: This is a digitally signed message part
