Re: [PATCH 1/1] verify-tag/verify-commit should exit unsuccessfully when signature is not trusted

From: Junio C Hamano <gitster@pobox.com>
Date: Aug 9, 2018, 18:40 UTC
Message-ID: <xmqqy3dfbcys.fsf@gitster-ct.c.googlers.com>
In-Reply-To: <20180809171219.GF1439@sigill.intra.peff.net>

Jeff King <peff@peff.net> writes:

Show 6 quoted lines

> I guess leaving it serves as a sort of cross-check if gpg would return a
> zero exit code but indicate in the status result that the signature was
> not good. Sort of a belt-and-suspenders, I guess (which might not be
> that implausible if we think about somebody wrapping gpg with a sloppy
> bit of shell code that loses the exit code -- it's their fault, but it
> might be nice for us to err on the conservative side).

OK, this time a real log message.

-- >8 --
Subject: [PATCH] gpg-interface: propagate exit status from gpg back to the callers

When gpg-interface API unified support for signature verification codepaths for signed tags and signed commits in mid 2015 at around v2.6.0-rc0~114, we accidentally loosened the GPG signature verification.

Before that change, signed commits were verified by looking for "G"ood signature from GPG, while ignoring the exit status of "gpg --verify" process, while signed tags were verified by simply passing the exit status of "gpg --verify" through. The unified code we currently have ignores the exit status of "gpg --verify" and returns successful verification when the signature matches an unexpired key regardless of the trust placed on the key (i.e. in addition to "G"ood ones, we accept "U"ntrusted ones).

Make these commands signal failure with their exit status when underlying "gpg --verify" (or the custom command specified by "gpg.program" configuration variable) does so. This essentially changes their behaviour in a backward incompatible way to reject signatures that have been made with untrusted keys even if they correctly verify, as that is how "gpg --verify" behaves.

Note that the code still overrides a zero exit status obtained from "gpg" (or gpg.program) if the output does not say the signature is good or computes correctly but made with untrusted keys, to catch a poorly written wrapper around "gpg" the user may give us.

We could exclude "U"ntrusted support from this fallback code, but that would be making two backward incompatible changes in a single commit, so let's avoid that for now. A follow-up change could do so if desired.

Helped-by: Vojtech Myslivec <vojtech.myslivec@nic.cz>
Helped-by: brian m. carlson <sandals@crustytoothpaste.net>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 gpg-interface.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/gpg-interface.c b/gpg-interface.c
index 0647bd6348..b5e64c55e2 100644
--- a/gpg-interface.c
+++ b/gpg-interface.c
@@ -81,12 +81,13 @@ int check_signature(const char *payload, size_t plen, const char *signature,
 	sigc->gpg_output = strbuf_detach(&gpg_output, NULL);
 	sigc->gpg_status = strbuf_detach(&gpg_status, NULL);
 	parse_gpg_output(sigc);
+	status |= sigc->result != 'G' && sigc->result != 'U';
 
  out:
 	strbuf_release(&gpg_status);
 	strbuf_release(&gpg_output);
 
-	return sigc->result != 'G' && sigc->result != 'U';
+	return !!status;
 }
 
 void print_signature_buffer(const struct signature_check *sigc, unsigned flags)

-- 
2.18.0-547-g1d89318c48