[PATCH 1/2] gpg-interface: handle missing " with " gracefully in parse_ssh_output()

["René Scharfe" <l.s.r@web.de>]

If the output of ssh-keygen starts with "Good \"git\" signature for ",
but is not followed by " with " for some reason, then parse_ssh_output()
uses -1 as the len parameter of xmemdupz(), which in turn will end the
program.  Reject the signature and carry on instead in that case.

Signed-off-by: René Scharfe <l.s.r@web.de>
---
This code was added after v2.33.0.
Patch formatted with --inter-hunk-context=2 for easier review.

Silly bonus question: What's the purpose of the "+ 1" and "- 1", which
seem to cancel each other out?

 gpg-interface.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/gpg-interface.c b/gpg-interface.c
index 800d8caa67..62d340e78a 100644
--- a/gpg-interface.c
+++ b/gpg-interface.c
@@ -387,17 +387,19 @@ static void parse_ssh_output(struct signature_check *sigc)
 	line = to_free = xmemdupz(sigc->output, strcspn(sigc->output, "\n"));

 	if (skip_prefix(line, "Good \"git\" signature for ", &line)) {
-		/* Valid signature and known principal */
-		sigc->result = 'G';
-		sigc->trust_level = TRUST_FULLY;
-
 		/* Search for the last "with" to get the full principal */
 		principal = line;
 		do {
 			search = strstr(line, " with ");
 			if (search)
 				line = search + 1;
 		} while (search != NULL);
+		if (line == principal)
+			goto cleanup;
+
+		/* Valid signature and known principal */
+		sigc->result = 'G';
+		sigc->trust_level = TRUST_FULLY;
 		sigc->signer = xmemdupz(principal, line - principal - 1);
 	} else if (skip_prefix(line, "Good \"git\" signature with ", &line)) {
 		/* Valid signature, but key unknown */
--
2.33.1