[PATCH v4 22/22] signed push: allow stale nonce in stateless mode

[Junio C Hamano <gitster@pobox.com>]

When operating with the stateless RPC mode, we will receive a nonce
issued by another instance of us that advertised our capability and
refs some time ago.  Update the logic to check received nonce to
detect this case, compute how much time has passed since the nonce
was issued and report the status with a new environment variable
GIT_PUSH_CERT_NONCE_SLOP to the hooks.

GIT_PUSH_CERT_NONCE_STATUS will report "SLOP" in such a case.  The
hooks are free to decide how large a slop it is willing to accept.

Strictly speaking, the "nonce" is not really a "nonce" anymore in
the stateless RPC mode, as it will happily take any "nonce" issued
by it (which is protected by HMAC and its secret key) as long as it
is fresh enough.  The degree of this security degradation, relative
to the native protocol, is about the same as the "we make sure that
the 'git push' decided to update our refs with new objects based on
the freshest observation of our refs by making sure the values they
claim the original value of the refs they ask us to update exactly
match the current state" security is loosened to accomodate the
stateless RPC mode in the existing code without this series, so
there is no need for those who are already using smart HTTP to push
to their repositories to be alarmed any more than they already are.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 Documentation/git-receive-pack.txt | 11 ++++++++
 builtin/receive-pack.c             | 57 ++++++++++++++++++++++++++++++++------
 2 files changed, 59 insertions(+), 9 deletions(-)

diff --git a/Documentation/git-receive-pack.txt b/Documentation/git-receive-pack.txt
index 2d4b452..2e5131d 100644
--- a/Documentation/git-receive-pack.txt
+++ b/Documentation/git-receive-pack.txt
@@ -89,6 +89,17 @@ the following environment variables:
 	"git push --signed" sent a bogus nonce.
 `OK`;;
 	"git push --signed" sent the nonce we asked it to send.
+`SLOP`;;
+	"git push --signed" sent a nonce different from what we
+	asked it to send now, but in a previous session.  See
+	`GIT_PUSH_CERT_NONCE_SLOP` environment variable.
+
+`GIT_PUSH_CERT_NONCE_SLOP`::
+	"git push --signed" sent a nonce different from what we
+	asked it to send now, but in a different session whose
+	starting time is different by this many seconds from the
+	current session.  Only meaningful when
+	`GIT_PUSH_CERT_NONCE_STATUS` says `SLOP`.
 
 This hook is called before any refname is updated and before any
 fast-forward checks are performed.
diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index a1823e5..86fb5a4 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -43,6 +43,8 @@ static int prefer_ofs_delta = 1;
 static int auto_update_server_info;
 static int auto_gc = 1;
 static int fix_thin = 1;
+static int stateless_rpc;
+static const char *service_dir;
 static const char *head_name;
 static void *head_name_to_free;
 static int sent_capabilities;
@@ -58,7 +60,9 @@ static const char *NONCE_UNSOLICITED = "UNSOLICITED";
 static const char *NONCE_BAD = "BAD";
 static const char *NONCE_MISSING = "MISSING";
 static const char *NONCE_OK = "OK";
+static const char *NONCE_SLOP = "SLOP";
 static const char *nonce_status;
+static long nonce_stamp_slop;
 
 static enum deny_action parse_deny_action(const char *var, const char *value)
 {
@@ -359,6 +363,8 @@ static const char *find_header(const char *msg, size_t len, const char *key)
 static const char *check_nonce(const char *buf, size_t len)
 {
 	const char *nonce = find_header(buf, len, "nonce");
+	unsigned long stamp, ostamp;
+	char *bohmac, *expect;
 
 	if (!nonce)
 		return NONCE_MISSING;
@@ -368,7 +374,39 @@ static const char *check_nonce(const char *buf, size_t len)
 		return NONCE_OK;
 
 	/* returned nonce MUST match what we gave out earlier */
-	return NONCE_BAD;
+	if (!stateless_rpc)
+		return NONCE_BAD;
+
+	/*
+	 * In stateless mode, we may be receiving a nonce issued
+	 * by another instance of the server that serving the same
+	 * repository, and the timestamps may not match, but the
+	 * nonce-seed and dir should match, so we can recompute
+	 * and report the time slop.
+	 */
+
+	/* nonce is concat(<seconds-since-epoch>, "-", <hmac>) */
+	if (*nonce <= '0' || '9' < *nonce)
+		return NONCE_BAD;
+	stamp = strtoul(nonce, &bohmac, 10);
+	if (bohmac == nonce || bohmac[1] != '-')
+		return NONCE_BAD;
+
+	expect = prepare_push_cert_nonce(service_dir, stamp);
+	if (strcmp(expect, nonce)) {
+		free(expect);
+		return NONCE_BAD;
+	}
+	free(expect);
+
+	/*
+	 * By how many seconds is this nonce stale?  Negative
+	 * value would mean it was issued by another server
+	 * with its clock skewed in the future.
+	 */
+	ostamp = strtoul(push_cert_nonce, NULL, 10);
+	nonce_stamp_slop = (long)ostamp - (long)stamp;
+	return NONCE_SLOP;
 }
 
 static void prepare_push_cert_sha1(struct child_process *proc)
@@ -417,6 +455,9 @@ static void prepare_push_cert_sha1(struct child_process *proc)
 		if (push_cert_nonce) {
 			argv_array_pushf(&env, "GIT_PUSH_CERT_NONCE=%s", push_cert_nonce);
 			argv_array_pushf(&env, "GIT_PUSH_CERT_NONCE_STATUS=%s", nonce_status);
+			if (nonce_status == NONCE_SLOP)
+				argv_array_pushf(&env, "GIT_PUSH_CERT_NONCE_SLOP=%ld",
+						 nonce_stamp_slop);
 		}
 		proc->env = env.argv;
 	}
@@ -1352,9 +1393,7 @@ static int delete_only(struct command *commands)
 int cmd_receive_pack(int argc, const char **argv, const char *prefix)
 {
 	int advertise_refs = 0;
-	int stateless_rpc = 0;
 	int i;
-	const char *dir = NULL;
 	struct command *commands;
 	struct sha1_array shallow = SHA1_ARRAY_INIT;
 	struct sha1_array ref = SHA1_ARRAY_INIT;
@@ -1387,21 +1426,21 @@ int cmd_receive_pack(int argc, const char **argv, const char *prefix)
 
 			usage(receive_pack_usage);
 		}
-		if (dir)
+		if (service_dir)
 			usage(receive_pack_usage);
-		dir = arg;
+		service_dir = arg;
 	}
-	if (!dir)
+	if (!service_dir)
 		usage(receive_pack_usage);
 
 	setup_path();
 
-	if (!enter_repo(dir, 0))
-		die("'%s' does not appear to be a git repository", dir);
+	if (!enter_repo(service_dir, 0))
+		die("'%s' does not appear to be a git repository", service_dir);
 
 	git_config(receive_pack_config, NULL);
 	if (cert_nonce_seed)
-		push_cert_nonce = prepare_push_cert_nonce(dir, time(NULL));
+		push_cert_nonce = prepare_push_cert_nonce(service_dir, time(NULL));
 
 	if (0 <= transfer_unpack_limit)
 		unpack_limit = transfer_unpack_limit;
-- 
2.1.0-399-g2df620b