Merge branch 'jc/push-cert'

Allow "git push" request to be signed, so that it can be verified and
audited, using the GPG signature of the person who pushed, that the
tips of branches at a public repository really point the commits
the pusher wanted to, without having to "trust" the server.

* jc/push-cert: (24 commits)
  receive-pack::hmac_sha1(): copy the entire SHA-1 hash out
  signed push: allow stale nonce in stateless mode
  signed push: teach smart-HTTP to pass "git push --signed" around
  signed push: fortify against replay attacks
  signed push: add "pushee" header to push certificate
  signed push: remove duplicated protocol info
  send-pack: send feature request on push-cert packet
  receive-pack: GPG-validate push certificates
  push: the beginning of "git push --signed"
  pack-protocol doc: typofix for PKT-LINE
  gpg-interface: move parse_signature() to where it should be
  gpg-interface: move parse_gpg_output() to where it should be
  send-pack: clarify that cmds_sent is a boolean
  send-pack: refactor inspecting and resetting status and sending commands
  send-pack: rename "new_refs" to "need_pack_data"
  receive-pack: factor out capability string generation
  send-pack: factor out capability string generation
  send-pack: always send capabilities
  send-pack: refactor decision to send update per ref
  send-pack: move REF_STATUS_REJECT_NODELETE logic a bit higher
  ...

1 files changed, 353 insertions, 40 deletions

diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index daf0600ca..a01ac2096 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -15,6 +15,8 @@
 #include "connected.h"
 #include "argv-array.h"
 #include "version.h"
+#include "tag.h"
+#include "gpg-interface.h"
 #include "sigchain.h"
 
 static const char receive_pack_usage[] = "git receive-pack <git-dir>";
@@ -42,11 +44,27 @@ 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;
 static int shallow_update;
 static const char *alt_shallow_file;
+static struct strbuf push_cert = STRBUF_INIT;
+static unsigned char push_cert_sha1[20];
+static struct signature_check sigcheck;
+static const char *push_cert_nonce;
+static const char *cert_nonce_seed;
+
+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 unsigned long nonce_stamp_slop_limit;
 
 static enum deny_action parse_deny_action(const char *var, const char *value)
 {
@@ -130,6 +148,14 @@ static int receive_pack_config(const char *var, const char *value, void *cb)
 		return 0;
 	}
 
+	if (strcmp(var, "receive.certnonceseed") == 0)
+		return git_config_string(&cert_nonce_seed, var, value);
+
+	if (strcmp(var, "receive.certnonceslop") == 0) {
+		nonce_stamp_slop_limit = git_config_ulong(var, value);
+		return 0;
+	}
+
 	return git_default_config(var, value, cb);
 }
 
@@ -138,15 +164,23 @@ static void show_ref(const char *path, const unsigned char *sha1)
 	if (ref_is_hidden(path))
 		return;
 
-	if (sent_capabilities)
+	if (sent_capabilities) {
 		packet_write(1, "%s %s\n", sha1_to_hex(sha1), path);
-	else
-		packet_write(1, "%s %s%c%s%s agent=%s\n",
-			     sha1_to_hex(sha1), path, 0,
-			     " report-status delete-refs side-band-64k quiet",
-			     prefer_ofs_delta ? " ofs-delta" : "",
-			     git_user_agent_sanitized());
-	sent_capabilities = 1;
+	} else {
+		struct strbuf cap = STRBUF_INIT;
+
+		strbuf_addstr(&cap,
+			      "report-status delete-refs side-band-64k quiet");
+		if (prefer_ofs_delta)
+			strbuf_addstr(&cap, " ofs-delta");
+		if (push_cert_nonce)
+			strbuf_addf(&cap, " push-cert=%s", push_cert_nonce);
+		strbuf_addf(&cap, " agent=%s", git_user_agent_sanitized());
+		packet_write(1, "%s %s%c%s\n",
+			     sha1_to_hex(sha1), path, 0, cap.buf);
+		strbuf_release(&cap);
+		sent_capabilities = 1;
+	}
 }
 
 static int show_ref_cb(const char *path, const unsigned char *sha1, int flag, void *unused)
@@ -253,6 +287,222 @@ static int copy_to_sideband(int in, int out, void *arg)
 	return 0;
 }
 
+#define HMAC_BLOCK_SIZE 64
+
+static void hmac_sha1(unsigned char *out,
+		      const char *key_in, size_t key_len,
+		      const char *text, size_t text_len)
+{
+	unsigned char key[HMAC_BLOCK_SIZE];
+	unsigned char k_ipad[HMAC_BLOCK_SIZE];
+	unsigned char k_opad[HMAC_BLOCK_SIZE];
+	int i;
+	git_SHA_CTX ctx;
+
+	/* RFC 2104 2. (1) */
+	memset(key, '\0', HMAC_BLOCK_SIZE);
+	if (HMAC_BLOCK_SIZE < key_len) {
+		git_SHA1_Init(&ctx);
+		git_SHA1_Update(&ctx, key_in, key_len);
+		git_SHA1_Final(key, &ctx);
+	} else {
+		memcpy(key, key_in, key_len);
+	}
+
+	/* RFC 2104 2. (2) & (5) */
+	for (i = 0; i < sizeof(key); i++) {
+		k_ipad[i] = key[i] ^ 0x36;
+		k_opad[i] = key[i] ^ 0x5c;
+	}
+
+	/* RFC 2104 2. (3) & (4) */
+	git_SHA1_Init(&ctx);
+	git_SHA1_Update(&ctx, k_ipad, sizeof(k_ipad));
+	git_SHA1_Update(&ctx, text, text_len);
+	git_SHA1_Final(out, &ctx);
+
+	/* RFC 2104 2. (6) & (7) */
+	git_SHA1_Init(&ctx);
+	git_SHA1_Update(&ctx, k_opad, sizeof(k_opad));
+	git_SHA1_Update(&ctx, out, 20);
+	git_SHA1_Final(out, &ctx);
+}
+
+static char *prepare_push_cert_nonce(const char *path, unsigned long stamp)
+{
+	struct strbuf buf = STRBUF_INIT;
+	unsigned char sha1[20];
+
+	strbuf_addf(&buf, "%s:%lu", path, stamp);
+	hmac_sha1(sha1, buf.buf, buf.len, cert_nonce_seed, strlen(cert_nonce_seed));;
+	strbuf_release(&buf);
+
+	/* RFC 2104 5. HMAC-SHA1-80 */
+	strbuf_addf(&buf, "%lu-%.*s", stamp, 20, sha1_to_hex(sha1));
+	return strbuf_detach(&buf, NULL);
+}
+
+/*
+ * NEEDSWORK: reuse find_commit_header() from jk/commit-author-parsing
+ * after dropping "_commit" from its name and possibly moving it out
+ * of commit.c
+ */
+static char *find_header(const char *msg, size_t len, const char *key)
+{
+	int key_len = strlen(key);
+	const char *line = msg;
+
+	while (line && line < msg + len) {
+		const char *eol = strchrnul(line, '\n');
+
+		if ((msg + len <= eol) || line == eol)
+			return NULL;
+		if (line + key_len < eol &&
+		    !memcmp(line, key, key_len) && line[key_len] == ' ') {
+			int offset = key_len + 1;
+			return xmemdupz(line + offset, (eol - line) - offset);
+		}
+		line = *eol ? eol + 1 : NULL;
+	}
+	return NULL;
+}
+
+static const char *check_nonce(const char *buf, size_t len)
+{
+	char *nonce = find_header(buf, len, "nonce");
+	unsigned long stamp, ostamp;
+	char *bohmac, *expect = NULL;
+	const char *retval = NONCE_BAD;
+
+	if (!nonce) {
+		retval = NONCE_MISSING;
+		goto leave;
+	} else if (!push_cert_nonce) {
+		retval = NONCE_UNSOLICITED;
+		goto leave;
+	} else if (!strcmp(push_cert_nonce, nonce)) {
+		retval = NONCE_OK;
+		goto leave;
+	}
+
+	if (!stateless_rpc) {
+		/* returned nonce MUST match what we gave out earlier */
+		retval = NONCE_BAD;
+		goto leave;
+	}
+
+	/*
+	 * 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.
+	 *
+	 * In addition, when a nonce issued by another instance has
+	 * timestamp within receive.certnonceslop seconds, we pretend
+	 * as if we issued that nonce when reporting to the hook.
+	 */
+
+	/* nonce is concat(<seconds-since-epoch>, "-", <hmac>) */
+	if (*nonce <= '0' || '9' < *nonce) {
+		retval = NONCE_BAD;
+		goto leave;
+	}
+	stamp = strtoul(nonce, &bohmac, 10);
+	if (bohmac == nonce || bohmac[0] != '-') {
+		retval = NONCE_BAD;
+		goto leave;
+	}
+
+	expect = prepare_push_cert_nonce(service_dir, stamp);
+	if (strcmp(expect, nonce)) {
+		/* Not what we would have signed earlier */
+		retval = NONCE_BAD;
+		goto leave;
+	}
+
+	/*
+	 * 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;
+
+	if (nonce_stamp_slop_limit &&
+	    abs(nonce_stamp_slop) <= nonce_stamp_slop_limit) {
+		/*
+		 * Pretend as if the received nonce (which passes the
+		 * HMAC check, so it is not a forged by third-party)
+		 * is what we issued.
+		 */
+		free((void *)push_cert_nonce);
+		push_cert_nonce = xstrdup(nonce);
+		retval = NONCE_OK;
+	} else {
+		retval = NONCE_SLOP;
+	}
+
+leave:
+	free(nonce);
+	free(expect);
+	return retval;
+}
+
+static void prepare_push_cert_sha1(struct child_process *proc)
+{
+	static int already_done;
+	struct argv_array env = ARGV_ARRAY_INIT;
+
+	if (!push_cert.len)
+		return;
+
+	if (!already_done) {
+		struct strbuf gpg_output = STRBUF_INIT;
+		struct strbuf gpg_status = STRBUF_INIT;
+		int bogs /* beginning_of_gpg_sig */;
+
+		already_done = 1;
+		if (write_sha1_file(push_cert.buf, push_cert.len, "blob", push_cert_sha1))
+			hashclr(push_cert_sha1);
+
+		memset(&sigcheck, '\0', sizeof(sigcheck));
+		sigcheck.result = 'N';
+
+		bogs = parse_signature(push_cert.buf, push_cert.len);
+		if (verify_signed_buffer(push_cert.buf, bogs,
+					 push_cert.buf + bogs, push_cert.len - bogs,
+					 &gpg_output, &gpg_status) < 0) {
+			; /* error running gpg */
+		} else {
+			sigcheck.payload = push_cert.buf;
+			sigcheck.gpg_output = gpg_output.buf;
+			sigcheck.gpg_status = gpg_status.buf;
+			parse_gpg_output(&sigcheck);
+		}
+
+		strbuf_release(&gpg_output);
+		strbuf_release(&gpg_status);
+		nonce_status = check_nonce(push_cert.buf, bogs);
+	}
+	if (!is_null_sha1(push_cert_sha1)) {
+		argv_array_pushf(&env, "GIT_PUSH_CERT=%s", sha1_to_hex(push_cert_sha1));
+		argv_array_pushf(&env, "GIT_PUSH_CERT_SIGNER=%s",
+				 sigcheck.signer ? sigcheck.signer : "");
+		argv_array_pushf(&env, "GIT_PUSH_CERT_KEY=%s",
+				 sigcheck.key ? sigcheck.key : "");
+		argv_array_pushf(&env, "GIT_PUSH_CERT_STATUS=%c", sigcheck.result);
+		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;
+	}
+}
+
 typedef int (*feed_fn)(void *, const char **, size_t *);
 static int run_and_feed_hook(const char *hook_name, feed_fn feed, void *feed_state)
 {
@@ -271,6 +521,8 @@ static int run_and_feed_hook(const char *hook_name, feed_fn feed, void *feed_sta
 	proc.in = -1;
 	proc.stdout_to_stderr = 1;
 
+	prepare_push_cert_sha1(&proc);
+
 	if (use_sideband) {
 		memset(&muxer, 0, sizeof(muxer));
 		muxer.proc = copy_to_sideband;
@@ -841,40 +1093,79 @@ static void execute_commands(struct command *commands,
 		      "the reported refs above");
 }
 
+static struct command **queue_command(struct command **tail,
+				      const char *line,
+				      int linelen)
+{
+	unsigned char old_sha1[20], new_sha1[20];
+	struct command *cmd;
+	const char *refname;
+	int reflen;
+
+	if (linelen < 83 ||
+	    line[40] != ' ' ||
+	    line[81] != ' ' ||
+	    get_sha1_hex(line, old_sha1) ||
+	    get_sha1_hex(line + 41, new_sha1))
+		die("protocol error: expected old/new/ref, got '%s'", line);
+
+	refname = line + 82;
+	reflen = linelen - 82;
+	cmd = xcalloc(1, sizeof(struct command) + reflen + 1);
+	hashcpy(cmd->old_sha1, old_sha1);
+	hashcpy(cmd->new_sha1, new_sha1);
+	memcpy(cmd->ref_name, refname, reflen);
+	cmd->ref_name[reflen] = '\0';
+	*tail = cmd;
+	return &cmd->next;
+}
+
+static void queue_commands_from_cert(struct command **tail,
+				     struct strbuf *push_cert)
+{
+	const char *boc, *eoc;
+
+	if (*tail)
+		die("protocol error: got both push certificate and unsigned commands");
+
+	boc = strstr(push_cert->buf, "\n\n");
+	if (!boc)
+		die("malformed push certificate %.*s", 100, push_cert->buf);
+	else
+		boc += 2;
+	eoc = push_cert->buf + parse_signature(push_cert->buf, push_cert->len);
+
+	while (boc < eoc) {
+		const char *eol = memchr(boc, '\n', eoc - boc);
+		tail = queue_command(tail, boc, eol ? eol - boc : eoc - eol);
+		boc = eol ? eol + 1 : eoc;
+	}
+}
+
 static struct command *read_head_info(struct sha1_array *shallow)
 {
 	struct command *commands = NULL;
 	struct command **p = &commands;
 	for (;;) {
 		char *line;
-		unsigned char old_sha1[20], new_sha1[20];
-		struct command *cmd;
-		char *refname;
-		int len, reflen;
+		int len, linelen;
 
 		line = packet_read_line(0, &len);
 		if (!line)
 			break;
 
 		if (len == 48 && starts_with(line, "shallow ")) {
-			if (get_sha1_hex(line + 8, old_sha1))
-				die("protocol error: expected shallow sha, got '%s'", line + 8);
-			sha1_array_append(shallow, old_sha1);
+			unsigned char sha1[20];
+			if (get_sha1_hex(line + 8, sha1))
+				die("protocol error: expected shallow sha, got '%s'",
+				    line + 8);
+			sha1_array_append(shallow, sha1);
 			continue;
 		}
 
-		if (len < 83 ||
-		    line[40] != ' ' ||
-		    line[81] != ' ' ||
-		    get_sha1_hex(line, old_sha1) ||
-		    get_sha1_hex(line + 41, new_sha1))
-			die("protocol error: expected old/new/ref, got '%s'",
-			    line);
-
-		refname = line + 82;
-		reflen = strlen(refname);
-		if (reflen + 82 < len) {
-			const char *feature_list = refname + reflen + 1;
+		linelen = strlen(line);
+		if (linelen < len) {
+			const char *feature_list = line + linelen + 1;
 			if (parse_feature_request(feature_list, "report-status"))
 				report_status = 1;
 			if (parse_feature_request(feature_list, "side-band-64k"))
@@ -882,13 +1173,34 @@ static struct command *read_head_info(struct sha1_array *shallow)
 			if (parse_feature_request(feature_list, "quiet"))
 				quiet = 1;
 		}
-		cmd = xcalloc(1, sizeof(struct command) + len - 80);
-		hashcpy(cmd->old_sha1, old_sha1);
-		hashcpy(cmd->new_sha1, new_sha1);
-		memcpy(cmd->ref_name, line + 82, len - 81);
-		*p = cmd;
-		p = &cmd->next;
+
+		if (!strcmp(line, "push-cert")) {
+			int true_flush = 0;
+			char certbuf[1024];
+
+			for (;;) {
+				len = packet_read(0, NULL, NULL,
+						  certbuf, sizeof(certbuf), 0);
+				if (!len) {
+					true_flush = 1;
+					break;
+				}
+				if (!strcmp(certbuf, "push-cert-end\n"))
+					break; /* end of cert */
+				strbuf_addstr(&push_cert, certbuf);
+			}
+
+			if (true_flush)
+				break;
+			continue;
+		}
+
+		p = queue_command(p, line, linelen);
 	}
+
+	if (push_cert.len)
+		queue_commands_from_cert(p, &push_cert);
+
 	return commands;
 }
 
@@ -1129,9 +1441,7 @@ 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;
@@ -1164,19 +1474,21 @@ 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(service_dir, time(NULL));
 
 	if (0 <= transfer_unpack_limit)
 		unpack_limit = transfer_unpack_limit;
@@ -1221,5 +1533,6 @@ int cmd_receive_pack(int argc, const char **argv, const char *prefix)
 		packet_flush(1);
 	sha1_array_clear(&shallow);
 	sha1_array_clear(&ref);
+	free((void *)push_cert_nonce);
 	return 0;
 }