[PATCH v2 6/8] promisor-remote: trust known remotes matching acceptFromServerUrl

[Christian Couder <christian.couder@gmail.com>]

A previous commit introduced the `promisor.acceptFromServerUrl` config
variable along with the machinery to parse and validate the URL glob
patterns and optional remote name prefixes it contains. However, these
URL patterns are not yet tied into the client's acceptance logic.

When a promisor remote is already configured locally, its fields (like
authentication tokens) may occasionally need to be refreshed by the
server. If `promisor.acceptFromServer` is set to the secure default
("None"), these updates are rejected, potentially causing future
fetches to fail.

To enable such targeted updates for trusted URLs, let's use the URL
patterns from `promisor.acceptFromServerUrl` as an additional URL
based allowlist.

Concretely, let's check the advertised URLs against the URL glob
patterns by introducing a new small helper function called
url_matches_accept_list(), which iterates over the glob patterns and
returns the first matching allowed_url entry (or NULL).

The URL matching is done component by component: scheme and port are
compared exactly, the host is matched with wildmatch() using the
WM_PATHNAME flag (so '*' cannot cross the '/' boundary into the path),
and the path is matched with wildmatch() without WM_PATHNAME (so '*'
can still match multi-level paths). Before matching, the advertised
URL is passed through url_normalize() so that case variations in the
scheme/host, percent-encoding tricks, and ".." path segments cannot
bypass the allowlist.

Let's then use this helper at the tail of should_accept_remote() so
that, when `accept == ACCEPT_NONE`, a known remote whose URL matches
the allowlist is still accepted.

To prepare for this new logic, let's also:

 - Add an 'accept_urls' parameter to should_accept_remote().

 - Replace the BUG() guard in the ACCEPT_KNOWN_URL case with an
   explicit 'if (accept == ACCEPT_KNOWN_URL) return' and a new
   BUG() guard in the ACCEPT_NONE case, so url_matches_accept_list()
   is only called in the ACCEPT_NONE case.

 - Call accept_from_server_url() from filter_promisor_remote()
   and relax its early return so that the function is entered when
   `accept_urls` has entries even if `accept == ACCEPT_NONE`.

With this, many organizations may only need something like:

  git config set --global \
          promisor.acceptFromServerUrl "https://my-org.com/*"

to accept only their own remotes. And if they need to accept additional
remotes in some specific repos, they can also set:

  git config set promisor.acceptFromServer knownUrl

and configure the additional remote manually only in the repos where
they are needed.

Let's then properly document `promisor.acceptFromServerUrl` in
"promisor.adoc" as an additive security allowlist for known remotes,
including the URL normalization behavior and the component-wise
matching, and let's mention it in "gitprotocol-v2.adoc".

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
---
 Documentation/config/promisor.adoc    | 52 ++++++++++++++
 Documentation/gitprotocol-v2.adoc     |  9 +--
 promisor-remote.c                     | 98 +++++++++++++++++++++++++--
 t/t5710-promisor-remote-capability.sh | 71 +++++++++++++++++++
 4 files changed, 220 insertions(+), 10 deletions(-)

diff --git a/Documentation/config/promisor.adoc b/Documentation/config/promisor.adoc
index b0fa43b839..efc066c3f2 100644
--- a/Documentation/config/promisor.adoc
+++ b/Documentation/config/promisor.adoc
@@ -51,6 +51,58 @@ promisor.acceptFromServer::
 	to "fetch" and "clone" requests from the client. Name and URL
 	comparisons are case sensitive. See linkgit:gitprotocol-v2[5].
 
+promisor.acceptFromServerUrl::
+	A glob pattern to specify which server-advertised URLs a
+	client is allowed to act on. When a URL matches, the client
+	will accept the advertised remote as a promisor remote and may
+	automatically accept field updates (such as authentication
+	tokens) from the server, even if `promisor.acceptFromServer`
+	is set to `none` (the default).
++
+This option can appear multiple times in config files. An advertised
+URL will be accepted if it matches _ANY_ glob pattern specified by
+this option in _ANY_ config file read by Git.
++
+Be _VERY_ careful with these patterns: `*` matches any sequence of
+characters within the 'host' and 'path' parts of a URL (but cannot
+cross part boundaries). An overly broad pattern is a major security
+risk, as a matching URL allows a server to update fields (such as
+authentication tokens) on known remotes without further confirmation.
+To minimize security risks, follow these guidelines:
++
+1. Start with a secure protocol scheme, like `https://` or `ssh://`.
++
+2. Only allow domain names or paths where you control and trust _ALL_
+   the content. Be especially careful with shared hosting platforms
+   like `github.com` or `gitlab.com`. A broad pattern like
+   `https://gitlab.com/*` is dangerous because it trusts every
+   repository on the entire platform. Always restrict such patterns to
+   your specific organization or namespace (e.g.,
+   `https://gitlab.com/your-org/*`).
++
+3. Never use globs at the end of domain names. For example,
+   `https://cdn.your-org.com/*` might be safe, but
+   `https://cdn.your-org.com*/*` is a major security risk because
+   the latter matches `https://cdn.your-org.com.hacker.net/repo`.
++
+4. Be careful using globs at the beginning of domain names. While the
+   code ensures a `*` in the host cannot cross into the path, a
+   pattern like `https://*.example.com/*` will still match any
+   subdomain. This is extremely dangerous on shared hosting platforms
+   (e.g., `https://*.github.io/*` trusts every user's site on the
+   entire platform).
++
+Before matching, both the advertised URL and the pattern are
+normalized: the scheme and host are lowercased, percent-encoded
+characters are decoded where possible, and path segments like `..`
+are resolved. The port must also match exactly (e.g.,
+`https://example.com:8080/*` will not match a URL advertised on
+port 9999).
++
+For the security implications of accepting a promisor remote, see the
+documentation of `promisor.acceptFromServer`. For details on the
+protocol, see linkgit:gitprotocol-v2[5].
+
 promisor.checkFields::
 	A comma or space separated list of additional remote related
 	field names. A client checks if the values of these fields
diff --git a/Documentation/gitprotocol-v2.adoc b/Documentation/gitprotocol-v2.adoc
index befa697d21..2beb70595f 100644
--- a/Documentation/gitprotocol-v2.adoc
+++ b/Documentation/gitprotocol-v2.adoc
@@ -866,10 +866,11 @@ the server advertised, the client shouldn't advertise the
 
 On the server side, the "promisor.advertise" and "promisor.sendFields"
 configuration options can be used to control what it advertises. On
-the client side, the "promisor.acceptFromServer" configuration option
-can be used to control what it accepts, and the "promisor.storeFields"
-option, to control what it stores. See the documentation of these
-configuration options in linkgit:git-config[1] for more information.
+the client side, the "promisor.acceptFromServer" and
+"promisor.acceptFromServerUrl" configuration options can be used to
+control what it accepts, and the "promisor.storeFields" option, to
+control what it stores. See the documentation of these configuration
+options in linkgit:git-config[1] for more information.
 
 Note that in the future it would be nice if the "promisor-remote"
 protocol capability could be used by the server, when responding to
diff --git a/promisor-remote.c b/promisor-remote.c
index 3f3924f587..72d5b94bf7 100644
--- a/promisor-remote.c
+++ b/promisor-remote.c
@@ -14,6 +14,7 @@
 #include "url.h"
 #include "urlmatch.h"
 #include "version.h"
+#include "wildmatch.h"
 
 struct promisor_remote_config {
 	struct promisor_remote *promisors;
@@ -742,8 +743,82 @@ static void load_accept_from_server_url(struct repository *repo,
 	}
 }
 
+static bool match_one_url(const struct url_info *pi, const struct url_info *ui)
+{
+	const char *pat = pi->url;
+	const char *url = ui->url;
+	char *p_str, *u_str;
+	bool res;
+
+	/*
+	 * Schemes must match exactly. They are case-folded by
+	 * url_normalize(), so strncmp() suffices.
+	 */
+	if (pi->scheme_len != ui->scheme_len || strncmp(pat, url, pi->scheme_len))
+		return false;
+
+	/*
+	 * Ports must match exactly. url_normalize() strips default
+	 * ports (like 443 for https), so length and content
+	 * comparisons are sufficient.
+	 */
+	if (pi->port_len != ui->port_len ||
+	    strncmp(pat + pi->port_off, url + ui->port_off, pi->port_len))
+		return false;
+
+	/*
+	 * Match host and path separately to prevent a '*' in the host
+	 * portion of the pattern from matching across the '/'
+	 * boundary into the path. Use WM_PATHNAME for the host so '*'
+	 * cannot cross '/' there, and 0 for the path so '*' can still
+	 * match multi-level paths.
+	 */
+
+	p_str = xstrndup(pat + pi->host_off, pi->host_len);
+	u_str = xstrndup(url + ui->host_off, ui->host_len);
+	res = !wildmatch(p_str, u_str, WM_PATHNAME);
+	free(p_str);
+	free(u_str);
+
+	if (!res)
+		return false;
+
+	p_str = xstrndup(pat + pi->path_off, pi->path_len);
+	u_str = xstrndup(url + ui->path_off, ui->path_len);
+	res = !wildmatch(p_str, u_str, 0);
+	free(p_str);
+	free(u_str);
+
+	return res;
+}
+
+static struct allowed_url *url_matches_accept_list(
+		struct string_list *accept_urls, const char *url)
+{
+	struct string_list_item *item;
+	struct url_info url_info;
+
+	url_info.url = url_normalize(url, &url_info);
+
+	if (!url_info.url)
+		return NULL;
+
+	for_each_string_list_item(item, accept_urls) {
+		struct allowed_url *allowed = item->util;
+
+		if (match_one_url(&allowed->pattern_info, &url_info)) {
+			free(url_info.url);
+			return allowed;
+		}
+	}
+
+	free(url_info.url);
+	return NULL;
+}
+
 static int should_accept_remote(enum accept_promisor accept,
 				struct promisor_info *advertised,
+				struct string_list *accept_urls,
 				struct string_list *config_info)
 {
 	struct promisor_info *p;
@@ -771,9 +846,6 @@ static int should_accept_remote(enum accept_promisor accept,
 	if (accept == ACCEPT_KNOWN_NAME)
 		return all_fields_match(advertised, config_info, p);
 
-	if (accept != ACCEPT_KNOWN_URL)
-		BUG("Unhandled 'enum accept_promisor' value '%d'", accept);
-
 	if (strcmp(p->url, remote_url)) {
 		warning(_("known remote named '%s' but with URL '%s' instead of '%s', "
 			  "ignoring this remote"),
@@ -781,7 +853,21 @@ static int should_accept_remote(enum accept_promisor accept,
 		return 0;
 	}
 
-	return all_fields_match(advertised, config_info, p);
+	if (accept == ACCEPT_KNOWN_URL)
+		return all_fields_match(advertised, config_info, p);
+
+	if (accept != ACCEPT_NONE)
+		BUG("Unhandled 'enum accept_promisor' value '%d'", accept);
+
+	/*
+	 * Even if accept == ACCEPT_NONE, we MUST trust this known
+	 * remote to update its token or other such fields if its URL
+	 * matches the acceptFromServerUrl allowlist!
+	 */
+	if (url_matches_accept_list(accept_urls, remote_url))
+		return all_fields_match(advertised, config_info, p);
+
+	return 0;
 }
 
 static int skip_field_name_prefix(const char *elem, const char *field_name, const char **value)
@@ -991,7 +1077,7 @@ static void filter_promisor_remote(struct repository *repo,
 	/* Load and validate the acceptFromServerUrl config */
 	load_accept_from_server_url(repo, &accept_urls);
 
-	if (accept == ACCEPT_NONE)
+	if (accept == ACCEPT_NONE && !accept_urls.nr)
 		return;
 
 	/* Parse remote info received */
@@ -1011,7 +1097,7 @@ static void filter_promisor_remote(struct repository *repo,
 			string_list_sort(&config_info);
 		}
 
-		if (should_accept_remote(accept, advertised, &config_info)) {
+		if (should_accept_remote(accept, advertised, &accept_urls, &config_info)) {
 			if (!store_info)
 				store_info = store_info_new(repo);
 			if (promisor_store_advertised_fields(advertised, store_info))
diff --git a/t/t5710-promisor-remote-capability.sh b/t/t5710-promisor-remote-capability.sh
index 3b39505380..0659b2ac15 100755
--- a/t/t5710-promisor-remote-capability.sh
+++ b/t/t5710-promisor-remote-capability.sh
@@ -387,6 +387,77 @@ test_expect_success "clone with 'KnownUrl' and empty url, so not advertised" '
 	check_missing_objects server 1 "$oid"
 '
 
+test_expect_success "clone with 'None' but URL allowlisted" '
+	git -C server config promisor.advertise true &&
+	test_when_finished "rm -rf client" &&
+
+	GIT_NO_LAZY_FETCH=0 git clone -c remote.lop.promisor=true \
+		-c remote.lop.fetch="+refs/heads/*:refs/remotes/lop/*" \
+		-c remote.lop.url="$TRASH_DIRECTORY_URL/lop" \
+		-c promisor.acceptfromserver=None \
+		-c promisor.acceptFromServerUrl="$ENCODED_TRASH_DIRECTORY_URL/*" \
+		--no-local --filter="blob:limit=5k" server client &&
+
+	# Check that the largest object is still missing on the server
+	check_missing_objects server 1 "$oid"
+'
+
+test_expect_success "clone with 'None' but URL not in allowlist" '
+	git -C server config promisor.advertise true &&
+	test_when_finished "rm -rf client" &&
+
+	GIT_NO_LAZY_FETCH=0 git clone -c remote.lop.promisor=true \
+		-c remote.lop.fetch="+refs/heads/*:refs/remotes/lop/*" \
+		-c remote.lop.url="$TRASH_DIRECTORY_URL/lop" \
+		-c promisor.acceptfromserver=None \
+		-c promisor.acceptFromServerUrl="https://example.com/*" \
+		--no-local --filter="blob:limit=5k" server client &&
+
+	# Check that the largest object is not missing on the server
+	check_missing_objects server 0 "" &&
+
+	# Reinitialize server so that the largest object is missing again
+	initialize_server 1 "$oid"
+'
+
+test_expect_success "clone with 'None' but URL allowlisted in one pattern out of two" '
+	git -C server config promisor.advertise true &&
+	test_when_finished "rm -rf client" &&
+
+	GIT_NO_LAZY_FETCH=0 git clone -c remote.lop.promisor=true \
+		-c remote.lop.fetch="+refs/heads/*:refs/remotes/lop/*" \
+		-c remote.lop.url="$TRASH_DIRECTORY_URL/lop" \
+		-c promisor.acceptfromserver=None \
+		-c promisor.acceptFromServerUrl="https://example.com/*" \
+		-c promisor.acceptFromServerUrl="$ENCODED_TRASH_DIRECTORY_URL/*" \
+		--no-local --filter="blob:limit=5k" server client &&
+
+	# Check that the largest object is still missing on the server
+	check_missing_objects server 1 "$oid"
+'
+
+test_expect_success "clone with 'None', URL allowlisted, but client has different URL" '
+	git -C server config promisor.advertise true &&
+	test_when_finished "rm -rf client" &&
+
+	# The client configures "lop" with a different URL (serverTwo) than
+	# what the server advertises (lop). Even though the advertised URL
+	# matches the allowlist, the remote is rejected because the
+	# configured URL does not match the advertised one.
+	GIT_NO_LAZY_FETCH=0 git clone -c remote.lop.promisor=true \
+		-c remote.lop.fetch="+refs/heads/*:refs/remotes/lop/*" \
+		-c remote.lop.url="$TRASH_DIRECTORY_URL/serverTwo" \
+		-c promisor.acceptfromserver=None \
+		-c promisor.acceptFromServerUrl="$ENCODED_TRASH_DIRECTORY_URL/*" \
+		--no-local --filter="blob:limit=5k" server client &&
+
+	# Check that the largest object is not missing on the server
+	check_missing_objects server 0 "" &&
+
+	# Reinitialize server so that the largest object is missing again
+	initialize_server 1 "$oid"
+'
+
 test_expect_success "clone with invalid promisor.acceptFromServerUrl" '
 	git -C server config promisor.advertise true &&
 	test_when_finished "rm -rf client" &&
-- 
2.54.0.19.gb68b9497aa