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From: "Glen Choo via GitGitGadget" <gitgitgadget@gmail.com>
To: git@vger.kernel.org
Cc: Taylor Blau <me@ttaylorr.com>,
	"brian m. carlson" <sandals@crustytoothpaste.net>,
	Derrick Stolee <derrickstolee@github.com>,
	Junio C Hamano <gitster@pobox.com>,
	Emily Shaffer <emilyshaffer@google.com>,
	Jonathan Tan <jonathantanmy@google.com>,
	Glen Choo <chooglen@google.com>, Glen Choo <chooglen@google.com>
Subject: [PATCH v5 5/5] setup.c: create `discovery.bare`
Date: Mon, 27 Jun 2022 18:36:34 +0000	[thread overview]
Message-ID: <14411512783fd4e2cdcc8513690377b29262f6b8.1656354994.git.gitgitgadget@gmail.com> (raw)
In-Reply-To: <pull.1261.v5.git.git.1656354994.gitgitgadget@gmail.com>

From: Glen Choo <chooglen@google.com>

There is a known social engineering attack that takes advantage of the
fact that a working tree can include an entire bare repository,
including a config file. A user could run a Git command inside the bare
repository thinking that the config file of the 'outer' repository would
be used, but in reality, the bare repository's config file (which is
attacker-controlled) is used, which may result in arbitrary code
execution. See [1] for a fuller description and deeper discussion.

A simple mitigation is to forbid bare repositories unless specified via
`--git-dir` or `GIT_DIR`. In environments that don't use bare
repositories, this would be minimally disruptive.

Create a config variable, `discovery.bare`, that tells Git whether or
not to die() when it discovers a bare repository. This only affects
repository discovery, thus it has no effect if discovery was not
done, e.g. if the user passes `--git-dir=my-dir`, discovery will be
skipped and my-dir will be used as the repo regardless of the
`discovery.bare` value.

This config is an enum of:

- "always": always allow bare repositories (this is the default)
- "never": never allow bare repositories

If we want to protect users from such attacks by default, neither value
will suffice - "always" provides no protection, but "never" is
impractical for bare repository users. A more usable default would be to
allow only non-embedded bare repositories ([2] contains one such
proposal), but detecting if a repository is embedded is potentially
non-trivial, so this work is not implemented in this series.

[1]: https://lore.kernel.org/git/kl6lsfqpygsj.fsf@chooglen-macbookpro.roam.corp.google.com
[2]: https://lore.kernel.org/git/5b969c5e-e802-c447-ad25-6acc0b784582@github.com

Signed-off-by: Glen Choo <chooglen@google.com>
---
 Documentation/config.txt           |  2 +
 Documentation/config/discovery.txt | 23 ++++++++++
 setup.c                            | 57 ++++++++++++++++++++++++-
 t/t0035-discovery-bare.sh          | 68 ++++++++++++++++++++++++++++++
 4 files changed, 149 insertions(+), 1 deletion(-)
 create mode 100644 Documentation/config/discovery.txt
 create mode 100755 t/t0035-discovery-bare.sh

diff --git a/Documentation/config.txt b/Documentation/config.txt
index e284b042f22..9a5e1329772 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -409,6 +409,8 @@ include::config/diff.txt[]
 
 include::config/difftool.txt[]
 
+include::config/discovery.txt[]
+
 include::config/extensions.txt[]
 
 include::config/fastimport.txt[]
diff --git a/Documentation/config/discovery.txt b/Documentation/config/discovery.txt
new file mode 100644
index 00000000000..bbcf89bb0b5
--- /dev/null
+++ b/Documentation/config/discovery.txt
@@ -0,0 +1,23 @@
+discovery.bare::
+	Specifies whether Git will work with a bare repository that it
+	found during repository discovery. If the repository is
+	specified directly via the --git-dir command-line option or the
+	GIT_DIR environment variable (see linkgit:git[1]), Git will
+	always use the specified repository, regardless of this value.
++
+This config setting is only respected in protected configuration (see
+<<SCOPES>>). This prevents the untrusted repository from tampering with
+this value.
++
+The currently supported values are:
++
+* `always`: Git always works with bare repositories
+* `never`: Git never works with bare repositories
++
+This defaults to `always`, but this default may change in the future.
++
+If you do not use bare repositories in your workflow, then it may be
+beneficial to set `discovery.bare` to `never` in your global config.
+This will protect you from attacks that involve cloning a repository
+that contains a bare repository and running a Git command within that
+directory.
diff --git a/setup.c b/setup.c
index c8e3c32814d..16938fd5a24 100644
--- a/setup.c
+++ b/setup.c
@@ -10,6 +10,10 @@
 static int inside_git_dir = -1;
 static int inside_work_tree = -1;
 static int work_tree_config_is_bogus;
+enum discovery_bare_allowed {
+	DISCOVERY_BARE_NEVER = 0,
+	DISCOVERY_BARE_ALWAYS,
+};
 
 static struct startup_info the_startup_info;
 struct startup_info *startup_info = &the_startup_info;
@@ -1142,6 +1146,46 @@ static int ensure_valid_ownership(const char *path)
 	return data.is_safe;
 }
 
+static int discovery_bare_cb(const char *key, const char *value, void *d)
+{
+	enum discovery_bare_allowed *discovery_bare_allowed = d;
+
+	if (strcmp(key, "discovery.bare"))
+		return 0;
+
+	if (!strcmp(value, "never")) {
+		*discovery_bare_allowed = DISCOVERY_BARE_NEVER;
+		return 0;
+	}
+	if (!strcmp(value, "always")) {
+		*discovery_bare_allowed = DISCOVERY_BARE_ALWAYS;
+		return 0;
+	}
+	return -1;
+}
+
+static enum discovery_bare_allowed get_discovery_bare(void)
+{
+	enum discovery_bare_allowed result = DISCOVERY_BARE_ALWAYS;
+	git_protected_config(discovery_bare_cb, &result);
+	return result;
+}
+
+static const char *discovery_bare_allowed_to_string(
+	enum discovery_bare_allowed discovery_bare_allowed)
+{
+	switch (discovery_bare_allowed) {
+	case DISCOVERY_BARE_NEVER:
+		return "never";
+	case DISCOVERY_BARE_ALWAYS:
+		return "always";
+	default:
+		BUG("invalid discovery_bare_allowed %d",
+		    discovery_bare_allowed);
+	}
+	return NULL;
+}
+
 enum discovery_result {
 	GIT_DIR_NONE = 0,
 	GIT_DIR_EXPLICIT,
@@ -1151,7 +1195,8 @@ enum discovery_result {
 	GIT_DIR_HIT_CEILING = -1,
 	GIT_DIR_HIT_MOUNT_POINT = -2,
 	GIT_DIR_INVALID_GITFILE = -3,
-	GIT_DIR_INVALID_OWNERSHIP = -4
+	GIT_DIR_INVALID_OWNERSHIP = -4,
+	GIT_DIR_DISALLOWED_BARE = -5,
 };
 
 /*
@@ -1248,6 +1293,8 @@ static enum discovery_result setup_git_directory_gently_1(struct strbuf *dir,
 		}
 
 		if (is_git_directory(dir->buf)) {
+			if (!get_discovery_bare())
+				return GIT_DIR_DISALLOWED_BARE;
 			if (!ensure_valid_ownership(dir->buf))
 				return GIT_DIR_INVALID_OWNERSHIP;
 			strbuf_addstr(gitdir, ".");
@@ -1394,6 +1441,14 @@ const char *setup_git_directory_gently(int *nongit_ok)
 		}
 		*nongit_ok = 1;
 		break;
+	case GIT_DIR_DISALLOWED_BARE:
+		if (!nongit_ok) {
+			die(_("cannot use bare repository '%s' (discovery.bare is '%s')"),
+			    dir.buf,
+			    discovery_bare_allowed_to_string(get_discovery_bare()));
+		}
+		*nongit_ok = 1;
+		break;
 	case GIT_DIR_NONE:
 		/*
 		 * As a safeguard against setup_git_directory_gently_1 returning
diff --git a/t/t0035-discovery-bare.sh b/t/t0035-discovery-bare.sh
new file mode 100755
index 00000000000..0b345d361e6
--- /dev/null
+++ b/t/t0035-discovery-bare.sh
@@ -0,0 +1,68 @@
+#!/bin/sh
+
+test_description='verify discovery.bare checks'
+
+. ./test-lib.sh
+
+pwd="$(pwd)"
+
+expect_accepted () {
+	git "$@" rev-parse --git-dir
+}
+
+expect_rejected () {
+	test_must_fail git "$@" rev-parse --git-dir 2>err &&
+	grep "discovery.bare" err
+}
+
+test_expect_success 'setup bare repo in worktree' '
+	git init outer-repo &&
+	git init --bare outer-repo/bare-repo
+'
+
+test_expect_success 'discovery.bare unset' '
+	(
+		cd outer-repo/bare-repo &&
+		expect_accepted
+	)
+'
+
+test_expect_success 'discovery.bare=always' '
+	git config --global discovery.bare always &&
+	(
+		cd outer-repo/bare-repo &&
+		expect_accepted
+	)
+'
+
+test_expect_success 'discovery.bare=never' '
+	git config --global discovery.bare never &&
+	(
+		cd outer-repo/bare-repo &&
+		expect_rejected
+	)
+'
+
+test_expect_success 'discovery.bare in the repository' '
+	(
+		cd outer-repo/bare-repo &&
+		# Temporarily set discovery.bare=always, otherwise git
+		# config fails with "fatal: not in a git directory"
+		# (like safe.directory)
+		git config --global discovery.bare always &&
+		git config discovery.bare always &&
+		git config --global discovery.bare never &&
+		expect_rejected
+	)
+'
+
+test_expect_success 'discovery.bare on the command line' '
+	git config --global discovery.bare never &&
+	(
+		cd outer-repo/bare-repo &&
+		expect_accepted -c discovery.bare=always &&
+		expect_rejected -c discovery.bare=
+	)
+'
+
+test_done
-- 
gitgitgadget

  parent reply	other threads:[~2022-06-27 18:41 UTC|newest]

Thread overview: 113+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-05-06 18:30 [PATCH] [RFC] setup.c: make bare repo discovery optional Glen Choo via GitGitGadget
2022-05-06 20:33 ` Junio C Hamano
2022-05-09 21:42 ` Taylor Blau
2022-05-09 22:54   ` Junio C Hamano
2022-05-09 23:57     ` Taylor Blau
2022-05-10  0:23       ` Junio C Hamano
2022-05-10 22:00   ` Glen Choo
2022-05-13 23:37 ` [PATCH v2 0/2] " Glen Choo via GitGitGadget
2022-05-13 23:37   ` [PATCH v2 1/2] " Glen Choo via GitGitGadget
2022-05-16 18:12     ` Glen Choo
2022-05-16 18:46     ` Derrick Stolee
2022-05-16 22:25       ` Taylor Blau
2022-05-17 20:24       ` Glen Choo
2022-05-17 21:51         ` Glen Choo
2022-05-13 23:37   ` [PATCH v2 2/2] setup.c: learn discovery.bareRepository=cwd Glen Choo via GitGitGadget
2022-05-16 18:49     ` Derrick Stolee
2022-05-16 16:40   ` [PATCH v2 0/2] setup.c: make bare repo discovery optional Junio C Hamano
2022-05-16 18:36     ` Glen Choo
2022-05-16 19:16       ` Junio C Hamano
2022-05-16 20:27         ` Glen Choo
2022-05-16 22:16           ` Junio C Hamano
2022-05-16 16:43   ` Junio C Hamano
2022-05-16 19:07   ` Derrick Stolee
2022-05-16 22:43     ` Taylor Blau
2022-05-16 23:19     ` Junio C Hamano
2022-05-17 18:56     ` Glen Choo
2022-05-27 21:09   ` [PATCH v3 0/5] config: introduce discovery.bare and protected config Glen Choo via GitGitGadget
2022-05-27 21:09     ` [PATCH v3 1/5] Documentation: define protected configuration Glen Choo via GitGitGadget
2022-05-27 23:29       ` Junio C Hamano
2022-06-02 12:42         ` Derrick Stolee
2022-06-02 16:53           ` Junio C Hamano
2022-06-02 17:39             ` Glen Choo
2022-06-03 15:57         ` Glen Choo
2022-05-27 21:09     ` [PATCH v3 2/5] config: read protected config with `git_protected_config()` Glen Choo via GitGitGadget
2022-05-28  0:28       ` Junio C Hamano
2022-05-31 17:43         ` Glen Choo
2022-06-01 15:58           ` Junio C Hamano
2022-06-02 12:56       ` Derrick Stolee
2022-05-27 21:09     ` [PATCH v3 3/5] setup.c: create `discovery.bare` Glen Choo via GitGitGadget
2022-05-28  0:59       ` Junio C Hamano
2022-06-02 13:11       ` Derrick Stolee
2022-05-27 21:09     ` [PATCH v3 4/5] config: include "-c" in protected config Glen Choo via GitGitGadget
2022-06-02 13:15       ` Derrick Stolee
2022-05-27 21:09     ` [PATCH v3 5/5] upload-pack: make uploadpack.packObjectsHook protected Glen Choo via GitGitGadget
2022-06-02 13:18       ` Derrick Stolee
2022-06-07 20:57     ` [PATCH v4 0/5] config: introduce discovery.bare and protected config Glen Choo via GitGitGadget
2022-06-07 20:57       ` [PATCH v4 1/5] Documentation/git-config.txt: add SCOPES section Glen Choo via GitGitGadget
2022-06-07 20:57       ` [PATCH v4 2/5] Documentation: define protected configuration Glen Choo via GitGitGadget
2022-06-22 21:58         ` Jonathan Tan
2022-06-23 18:21           ` Glen Choo
2022-06-07 20:57       ` [PATCH v4 3/5] config: read protected config with `git_protected_config()` Glen Choo via GitGitGadget
2022-06-07 22:49         ` Junio C Hamano
2022-06-08  0:22           ` Glen Choo
2022-06-07 20:57       ` [PATCH v4 4/5] safe.directory: use git_protected_config() Glen Choo via GitGitGadget
2022-06-07 20:57       ` [PATCH v4 5/5] setup.c: create `discovery.bare` Glen Choo via GitGitGadget
2022-06-07 21:37         ` Glen Choo
2022-06-22 22:03       ` [PATCH v4 0/5] config: introduce discovery.bare and protected config Jonathan Tan
2022-06-23 17:13         ` Glen Choo
2022-06-23 18:32           ` Junio C Hamano
2022-06-27 17:34             ` Glen Choo
2022-06-27 18:19       ` Glen Choo
2022-06-27 18:36       ` [PATCH v5 " Glen Choo via GitGitGadget
2022-06-27 18:36         ` [PATCH v5 1/5] Documentation/git-config.txt: add SCOPES section Glen Choo via GitGitGadget
2022-06-27 18:36         ` [PATCH v5 2/5] Documentation: define protected configuration Glen Choo via GitGitGadget
2022-06-27 18:36         ` [PATCH v5 3/5] config: learn `git_protected_config()` Glen Choo via GitGitGadget
2022-06-27 18:36         ` [PATCH v5 4/5] safe.directory: use git_protected_config() Glen Choo via GitGitGadget
2022-06-27 18:36         ` Glen Choo via GitGitGadget [this message]
2022-06-30 13:20           ` [PATCH v5 5/5] setup.c: create `discovery.bare` Ævar Arnfjörð Bjarmason
2022-06-30 17:28             ` Glen Choo
2022-06-30 18:13         ` [PATCH v6 0/5] config: introduce discovery.bare and protected config Glen Choo via GitGitGadget
2022-06-30 18:13           ` [PATCH v6 1/5] Documentation/git-config.txt: add SCOPES section Glen Choo via GitGitGadget
2022-06-30 22:32             ` Taylor Blau
2022-07-06 17:44               ` Glen Choo
2022-06-30 18:13           ` [PATCH v6 2/5] Documentation: define protected configuration Glen Choo via GitGitGadget
2022-06-30 23:49             ` Taylor Blau
2022-07-06 18:21               ` Glen Choo
2022-06-30 18:13           ` [PATCH v6 3/5] config: learn `git_protected_config()` Glen Choo via GitGitGadget
2022-07-01  1:22             ` Taylor Blau
2022-07-06 22:42               ` Glen Choo
2022-06-30 18:13           ` [PATCH v6 4/5] safe.directory: use git_protected_config() Glen Choo via GitGitGadget
2022-06-30 18:13           ` [PATCH v6 5/5] setup.c: create `discovery.bare` Glen Choo via GitGitGadget
2022-07-01  1:30             ` Taylor Blau
2022-07-07 19:55               ` Glen Choo
2022-06-30 22:13           ` [PATCH v6 0/5] config: introduce discovery.bare and protected config Taylor Blau
2022-06-30 23:07           ` Ævar Arnfjörð Bjarmason
2022-07-01 17:37             ` Glen Choo
2022-07-08 21:58               ` Ævar Arnfjörð Bjarmason
2022-07-12 20:47                 ` Glen Choo
2022-07-12 23:53                   ` Ævar Arnfjörð Bjarmason
2022-07-07 23:01           ` [PATCH v7 " Glen Choo via GitGitGadget
2022-07-07 23:01             ` [PATCH v7 1/5] Documentation/git-config.txt: add SCOPES section Glen Choo via GitGitGadget
2022-07-07 23:43               ` Junio C Hamano
2022-07-08 17:01                 ` Glen Choo
2022-07-08 19:01                   ` Junio C Hamano
2022-07-08 21:38                     ` Glen Choo
2022-07-07 23:01             ` [PATCH v7 2/5] Documentation: define protected configuration Glen Choo via GitGitGadget
2022-07-08  0:39               ` Junio C Hamano
2022-07-07 23:01             ` [PATCH v7 3/5] config: learn `git_protected_config()` Glen Choo via GitGitGadget
2022-07-07 23:01             ` [PATCH v7 4/5] safe.directory: use git_protected_config() Glen Choo via GitGitGadget
2022-07-07 23:01             ` [PATCH v7 5/5] setup.c: create `discovery.bare` Glen Choo via GitGitGadget
2022-07-08  1:07             ` [PATCH v7 0/5] config: introduce discovery.bare and protected config Junio C Hamano
2022-07-08 20:35               ` Glen Choo
2022-07-12 22:11                 ` Glen Choo
2022-07-14 21:27             ` [PATCH v8 0/5] config: introduce safe.bareRepository " Glen Choo via GitGitGadget
2022-07-14 21:27               ` [PATCH v8 1/5] Documentation/git-config.txt: add SCOPES section Glen Choo via GitGitGadget
2022-07-14 21:27               ` [PATCH v8 2/5] Documentation: define protected configuration Glen Choo via GitGitGadget
2022-07-14 21:27               ` [PATCH v8 3/5] config: learn `git_protected_config()` Glen Choo via GitGitGadget
2022-07-25 18:26                 ` SANITIZE=address failure on master (was: [PATCH v8 3/5] config: learn `git_protected_config()`) Ævar Arnfjörð Bjarmason
2022-07-25 20:15                   ` Glen Choo
2022-07-25 20:41                     ` Ævar Arnfjörð Bjarmason
2022-07-25 20:56                       ` Glen Choo
2022-07-14 21:28               ` [PATCH v8 4/5] safe.directory: use git_protected_config() Glen Choo via GitGitGadget
2022-07-14 21:28               ` [PATCH v8 5/5] setup.c: create `safe.bareRepository` Glen Choo via GitGitGadget

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