[PATCH v6 5/5] setup.c: create `discovery.bare`
- From
Glen Choo via GitGitGadget <gitgitgadget@gmail.com>
- Date
- Jun 30, 2022, 18:13 UTC
- Message-ID
- <a1323d963f917df661a8701c305d84e781a8f550.1656612839.git.gitgitgadget@gmail.com>
- In-Reply-To
- <pull.1261.v6.git.git.1656612839.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 | 52 +++++++++++++++++++++++++++ 4 files changed, 133 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..8f802746530 --- /dev/null +++ b/t/t0035-discovery-bare.sh @@ -0,0 +1,52 @@ +#!/bin/sh + +test_description='verify discovery.bare checks' + +TEST_PASSES_SANITIZE_LEAK=true +. ./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 -F "cannot use bare repository" 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' ' + expect_accepted -C outer-repo/bare-repo +' + +test_expect_success 'discovery.bare=always' ' + test_config_global discovery.bare always && + expect_accepted -C outer-repo/bare-repo +' + +test_expect_success 'discovery.bare=never' ' + test_config_global discovery.bare never && + expect_rejected -C outer-repo/bare-repo +' + +test_expect_success 'discovery.bare in the repository' ' + # discovery.bare must not be "never", otherwise git config fails + # with "fatal: not in a git directory" (like safe.directory) + test_config -C outer-repo/bare-repo discovery.bare always && + test_config_global discovery.bare never && + expect_rejected -C outer-repo/bare-repo +' + +test_expect_success 'discovery.bare on the command line' ' + test_config_global discovery.bare never && + expect_accepted -C outer-repo/bare-repo \ + -c discovery.bare=always +' + +test_done
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