From: Glen Choo <chooglen@xxxxxxxxxx> 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, `safe.bareRepository`, that tells Git whether or not to die() when working with a bare repository. This config is an enum of: - "all": allow all bare repositories (this is the default) - "explicit": only allow bare repositories specified via --git-dir or GIT_DIR. If we want to protect users from such attacks by default, neither value will suffice - "all" provides no protection, but "explicit" 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@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx [2]: https://lore.kernel.org/git/5b969c5e-e802-c447-ad25-6acc0b784582@xxxxxxxxxx Signed-off-by: Glen Choo <chooglen@xxxxxxxxxx> --- Documentation/config/safe.txt | 19 +++++++++++ setup.c | 57 ++++++++++++++++++++++++++++++++- t/t0035-safe-bare-repository.sh | 54 +++++++++++++++++++++++++++++++ 3 files changed, 129 insertions(+), 1 deletion(-) create mode 100755 t/t0035-safe-bare-repository.sh diff --git a/Documentation/config/safe.txt b/Documentation/config/safe.txt index f72b4408798..bde7f31459b 100644 --- a/Documentation/config/safe.txt +++ b/Documentation/config/safe.txt @@ -1,3 +1,22 @@ +safe.bareRepository:: + Specifies which bare repositories Git will work with. The currently + supported values are: ++ +* `all`: Git works with all bare repositories. This is the default. +* `explicit`: Git only works with bare repositories specified via + the top-level `--git-dir` command-line option, or the `GIT_DIR` + environment variable (see linkgit:git[1]). ++ +If you do not use bare repositories in your workflow, then it may be +beneficial to set `safe.bareRepository` to `explicit` 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. ++ +This config setting is only respected in protected configuration (see +<<SCOPES>>). This prevents the untrusted repository from tampering with +this value. + safe.directory:: These config entries specify Git-tracked directories that are considered safe even if they are owned by someone other than the diff --git a/setup.c b/setup.c index ec5b9139e32..8c683e92b62 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 allowed_bare_repo { + ALLOWED_BARE_REPO_EXPLICIT = 0, + ALLOWED_BARE_REPO_ALL, +}; static struct startup_info the_startup_info; struct startup_info *startup_info = &the_startup_info; @@ -1160,6 +1164,46 @@ static int ensure_valid_ownership(const char *gitfile, return data.is_safe; } +static int allowed_bare_repo_cb(const char *key, const char *value, void *d) +{ + enum allowed_bare_repo *allowed_bare_repo = d; + + if (strcasecmp(key, "safe.bareRepository")) + return 0; + + if (!strcmp(value, "explicit")) { + *allowed_bare_repo = ALLOWED_BARE_REPO_EXPLICIT; + return 0; + } + if (!strcmp(value, "all")) { + *allowed_bare_repo = ALLOWED_BARE_REPO_ALL; + return 0; + } + return -1; +} + +static enum allowed_bare_repo get_allowed_bare_repo(void) +{ + enum allowed_bare_repo result = ALLOWED_BARE_REPO_ALL; + git_protected_config(allowed_bare_repo_cb, &result); + return result; +} + +static const char *allowed_bare_repo_to_string( + enum allowed_bare_repo allowed_bare_repo) +{ + switch (allowed_bare_repo) { + case ALLOWED_BARE_REPO_EXPLICIT: + return "explicit"; + case ALLOWED_BARE_REPO_ALL: + return "all"; + default: + BUG("invalid allowed_bare_repo %d", + allowed_bare_repo); + } + return NULL; +} + enum discovery_result { GIT_DIR_NONE = 0, GIT_DIR_EXPLICIT, @@ -1169,7 +1213,8 @@ 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, }; /* @@ -1297,6 +1342,8 @@ static enum discovery_result setup_git_directory_gently_1(struct strbuf *dir, } if (is_git_directory(dir->buf)) { + if (get_allowed_bare_repo() == ALLOWED_BARE_REPO_EXPLICIT) + return GIT_DIR_DISALLOWED_BARE; if (!ensure_valid_ownership(NULL, NULL, dir->buf)) return GIT_DIR_INVALID_OWNERSHIP; strbuf_addstr(gitdir, "."); @@ -1443,6 +1490,14 @@ 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' (safe.bareRepository is '%s')"), + dir.buf, + allowed_bare_repo_to_string(get_allowed_bare_repo())); + } + *nongit_ok = 1; + break; case GIT_DIR_NONE: /* * As a safeguard against setup_git_directory_gently_1 returning diff --git a/t/t0035-safe-bare-repository.sh b/t/t0035-safe-bare-repository.sh new file mode 100755 index 00000000000..ecbdc8238db --- /dev/null +++ b/t/t0035-safe-bare-repository.sh @@ -0,0 +1,54 @@ +#!/bin/sh + +test_description='verify safe.bareRepository 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 'safe.bareRepository unset' ' + expect_accepted -C outer-repo/bare-repo +' + +test_expect_success 'safe.bareRepository=all' ' + test_config_global safe.bareRepository all && + expect_accepted -C outer-repo/bare-repo +' + +test_expect_success 'safe.bareRepository=explicit' ' + test_config_global safe.bareRepository explicit && + expect_rejected -C outer-repo/bare-repo +' + +test_expect_success 'safe.bareRepository in the repository' ' + # safe.bareRepository must not be "explicit", otherwise + # git config fails with "fatal: not in a git directory" (like + # safe.directory) + test_config -C outer-repo/bare-repo safe.bareRepository \ + all && + test_config_global safe.bareRepository explicit && + expect_rejected -C outer-repo/bare-repo +' + +test_expect_success 'safe.bareRepository on the command line' ' + test_config_global safe.bareRepository explicit && + expect_accepted -C outer-repo/bare-repo \ + -c safe.bareRepository=all +' + +test_done -- gitgitgadget
