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, `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. `--git-dir` was passed). 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@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx [2]: https://lore.kernel.org/git/5b969c5e-e802-c447-ad25-6acc0b784582@xxxxxxxxxx Signed-off-by: Glen Choo <chooglen@xxxxxxxxxx> --- Documentation/config.txt | 2 + Documentation/config/discovery.txt | 19 +++++++++ setup.c | 66 +++++++++++++++++++++++++++++- t/t0035-discovery-bare.sh | 64 +++++++++++++++++++++++++++++ 4 files changed, 150 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 07832de1a6c..34133288d75 100644 --- a/Documentation/config.txt +++ b/Documentation/config.txt @@ -415,6 +415,8 @@ 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..fbe93597e7c --- /dev/null +++ b/Documentation/config/discovery.txt @@ -0,0 +1,19 @@ +discovery.bare:: + '(Protected config only)' Specifies whether Git will work with a + bare repository that it found during repository discovery. This + has no effect if the repository is specified directly via the + --git-dir command-line option or the GIT_DIR environment + variable (see linkgit:git[1]). ++ +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 847d47f9195..6686743ab7d 100644 --- a/setup.c +++ b/setup.c @@ -10,6 +10,13 @@ static int inside_git_dir = -1; static int inside_work_tree = -1; static int work_tree_config_is_bogus; +enum discovery_bare_config { + DISCOVERY_BARE_UNKNOWN = -1, + DISCOVERY_BARE_NEVER = 0, + DISCOVERY_BARE_ALWAYS, +}; +static enum discovery_bare_config discovery_bare_config = + DISCOVERY_BARE_UNKNOWN; static struct startup_info the_startup_info; struct startup_info *startup_info = &the_startup_info; @@ -1133,6 +1140,52 @@ 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) +{ + if (strcmp(key, "discovery.bare")) + return 0; + + if (!strcmp(value, "never")) { + discovery_bare_config = DISCOVERY_BARE_NEVER; + return 0; + } + if (!strcmp(value, "always")) { + discovery_bare_config = DISCOVERY_BARE_ALWAYS; + return 0; + } + return -1; +} + +static int check_bare_repo_allowed(void) +{ + if (discovery_bare_config == DISCOVERY_BARE_UNKNOWN) { + discovery_bare_config = DISCOVERY_BARE_ALWAYS; + git_protected_config(discovery_bare_cb, NULL); + } + switch (discovery_bare_config) { + case DISCOVERY_BARE_NEVER: + return 0; + case DISCOVERY_BARE_ALWAYS: + return 1; + case DISCOVERY_BARE_UNKNOWN: + BUG("invalid discovery_bare_config %d", discovery_bare_config); + } + return 0; +} + +static const char *discovery_bare_config_to_string(void) +{ + switch (discovery_bare_config) { + case DISCOVERY_BARE_NEVER: + return "never"; + case DISCOVERY_BARE_ALWAYS: + return "always"; + case DISCOVERY_BARE_UNKNOWN: + BUG("invalid discovery_bare_config %d", discovery_bare_config); + } + return NULL; +} + enum discovery_result { GIT_DIR_NONE = 0, GIT_DIR_EXPLICIT, @@ -1142,7 +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, }; /* @@ -1239,6 +1293,8 @@ static enum discovery_result setup_git_directory_gently_1(struct strbuf *dir, } if (is_git_directory(dir->buf)) { + if (!check_bare_repo_allowed()) + return GIT_DIR_DISALLOWED_BARE; if (!ensure_valid_ownership(dir->buf)) return GIT_DIR_INVALID_OWNERSHIP; strbuf_addstr(gitdir, "."); @@ -1385,6 +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_config_to_string()); + } + *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..94c2f76d774 --- /dev/null +++ b/t/t0035-discovery-bare.sh @@ -0,0 +1,64 @@ +#!/bin/sh + +test_description='verify discovery.bare checks' + +. ./test-lib.sh + +pwd="$(pwd)" + +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 && + git rev-parse --git-dir + ) +' + +test_expect_success 'discovery.bare=always' ' + git config --global discovery.bare always && + ( + cd outer-repo/bare-repo && + git rev-parse --git-dir + ) +' + +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 && + test_must_fail git -c discovery.bare=always rev-parse --git-dir 2>err && + grep "discovery.bare" err + ) +' + +test_done -- gitgitgadget
