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setup.c: create safe.bareRepository

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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@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>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Glen Choo
2022-07-14 21:28:01 +00:00
committed by Junio C Hamano
parent 6061601d9f
commit 8d1a744820
3 changed files with 129 additions and 1 deletions
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57
setup.c
View File

@ -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