Re: [PATCH] fsck: detect bare repos in trees and warn

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

Hi Glen,

On Wed, 6 Apr 2022, Glen Choo wrote:

> Git tries not to distribute configs in-repo because they are a security
> risk. However, an attacker can do exactly this if they embed a bare
> repo inside of another repo.
>
> Teach fsck to detect whether a tree object contains a bare repo (as
> determined by setup.c) and warn. This will help hosting sites detect and
> prevent transmission of such malicious repos.
>
> See [1] for a more in-depth discussion, including future steps and
> alternatives.
>
> [1] https://lore.kernel.org/git/kl6lsfqpygsj.fsf@chooglen-macbookpro.roam.corp.google.com/

Out of curiosity: does this new check trigger with
https://github.com/libgit2/libgit2? AFAIR it has embedded repositories
that are used in its test suite. In other words, libgit2 has a legitimate
use case for embedded bare repositories, I believe.

Thank you,
Johannes

>
> Signed-off-by: Glen Choo <chooglen@google.com>
> ---
>  fsck.c          | 19 +++++++++++++++++++
>  fsck.h          |  1 +
>  setup.c         |  4 ++++
>  t/t1450-fsck.sh | 36 ++++++++++++++++++++++++++++++++++++
>  4 files changed, 60 insertions(+)
>
> diff --git a/fsck.c b/fsck.c
> index 3ec500d707..11c11c348a 100644
> --- a/fsck.c
> +++ b/fsck.c
> @@ -573,6 +573,9 @@ static int fsck_tree(const struct object_id *tree_oid,
>  	int has_bad_modes = 0;
>  	int has_dup_entries = 0;
>  	int not_properly_sorted = 0;
> +	int has_head = 0;
> +	int has_refs_entry = 0;
> +	int has_objects_entry = 0;
>  	struct tree_desc desc;
>  	unsigned o_mode;
>  	const char *o_name;
> @@ -602,6 +605,12 @@ static int fsck_tree(const struct object_id *tree_oid,
>  		has_dotdot |= !strcmp(name, "..");
>  		has_dotgit |= is_hfs_dotgit(name) || is_ntfs_dotgit(name);
>  		has_zero_pad |= *(char *)desc.buffer == '0';
> +		has_head |= !strcasecmp(name, "HEAD")
> +			&& (S_ISLNK(mode) || S_ISREG(mode));
> +		has_refs_entry |= !strcasecmp(name, "refs")
> +			&& (S_ISLNK(mode) || S_ISDIR(mode));
> +		has_objects_entry |= !strcasecmp(name, "objects")
> +			&& (S_ISLNK(mode) || S_ISDIR(mode));
>
>  		if (is_hfs_dotgitmodules(name) || is_ntfs_dotgitmodules(name)) {
>  			if (!S_ISLNK(mode))
> @@ -739,6 +748,16 @@ static int fsck_tree(const struct object_id *tree_oid,
>  		retval += report(options, tree_oid, OBJ_TREE,
>  				 FSCK_MSG_TREE_NOT_SORTED,
>  				 "not properly sorted");
> +	/*
> +	 * Determine if this tree looks like a bare repository according
> +	 * to the rules of setup.c. If those are changed, this should be
> +	 * changed too.
> +	 */
> +	if (has_head && has_refs_entry && has_objects_entry)
> +		retval += report(options, tree_oid, OBJ_TREE,
> +				 FSCK_MSG_EMBEDDED_BARE_REPO,
> +				 "contains bare repository");
> +
>  	return retval;
>  }
>
> diff --git a/fsck.h b/fsck.h
> index d07f7a2459..3f0f73b0f3 100644
> --- a/fsck.h
> +++ b/fsck.h
> @@ -65,6 +65,7 @@ enum fsck_msg_type {
>  	FUNC(NULL_SHA1, WARN) \
>  	FUNC(ZERO_PADDED_FILEMODE, WARN) \
>  	FUNC(NUL_IN_COMMIT, WARN) \
> +	FUNC(EMBEDDED_BARE_REPO, WARN) \
>  	/* infos (reported as warnings, but ignored by default) */ \
>  	FUNC(GITMODULES_PARSE, INFO) \
>  	FUNC(GITIGNORE_SYMLINK, INFO) \
> diff --git a/setup.c b/setup.c
> index 04ce33cdcd..2600548776 100644
> --- a/setup.c
> +++ b/setup.c
> @@ -336,6 +336,10 @@ int get_common_dir_noenv(struct strbuf *sb, const char *gitdir)
>   *  - either a HEAD symlink or a HEAD file that is formatted as
>   *    a proper "ref:", or a regular file HEAD that has a properly
>   *    formatted sha1 object name.
> + *
> + * fsck.c checks for bare repositories in trees using similar rules, but a
> + * duplicated implementation. If these are changed, the correspnding code in
> + * fsck.c should change too.
>   */
>  int is_git_directory(const char *suspect)
>  {
> diff --git a/t/t1450-fsck.sh b/t/t1450-fsck.sh
> index de50c0ea01..a65827bc03 100755
> --- a/t/t1450-fsck.sh
> +++ b/t/t1450-fsck.sh
> @@ -563,6 +563,42 @@ dot-backslash-case .\\\\.GIT\\\\foobar
>  dotgit-case-backslash .git\\\\foobar
>  EOF
>
> +test_expect_success "fsck notices bare repo" '
> +(
> +	mkdir -p embedded-bare-repo/bare &&
> +	git init embedded-bare-repo &&
> +	(
> +		cd embedded-bare-repo/bare &&
> +		echo content >HEAD &&
> +		mkdir refs/ objects/ &&
> +		echo content >refs/foo &&
> +		echo content >objects/foo &&
> +		git add . &&
> +		git commit -m base &&
> +		bad_tree=$(git rev-parse HEAD:bare) &&
> +		git fsck 2>out &&
> +		test_i18ngrep "warning.*tree $bad_tree: embeddedBareRepo: contains bare repository" out
> +	)
> +)'
> +
> +test_expect_success "fsck notices bare repo with odd casing" '
> +(
> +	mkdir -p embedded-bare-repo-case/bare &&
> +	git init embedded-bare-repo-case &&
> +	(
> +		cd embedded-bare-repo-case/bare &&
> +		echo content >heAD &&
> +		mkdir Refs/ objectS/ &&
> +		echo content >Refs/foo &&
> +		echo content >objectS/foo &&
> +		git add . &&
> +		git commit -m base &&
> +		bad_tree=$(git rev-parse HEAD:bare) &&
> +		git fsck 2>out &&
> +		test_i18ngrep "warning.*tree $bad_tree: embeddedBareRepo: contains bare repository" out
> +	)
> +)'
> +
>  test_expect_success 'fsck allows .??it' '
>  	(
>  		git init not-dotgit &&
>
> base-commit: 805e0a68082a217f0112db9ee86a022227a9c81b
> --
> 2.33.GIT
>
>