Re: Security of .git/config and .git/hooks

From: Junio C Hamano <gitster@pobox.com>
Date: Oct 3, 2017, 01:12 UTC
Message-ID: <xmqqtvzhkpal.fsf@gitster.mtv.corp.google.com>
In-Reply-To: <20171002234517.GV19555@aiede.mtv.corp.google.com>

Jonathan Nieder <jrnieder@gmail.com> writes:

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> Proposed fix: because of case (1), I would like a way to tell Git to
> stop trusting any files in .git.  That is:
>
>  1. Introduce a (configurable) list of "safe" configuration items that
>     can be set in .git/config and don't respect any others.

The list of "safe" things are configurable by having something in ~/.gitconfig, perhaps?

How would this work, from the end-user's point of view, with "git config --global" and "git config --local"?

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>  2. But what if I want to set a different pager per-repository?
>     I think we could do this using configuration "profiles".
>     My ~/.config/git/profiles/ directory would contain git-style
>     config files for repositories to include.  Repositories could
>     then contain
>
> 	[include]
> 		path = ~/.config/git/profiles/fancy-log-pager
>
>     to make use of those settings.  The facility (1) would
>     special-case this directory to allow it to set "unsafe" settings
>     since files there are assumed not to be under the control of an
>     attacker.

Meaning, "include" is not in "safe" category, but a value that begins with "~/.config/git/" are excempt???

>  3. Likewise for hooks: my ~/.config/git/hooks/ directory would
>     contain hooks for repositories to make use of.  Repositories could
>     symlink to hook files from there to make use of them.

I am not sure what this means. .git/hooks/pre-commit being a symbolic link to "~/.config/git/hooks/pre-commit-fancy" (i.e. readlink gives the path with tilde unexpanded), so that the attacked sysadmin will not run it from ~attacker/.config/git/hooks?

And the code that finds a hook to run sees .git/hooks/pre-commit, resolves the symlink manually and makes sure it leads to somewhere inside ~/.config/... (otherwise it rejects) and then uses the pointed-at copy?

At that point, we are not taking any advantage of symbolic-link-ness of the entity, so .git/hooks/pre-commit being a text file that has a single like, e.g.

	# safe-hook: pre-commit-fancy

may be sufficient (and we do not have to worry about systems without symbolic links)? The machinery that used to manually resolved symlink instead reads it, finds "pre-commit-fancy" in ~/.config/git/hooks/ and runs it, and you get the same behaviour, no?

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> One downside of (3) is its reliance on symlinks.  Some alternatives:
>
>  3b. Use core.hooksPath configuration instead.  Rely on (2).
>  3c. Introduce new hook.* configuration to be used instead of hook
>      scripts.  Rely on (2).

I guess I invented 3d. without reading ahead X-<. None of the 3x variants other than 3 proper will not work for scripts and existing code that sees that .git/hooks/pre-commit is an executable and runs it, and 3 proper will not work without symbolic links, so this means we'd need "git locate-hook pre-commit" (and underlying locate_hook() helper API) that returns "/home/me/.git/config/hook/pre-commit-fancy" or fails when we do this transition. In an unconverted repository, it may return $PWD/.git/hooks/pre-commit, or failure if we are running under the paranoid mode.

Sounds workable.