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

[Junio C Hamano <gitster@pobox.com>]

Jonathan Nieder <jrnieder@gmail.com> writes:

> 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"?

>  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?

> 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.