Re: [PATCH 4/5] attr: do not respect symlinks for in-tree .gitattributes

From: Duy Nguyen <pclouds@gmail.com>
Date: Nov 9, 2016, 09:22 UTC
Message-ID: <CACsJy8ASMBk+Yak7LyybANFYkoU_Poi1ZGY=ufKtq1vSkoYCXQ@mail.gmail.com>
In-Reply-To: <20161108222127.mejb74maewzhn3qg@sigill.intra.peff.net>

On Wed, Nov 9, 2016 at 5:21 AM, Jeff King <peff@peff.net> wrote:

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> On Tue, Nov 08, 2016 at 08:38:55AM +0700, Duy Nguyen wrote:
>
>> > Another approach is to have a config option to disallow symlinks to
>> > destinations outside of the repository tree (I'm not sure if it should
>> > be on or off by default, though).
>>
>> Let's err on the safe side and disable symlinks to outside repo by
>> default (or even all symlinks on .gitattributes and .gitignore as the
>> first step)
>
> Both of those are actually much harder than you might think.
>
> For matching specific names, we have to deal with case-folding.  It's
> easy to hit the common ones like ".GITIGNORE" with fspathcmp(). But if
> this is actually protection against malicious repositories, we have to
> match all of the horrible filesystem-specific junk that we did for
> ".git".

We could realpath() it and check if the result path is inside realpath($GIT_WORK_TREE). The real work would be done by OS. We will need to check if it points to .git/something, but I think we have that covered. The approach is a bit heavy for such a sanity check though

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> Symlinks are likewise tricky.  If we see that a symlink points to
> "foo/../bar", then we don't know if it leaves the repository unless we
> also look at "foo" to see if it is also a symlink. So you really end up
> having to resolve the symlink yourself (and when checking out multiple
> files, there's an ordering dependency).

We do have this dependency problem right now (e.g. files A and .gitattributes are checked out at the same time and .gitattributes has some attribute on A). It looks like we resolve it by reading the index version at checkout time. We probably can do the same for gitattribute symlinks.

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> I think it might be enough to check:
>
>   - leading "../" tokens in the symlink's destination can be checked
>     against the symlink's path. So "../foo" is OK for path "one/two",
>     but not for path "one".
>
>   - interior "../" can be disallowed entirely. Technically
>     "foo/../bar/../baz" _can_ be a fine symlink destination, but why?
>     It's identical to "baz" unless you are following a bunch of interior
>     symlinks. And if those are interior symlinks, it's still confusing
>     and unnecessarily obfuscated, and a good sign that somebody is
>     trying to do something tricky.

Sounds good.

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> So one reasonable fix might be to have a config option like
> "core.saneSymlinks" that enforces both of those rules for _all_ symlinks
> that we checkout to the working tree. And it could either refuse to
> check them out, or replace them with a file containing the symlink
> content (as we do on systems that don't support symlinks, IIRC).

I wonder if anyone want core.saneSymlinks on, but they have some links that do not meet the above checks and still want to follow them anyway. One way to add such an exception is mark the path with an attribute "follow". Yeah I have a dependency loop :(

-- 
Duy