Re: Is git clone followed by git verify-tag meaningful?

From: Jeff King <peff@peff.net>
Date: Aug 28, 2019, 23:47 UTC
Message-ID: <20190828234706.GB25355@sigill.intra.peff.net>
In-Reply-To: <20190828203224.GE26001@chatter.i7.local>

On Wed, Aug 28, 2019 at 04:32:24PM -0400, Konstantin Ryabitsev wrote:

Show 5 quoted lines

> If I know that a project uses tag signing, would "git clone" followed by
> "git verify-tag" be meaningful without a "git fsck" in-between? I.e. if an
> attacker has control over the remote server, can they sneak in any badness
> into any of the resulting files and still have the clone, checkout, and
> verify-tag return success unless the repository is fsck'd before verify-tag?

It depends on your definition of badness. :)

Generally, Git clients do not trust the server much at all (not only to be no malicious, but also not to accidentally introduce bit errors).

Even without the fsck, we will compute the sha1 of each object (we must, because the other side doesn't send it at all), and that we have all objects reachable from the refs. So verifying the tag at that point demonstrates a signature on the tag object, which refers to probably some commit via sha1, which refers to actual trees and blobs by a chain of sha1s. If you believe in the integrity of sha1, then it has effectively signed all of that content.

Likewise, Git does not necessarily trust what is in the objects. A malicious repository could claim to store an entry for ".git/config" or "/etc/passwd". Without any further action from you, we'd detect and reject those during a checkout.

If you want to analyze each object for such malformed bits before the checkout, you can do so with "git fsck". But consider instead setting transfer.fsckObjects to check the objects while they're being indexed by the initial clone (i.e., having their sha1's computed). It's effectively free to do it at that point, whereas a later fsck has to access each object again (this takes on the order of minutes of CPU for the kernel).

I don't think there's any real safety in doing so for the case you've described (there's no bad pattern that fsck knows about that the actual checkout code does not). But it does give you an early warning, and is especially help if you're not planning to check things out yourself, but want to avoid hosting malicious repos.

-Peff