http: make redirects more obvious

We instruct curl to always follow HTTP redirects. This is convenient, but it creates opportunities for malicious servers to create confusing situations. For instance, imagine Alice is a git user with access to a private repository on Bob's server. Mallory runs her own server and wants to access objects from Bob's repository. Mallory may try a few tricks that involve asking Alice to clone from her, build on top, and then push the result: 1. Mallory may simply redirect all fetch requests to Bob's server. Git will transparently follow those redirects and fetch Bob's history, which Alice may believe she got from Mallory. The subsequent push seems like it is just feeding Mallory back her own objects, but is actually leaking Bob's objects. There is nothing in git's output to indicate that Bob's repository was involved at all. The downside (for Mallory) of this attack is that Alice will have received Bob's entire repository, and is likely to notice that when building on top of it. 2. If Mallory happens to know the sha1 of some object X in Bob's repository, she can instead build her own history that references that object. She then runs a dumb http server, and Alice's client will fetch each object individually. When it asks for X, Mallory redirects her to Bob's server. The end result is that Alice obtains objects from Bob, but they may be buried deep in history. Alice is less likely to notice. Both of these attacks are fairly hard to pull off. There's a social component in getting Mallory to convince Alice to work with her. Alice may be prompted for credentials in accessing Bob's repository (but not always, if she is using a credential helper that caches). Attack (1) requires a certain amount of obliviousness on Alice's part while making a new commit. Attack (2) requires that Mallory knows a sha1 in Bob's repository, that Bob's server supports dumb http, and that the object in question is loose on Bob's server. But we can probably make things a bit more obvious without any loss of functionality. This patch does two things to that end. First, when we encounter a whole-repo redirect during the initial ref discovery, we now inform the user on stderr, making attack (1) much more obvious. Second, the decision to follow redirects is now configurable. The truly paranoid can set the new http.followRedirects to false to avoid any redirection entirely. But for a more practical default, we will disallow redirects only after the initial ref discovery. This is enough to thwart attacks similar to (2), while still allowing the common use of redirects at the repository level. Since c93c92f30 (http: update base URLs when we see redirects, 2013-09-28) we re-root all further requests from the redirect destination, which should generally mean that no further redirection is necessary. As an escape hatch, in case there really is a server that needs to redirect individual requests, the user can set http.followRedirects to "true" (and this can be done on a per-server basis via http.*.followRedirects config). Reported-by: Jann Horn <jannh@google.com> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>