docs: document security issues around untrusted .git dirs

For a long time our general philosophy has been that it's unsafe to run
arbitrary Git commands if you don't trust the hooks or config in .git,
but that running upload-pack should be OK. E.g., see 1456b04 (Remove
post-upload-hook, 2009-12-10), or the design of uploadpack.packObjectsHook.

But we never really documented this (and even the discussions that led
to 1456b04 were not on the public list!). Let's try to make our
approach more clear, but also be realistic that even upload-pack carries
some risk.

Helped-by: Filip Hejsek <filip.hejsek@gmail.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: peff

Documentation/git-upload-pack.txt

@@ -71,6 +71,21 @@ This is implemented by having `upload-pack` internally set the
 you trust it), you can explicitly set `GIT_NO_LAZY_FETCH` to
 `0`.
 
+SECURITY
+--------
+
+Most Git commands should not be run in an untrusted `.git` directory
+(see the section `SECURITY` in linkgit:git[1]). `upload-pack` tries to
+avoid any dangerous configuration options or hooks from the repository
+it's serving, making it safe to clone an untrusted directory and run
+commands on the resulting clone.
+
+For an extra level of safety, you may be able to run `upload-pack` as an
+alternate user. The details will be platform dependent, but on many
+systems you can run:
+
+    git clone --no-local --upload-pack='sudo -u nobody git-upload-pack' ...
+
 SEE ALSO
 --------
 linkgit:gitnamespaces[7]

Documentation/git.txt

@@ -1032,6 +1032,37 @@ The index is also capable of storing multiple entries (called "stages")
 for a given pathname.  These stages are used to hold the various
 unmerged version of a file when a merge is in progress.
 
+SECURITY
+--------
+
+Some configuration options and hook files may cause Git to run arbitrary
+shell commands. Because configuration and hooks are not copied using
+`git clone`, it is generally safe to clone remote repositories with
+untrusted content, inspect them with `git log`, and so on.
+
+However, it is not safe to run Git commands in a `.git` directory (or
+the working tree that surrounds it) when that `.git` directory itself
+comes from an untrusted source. The commands in its config and hooks
+are executed in the usual way.
+
+By default, Git will refuse to run when the repository is owned by
+someone other than the user running the command. See the entry for
+`safe.directory` in linkgit:git-config[1]. While this can help protect
+you in a multi-user environment, note that you can also acquire
+untrusted repositories that are owned by you (for example, if you
+extract a zip file or tarball from an untrusted source). In such cases,
+you'd need to "sanitize" the untrusted repository first.
+
+If you have an untrusted `.git` directory, you should first clone it
+with `git clone --no-local` to obtain a clean copy. Git does restrict
+the set of options and hooks that will be run by `upload-pack`, which
+handles the server side of a clone or fetch, but beware that the
+surface area for attack against `upload-pack` is large, so this does
+carry some risk. The safest thing is to serve the repository as an
+unprivileged user (either via linkgit:git-daemon[1], ssh, or using
+other tools to change user ids). See the discussion in the `SECURITY`
+section of linkgit:git-upload-pack[1].
+
 FURTHER DOCUMENTATION
 ---------------------