setup_git_directory(): add an owner check for the top-level directory

It poses a security risk to search for a git directory outside of the
directories owned by the current user.

For example, it is common e.g. in computer pools of educational
institutes to have a "scratch" space: a mounted disk with plenty of
space that is regularly swiped where any authenticated user can create
a directory to do their work. Merely navigating to such a space with a
Git-enabled `PS1` when there is a maliciously-crafted `/scratch/.git/`
can lead to a compromised account.

The same holds true in multi-user setups running Windows, as `C:\` is
writable to every authenticated user by default.

To plug this vulnerability, we stop Git from accepting top-level
directories owned by someone other than the current user. We avoid
looking at the ownership of each and every directories between the
current and the top-level one (if there are any between) to avoid
introducing a performance bottleneck.

This new default behavior is obviously incompatible with the concept of
shared repositories, where we expect the top-level directory to be owned
by only one of its legitimate users. To re-enable that use case, we add
support for adding exceptions from the new default behavior via the
config setting `safe.directory`.

The `safe.directory` config setting is only respected in the system and
global configs, not from repository configs or via the command-line, and
can have multiple values to allow for multiple shared repositories.

We are particularly careful to provide a helpful message to any user
trying to use a shared repository.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Documentation/config.txt

@@ -438,6 +438,8 @@ include::config/rerere.txt[]
 
 include::config/reset.txt[]
 
+include::config/safe.txt[]
+
 include::config/sendemail.txt[]
 
 include::config/sequencer.txt[]

Documentation/config/safe.txt

@@ -0,0 +1,21 @@
+safe.directory::
+	These config entries specify Git-tracked directories that are
+	considered safe even if they are owned by someone other than the
+	current user. By default, Git will refuse to even parse a Git
+	config of a repository owned by someone else, let alone run its
+	hooks, and this config setting allows users to specify exceptions,
+	e.g. for intentionally shared repositories (see the `--shared`
+	option in linkgit:git-init[1]).
++
+This is a multi-valued setting, i.e. you can add more than one directory
+via `git config --add`. To reset the list of safe directories (e.g. to
+override any such directories specified in the system config), add a
+`safe.directory` entry with an empty value.
++
+This config setting is only respected when specified in a system or global
+config, not when it is specified in a repository config or via the command
+line option `-c safe.directory=<path>`.
++
+The value of this setting is interpolated, i.e. `~/<path>` expands to a
+path relative to the home directory and `%(prefix)/<path>` expands to a
+path relative to Git's (runtime) prefix.

setup.c

@@ -5,6 +5,7 @@
 #include "string-list.h"
 #include "chdir-notify.h"
 #include "promisor-remote.h"
+#include "quote.h"
 
 static int inside_git_dir = -1;
 static int inside_work_tree = -1;
@@ -1024,6 +1025,42 @@ static int canonicalize_ceiling_entry(struct string_list_item *item,
 	}
 }
 
+struct safe_directory_data {
+	const char *path;
+	int is_safe;
+};
+
+static int safe_directory_cb(const char *key, const char *value, void *d)
+{
+	struct safe_directory_data *data = d;
+
+	if (!value || !*value)
+		data->is_safe = 0;
+	else {
+		const char *interpolated = NULL;
+
+		if (!git_config_pathname(&interpolated, key, value) &&
+		    !fspathcmp(data->path, interpolated ? interpolated : value))
+			data->is_safe = 1;
+
+		free((char *)interpolated);
+	}
+
+	return 0;
+}
+
+static int ensure_valid_ownership(const char *path)
+{
+	struct safe_directory_data data = { .path = path };
+
+	if (is_path_owned_by_current_user(path))
+		return 1;
+
+	read_very_early_config(safe_directory_cb, &data);
+
+	return data.is_safe;
+}
+
 enum discovery_result {
 	GIT_DIR_NONE = 0,
 	GIT_DIR_EXPLICIT,
@@ -1032,7 +1069,8 @@ enum discovery_result {
 	/* these are errors */
 	GIT_DIR_HIT_CEILING = -1,
 	GIT_DIR_HIT_MOUNT_POINT = -2,
-	GIT_DIR_INVALID_GITFILE = -3
+	GIT_DIR_INVALID_GITFILE = -3,
+	GIT_DIR_INVALID_OWNERSHIP = -4
 };
 
 /*
@@ -1122,11 +1160,15 @@ static enum discovery_result setup_git_directory_gently_1(struct strbuf *dir,
 		}
 		strbuf_setlen(dir, offset);
 		if (gitdirenv) {
+			if (!ensure_valid_ownership(dir->buf))
+				return GIT_DIR_INVALID_OWNERSHIP;
 			strbuf_addstr(gitdir, gitdirenv);
 			return GIT_DIR_DISCOVERED;
 		}
 
 		if (is_git_directory(dir->buf)) {
+			if (!ensure_valid_ownership(dir->buf))
+				return GIT_DIR_INVALID_OWNERSHIP;
 			strbuf_addstr(gitdir, ".");
 			return GIT_DIR_BARE;
 		}
@@ -1253,6 +1295,19 @@ const char *setup_git_directory_gently(int *nongit_ok)
 			    dir.buf);
 		*nongit_ok = 1;
 		break;
+	case GIT_DIR_INVALID_OWNERSHIP:
+		if (!nongit_ok) {
+			struct strbuf quoted = STRBUF_INIT;
+
+			sq_quote_buf_pretty(&quoted, dir.buf);
+			die(_("unsafe repository ('%s' is owned by someone else)\n"
+			      "To add an exception for this directory, call:\n"
+			      "\n"
+			      "\tgit config --global --add safe.directory %s"),
+			    dir.buf, quoted.buf);
+		}
+		*nongit_ok = 1;
+		break;
 	case GIT_DIR_NONE:
 		/*
 		 * As a safeguard against setup_git_directory_gently_1 returning