setup: tighten ownership checks post CVE-2022-24765

8959555cee (setup_git_directory(): add an owner check for the top-level
directory, 2022-03-02), adds a function to check for ownership of
repositories using a directory that is representative of it, and ways to
add exempt a specific repository from said check if needed, but that
check didn't account for owership of the gitdir, or (when used) the
gitfile that points to that gitdir.

An attacker could create a git repository in a directory that they can
write into but that is owned by the victim to work around the fix that
was introduced with CVE-2022-24765 to potentially run code as the
victim.

An example that could result in privilege escalation to root in *NIX would
be to set a repository in a shared tmp directory by doing (for example):

  $ git -C /tmp init

To avoid that, extend the ensure_valid_ownership function to be able to
check for all three paths.

This will have the side effect of tripling the number of stat() calls
when a repository is detected, but the effect is expected to be likely
minimal, as it is done only once during the directory walk in which Git
looks for a repository.

Additionally make sure to resolve the gitfile (if one was used) to find
the relevant gitdir for checking.

While at it change the message printed on failure so it is clear we are
referring to the repository by its worktree (or gitdir if it is bare) and
not to a specific directory.

Helped-by: Junio C Hamano <junio@pobox.com>
Helped-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>

setup.c

@@ -1054,14 +1054,32 @@ static int safe_directory_cb(const char *key, const char *value, void *d)
 	return 0;
 }
 
-static int ensure_valid_ownership(const char *path)
+/*
+ * Check if a repository is safe, by verifying the ownership of the
+ * worktree (if any), the git directory, and the gitfile (if any).
+ *
+ * Exemptions for known-safe repositories can be added via `safe.directory`
+ * config settings; for non-bare repositories, their worktree needs to be
+ * added, for bare ones their git directory.
+ */
+static int ensure_valid_ownership(const char *gitfile,
+				const char *worktree, const char *gitdir)
 {
-	struct safe_directory_data data = { .path = path };
+	struct safe_directory_data data = {
+		.path = worktree ? worktree : gitdir
+	};
 
 	if (!git_env_bool("GIT_TEST_ASSUME_DIFFERENT_OWNER", 0) &&
-	    is_path_owned_by_current_user(path))
+	   (!gitfile || is_path_owned_by_current_user(gitfile)) &&
+	   (!worktree || is_path_owned_by_current_user(worktree)) &&
+	   (!gitdir || is_path_owned_by_current_user(gitdir)))
 		return 1;
 
+	/*
+	 * data.path is the "path" that identifies the repository and it is
+	 * constant regardless of what failed above. data.is_safe should be
+	 * initialized to false, and might be changed by the callback.
+	 */
 	read_very_early_config(safe_directory_cb, &data);
 
 	return data.is_safe;
@@ -1149,6 +1167,8 @@ static enum discovery_result setup_git_directory_gently_1(struct strbuf *dir,
 		current_device = get_device_or_die(dir->buf, NULL, 0);
 	for (;;) {
 		int offset = dir->len, error_code = 0;
+		char *gitdir_path = NULL;
+		char *gitfile = NULL;
 
 		if (offset > min_offset)
 			strbuf_addch(dir, '/');
@@ -1159,21 +1179,50 @@ static enum discovery_result setup_git_directory_gently_1(struct strbuf *dir,
 			if (die_on_error ||
 			    error_code == READ_GITFILE_ERR_NOT_A_FILE) {
 				/* NEEDSWORK: fail if .git is not file nor dir */
-				if (is_git_directory(dir->buf))
+				if (is_git_directory(dir->buf)) {
 					gitdirenv = DEFAULT_GIT_DIR_ENVIRONMENT;
+					gitdir_path = xstrdup(dir->buf);
+				}
 			} else if (error_code != READ_GITFILE_ERR_STAT_FAILED)
 				return GIT_DIR_INVALID_GITFILE;
-		}
+		} else
+			gitfile = xstrdup(dir->buf);
+		/*
+		 * Earlier, we tentatively added DEFAULT_GIT_DIR_ENVIRONMENT
+		 * to check that directory for a repository.
+		 * Now trim that tentative addition away, because we want to
+		 * focus on the real directory we are in.
+		 */
 		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;
+			enum discovery_result ret;
+
+			if (ensure_valid_ownership(gitfile,
+						 dir->buf,
+				 (gitdir_path ? gitdir_path : gitdirenv))) {
+				strbuf_addstr(gitdir, gitdirenv);
+				ret = GIT_DIR_DISCOVERED;
+			} else
+				ret = GIT_DIR_INVALID_OWNERSHIP;
+
+			/*
+			 * Earlier, during discovery, we might have allocated
+			 * string copies for gitdir_path or gitfile so make
+			 * sure we don't leak by freeing them now, before
+			 * leaving the loop and function.
+			 *
+			 * Note: gitdirenv will be non-NULL whenever these are
+			 * allocated, therefore we need not take care of releasing
+			 * them outside of this conditional block.
+			 */
+			free(gitdir_path);
+			free(gitfile);
+
+			return ret;
 		}
 
 		if (is_git_directory(dir->buf)) {
-			if (!ensure_valid_ownership(dir->buf))
+			if (!ensure_valid_ownership(NULL, NULL, dir->buf))
 				return GIT_DIR_INVALID_OWNERSHIP;
 			strbuf_addstr(gitdir, ".");
 			return GIT_DIR_BARE;
@@ -1306,7 +1355,7 @@ const char *setup_git_directory_gently(int *nongit_ok)
 			struct strbuf quoted = STRBUF_INIT;
 
 			sq_quote_buf_pretty(&quoted, dir.buf);
-			die(_("unsafe repository ('%s' is owned by someone else)\n"
+			die(_("detected dubious ownership in repository at '%s'\n"
 			      "To add an exception for this directory, call:\n"
 			      "\n"
 			      "\tgit config --global --add safe.directory %s"),