sudoedit: do not permit editor arguments to include "--" (CVE-2023-22809)

We use "--" to separate the editor and arguments from the files to edit.
If the editor arguments include "--", sudo can be tricked into allowing
the user to edit a file not permitted by the security policy.
Thanks to Matthieu Barjole and Victor Cutillas of Synacktiv
(https://synacktiv.com) for finding this bug.

plugins/sudoers/editor.c

@@ -131,7 +131,7 @@ resolve_editor(const char *ed, size_t edlen, int nfiles, char * const *files,
     const char *tmp, *cp, *ep = NULL;
     const char *edend = ed + edlen;
     struct stat user_editor_sb;
-    int nargc;
+    int nargc = 0;
     debug_decl(resolve_editor, SUDOERS_DEBUG_UTIL);
 
     /*
@@ -149,10 +149,7 @@ resolve_editor(const char *ed, size_t edlen, int nfiles, char * const *files,
     /* If we can't find the editor in the user's PATH, give up. */
     if (find_path(editor, &editor_path, &user_editor_sb, getenv("PATH"), NULL,
 	    0, allowlist) != FOUND) {
-	sudoers_gc_remove(GC_PTR, editor);
-	free(editor);
-	errno = ENOENT;
-	debug_return_str(NULL);
+	goto bad;
     }
 
     /* Count rest of arguments and allocate editor argv. */
@@ -173,6 +170,17 @@ resolve_editor(const char *ed, size_t edlen, int nfiles, char * const *files,
 	nargv[nargc] = copy_arg(cp, ep - cp);
 	if (nargv[nargc] == NULL)
 	    goto oom;
+
+	/*
+	 * We use "--" to separate the editor and arguments from the files
+	 * to edit.  The editor arguments themselves may not contain "--".
+	 */
+	if (strcmp(nargv[nargc], "--") == 0) {
+	    sudo_warnx(U_("ignoring editor: %.*s"), (int)edlen, ed);
+	    sudo_warnx("%s", U_("editor arguments may not contain \"--\""));
+	    errno = EINVAL;
+	    goto bad;
+	}
     }
     if (nfiles != 0) {
 	nargv[nargc++] = (char *)"--";
@@ -186,6 +194,7 @@ resolve_editor(const char *ed, size_t edlen, int nfiles, char * const *files,
     debug_return_str(editor_path);
 oom:
     sudo_warnx(U_("%s: %s"), __func__, U_("unable to allocate memory"));
+bad:
     sudoers_gc_remove(GC_PTR, editor);
     free(editor);
     free(editor_path);

plugins/sudoers/sudoers.c

@@ -802,21 +802,32 @@ sudoers_policy_main(int argc, char * const argv[], int pwflag, char *env_add[],
 
     /* Note: must call audit before uid change. */
     if (ISSET(sudo_mode, MODE_EDIT)) {
+	const char *env_editor = NULL;
 	char **edit_argv;
 	int edit_argc;
-	const char *env_editor;
 
 	free(safe_cmnd);
 	safe_cmnd = find_editor(NewArgc - 1, NewArgv + 1, &edit_argc,
 	    &edit_argv, NULL, &env_editor);
 	if (safe_cmnd == NULL) {
-	    if (errno != ENOENT)
+	    switch (errno) {
+	    case ENOENT:
+		audit_failure(NewArgv, N_("%s: command not found"),
+		    env_editor ? env_editor : def_editor);
+		sudo_warnx(U_("%s: command not found"),
+		    env_editor ? env_editor : def_editor);
+		goto bad;
+	    case EINVAL:
+		if (def_env_editor && env_editor != NULL) {
+		    /* User tried to do something funny with the editor. */
+		    log_warningx(SLOG_NO_STDERR|SLOG_AUDIT|SLOG_SEND_MAIL,
+			"invalid user-specified editor: %s", env_editor);
+		    goto bad;
+		}
+		FALLTHROUGH;
+	    default:
 		goto done;
-	    audit_failure(NewArgv, N_("%s: command not found"),
-		env_editor ? env_editor : def_editor);
-	    sudo_warnx(U_("%s: command not found"),
-		env_editor ? env_editor : def_editor);
-	    goto bad;
+	    }
 	}
 	/* find_editor() already g/c'd edit_argv[] */
 	if (NewArgv != saved_argv) {

plugins/sudoers/visudo.c

@@ -365,7 +365,7 @@ static char *
 get_editor(int *editor_argc, char ***editor_argv)
 {
     char *editor_path = NULL, **allowlist = NULL;
-    const char *env_editor;
+    const char *env_editor = NULL;
     static const char *files[] = { "+1", "sudoers" };
     unsigned int allowlist_len = 0;
     debug_decl(get_editor, SUDOERS_DEBUG_UTIL);
@@ -399,7 +399,11 @@ get_editor(int *editor_argc, char ***editor_argv)
     if (editor_path == NULL) {
 	if (def_env_editor && env_editor != NULL) {
 	    /* We are honoring $EDITOR so this is a fatal error. */
-	    sudo_fatalx(U_("specified editor (%s) doesn't exist"), env_editor);
+	    if (errno == ENOENT) {
+		sudo_warnx(U_("specified editor (%s) doesn't exist"),
+		    env_editor);
+	    }
+	    exit(EXIT_FAILURE);
 	}
 	sudo_fatalx(U_("no editor found (editor path = %s)"), def_editor);
     }