Re: [PATCH/RFC/GSoC 1/3] path.c: implement xdg_runtime_dir()

> On Wed, Mar 16, 2016 at 06:07:43PM +0800, Hui Yiqun wrote:
>
>> +     if (runtime_dir && *runtime_dir)
>> +             git_runtime_dir = mkpathdup("%s/git/", runtime_dir);
>> +     else
>> +             git_runtime_dir = mkpathdup("/tmp/git-%d", uid);
>
> Here we allocate the string, but later we may return NULL on error,
> leaking the allocated memory.

>
>> +     if (!lstat(git_runtime_dir, &st)) {
>> +             /*
>> +              * As described in XDG base dir spec[1], the subdirectory
>> +              * under $XDG_RUNTIME_DIR or its fallback MUST be owned by
>> +              * the user, and its unix access mode MUST be 0700.
>> +              *
>> +              * Calling chmod or chown silently may cause security
>> +              * problem if somebody chdir to it, sleep, and then, try
>> +              * to open our protected runtime cache or socket.
>> +              * So we just put warning and left it to user to solve.
>> +              *
>> +              * [1]https://specifications.freedesktop.org/basedir-spec/
>> +              * basedir-spec-latest.html
>> +              */
>
> OK. I think these checks should be sufficient to deal with the /tmp race
> I mentioned elsewhere in the thread (assuming that an attacker cannot
> flip the uid back and forth in the same way, but that should be true on
> Unix systems).
>
>> +             if ((st.st_mode & 0777) != S_IRWXU) {
>> +                     fprintf(stderr,
>> +                                     "permission of runtime directory '%s' "
>> +                                     "MUST be 0700 instead of 0%o\n",
>> +                                     git_runtime_dir, (st.st_mode & 0777));
>> +                     return NULL;
>> +             } else if (st.st_uid != uid) {
>> +                     fprintf(stderr,
>> +                                     "owner of runtime directory '%s' "
>> +                                     "MUST be %d instead of %d\n",
>> +                                     git_runtime_dir, uid, st.st_uid);
>> +                     return NULL;
>> +             }
>
> Should these be using warning(), rather than a raw fprintf?

>
>> +     } else {
>> +             if (safe_create_leading_directories_const(git_runtime_dir) < 0) {
>> +                     fprintf(stderr,
>> +                                     "unable to create directories for '%s'\n",
>> +                                     git_runtime_dir);
>> +                     return NULL;
>> +             }
>> +             if (mkdir(git_runtime_dir, 0700) < 0) {
>> +                     fprintf(stderr,
>> +                                     "unable to mkdir '%s'\n", git_runtime_dir);
>> +                     return NULL;
>> +             }
>> +     }
>
> And this retains the un-racy mkdir(). Good.
>
>> +     free(git_runtime_dir);
>> +     return mkpathdup("%s/%s", git_runtime_dir, filename);
>
> This mkpathdup accesses the string we just freed?
>
> It might be easier to just use a strbuf here, and then you can append to
> it at the end.