On Fri, Nov 6, 2015 at 4:48 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: > On Fri, Nov 6, 2015 at 4:16 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote: >> On Fri, Nov 6, 2015 at 4:11 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote: >>> On Fri, Nov 6, 2015 at 2:30 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: >>>> On Fri, Nov 6, 2015 at 1:59 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote: >>>>> Adding Ted, who might know how this all hooks together. (The context >>>>> is that a write() or truncate() on a setgid file clears the setgid, >>>>> but mmap writes don't.) >>>>> >>>>> On Wed, Nov 4, 2015 at 9:59 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: >>>>>> On Tue, Nov 3, 2015 at 10:58 PM, Willy Tarreau <w@xxxxxx> wrote: >>>>>>> On Tue, Nov 03, 2015 at 03:29:55PM -0800, Kees Cook wrote: >>>>>>>> Using "write" does kill the set-gid bit. I haven't looked at >>>>>>>> why. >>>>>>>> Al or anyone else, is there a meaningful distinction here? >>>>>>> >>>>>>> I remember this one, I got caught once while trying to put a shell into >>>>>>> a suid-writable file to get some privileges someone forgot to offer me :-) >>>>>>> >>>>>>> It's done by should_remove_suid() which is called upon write() and truncate(). >>>>> >>>>> file_remove_privs() seems to be the right entry point. >>>>> __generic_file_write_iter in mm/filemap.c calls it, though. Are these >>>>> callbacks not used for mmap writes? >>>> >>>> They're certainly not used early enough -- we need to remove suid when >>>> the page becomes writable via mmap (wp_page_shared), not when >>>> writeback happens, or at least not only when writeback happens. >>> >>> Well, I'm shy about the change there. For example, we don't strip in >>> on open(RDWR), just on write(). >> >> I take it back. Hooking wp_page_shared looks expensive. :) Maybe we do >> need to hook the mmap? > > But file_update_time already pokes at the same (or nearby) cachelines, > I think -- why would it be expensive? The whole thing could be > guarded by if (unlikely(is setuid)), right? Yeah, true. I added file_remove_privs calls near all the file_update_time calls, to no effect. Added to wp_page_shared too, nothing. Hmmm. -- Kees Cook Chrome OS Security -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
