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? >> >>> Should the >>> mmap MAP_SHARED-write trigger the loss of the set-gid bit too? While >>> holding the file open with either open or mmap, I get a Text-in-use >>> error, so I would kind of expect the same behavior between either >>> close() and munmap(). I wonder if this is a bug, and if so, then your >>> link patch is indeed useful again. :) >> >> I don't see how this could be done with mmap(). Maybe we have a way to know >> when the first write is performed via this path, I have no idea. > > do_wp_page might be a decent bet. Or wp_page_shared? Can we get back to a file from the mm at that point? -Kees -- 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
